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Java Programming Tutorial

OOP Exercises

Exercises on Classes

Ex: The Circle Class (An Introduction to Classes and Instances)

This first exercise shall lead you through all the basic concepts in OOP.

ExerciseOOP_Circle.png

A class called circle is designed as shown in the following class diagram. It contains:

  • Two private instance variables: radius (of the type double) and color (of the type String), with default value of 1.0 and "red", respectively.
  • Two overloaded constructors - a default constructor with no argument, and a constructor which takes a double argument for radius.
  • Two public methods: getRadius() and getArea(), which return the radius and area of this instance, respectively.

The source codes for Circle.java is as follows:

/*
 * The Circle class models a circle with a radius and color.
 */
public class Circle {  // Save as "Circle.java"
   // private instance variable, not accessible from outside this class
   private double radius;
   private String color;
   
   // The default constructor with no argument.
   // It sets the radius and color to their default value.
   public Circle() {
      radius = 1.0;
      color = "red";
   }
   
   // 2nd constructor with given radius, but color default
   public Circle(double r) {
      radius = r;
      color = "red";
   }
   
   // A public method for retrieving the radius
   public double getRadius() {
     return radius; 
   }
   
   // A public method for computing the area of circle
   public double getArea() {
      return radius*radius*Math.PI;
   }
}

Compile "Circle.java". Can you run the Circle class? Why?

This Circle class does not have a main() method. Hence, it cannot be run directly. This Circle class is a “building block” and is meant to be used in another program.

Let us write a test program called TestCircle (in another source file called TestCircle.java) which uses the Circle class, as follows:

public class TestCircle {  // Save as "TestCircle.java"
   public static void main(String[] args) {
      // Declare an instance of Circle class called c1.
      // Construct the instance c1 by invoking the "default" constructor
      // which sets its radius and color to their default value.
      Circle c1 = new Circle();
      // Invoke public methods on instance c1, via dot operator.
      System.out.println("The circle has radius of " 
         + c1.getRadius() + " and area of " + c1.getArea());
   
      // Declare an instance of class circle called c2.
      // Construct the instance c2 by invoking the second constructor
      // with the given radius and default color.
      Circle c2 = new Circle(2.0);
      // Invoke public methods on instance c2, via dot operator.
      System.out.println("The circle has radius of " 
         + c2.getRadius() + " and area of " + c2.getArea());
   }
}

Now, run the TestCircle and study the results.

More Basic OOP Concepts
  1. Constructor: Modify the class Circle to include a third constructor for constructing a Circle instance with two arguments - a double for radius and a String for color.
    // 3rd constructor to construct a new instance of Circle with the given radius and color
    public Circle (double r, String c) { ...... }
    Modify the test program TestCircle to construct an instance of Circle using this constructor.
  2. Getter: Add a getter for variable color for retrieving the color of this instance.
    // Getter for instance variable color
    public String getColor() { ...... }
    Modify the test program to test this method.
  3. public vs. private: In TestCircle, can you access the instance variable radius directly (e.g., System.out.println(c1.radius)); or assign a new value to radius (e.g., c1.radius=5.0)? Try it out and explain the error messages.
  4. Setter: Is there a need to change the values of radius and color of a Circle instance after it is constructed? If so, add two public methods called setters for changing the radius and color of a Circle instance as follows:
    // Setter for instance variable radius
    public void setRadius(double newRadius) {
       radius = newRadius;
    }
     
    // Setter for instance variable color
    public void setColor(String newColor) { ...... }
    Modify the TestCircle to test these methods, e.g.,
    Circle c4 = new Circle();   // construct an instance of Circle
    c4.setRadius(5.0);          // change radius
    System.out.println("radius is: " + c4.getRadius()); // Print radius via getter
    c4.setColor(......);        // Change color
    System.out.println("color is: " + c4.getColor());   // Print color via getter
    
    // You cannot do the following because setRadius() returns void,
    // which cannot be printed.
    System.out.println(c4.setRadius(4.0));
  5. Keyword "this": Instead of using variable names such as r (for radius) and c (for color) in the methods' arguments, it is better to use variable names radius (for radius) and color (for color) and use the special keyword "this" to resolve the conflict between instance variables and methods' arguments. For example,
    // Instance variable
    private double radius;
    
    // Constructor
    public Circle(double radius) {
       this.radius = radius;   // "this.radius" refers to the instance variable
                               // "radius" refers to the method's parameter
       color = .......   
    }
    
    // Setter of radius
    public void setRadius(double radius) {
       this.radius = radius;   // "this.radius" refers to the instance variable
                               // "radius" refers to the method's argument
    }
    Modify ALL the constructors and setters in the Circle class to use the keyword "this".
  6. Method toString(): Every well-designed Java class should contain a public method called toString() that returns a short description of the instance (in a return type of String). The toString() method can be called explicitly (via instanceName.toString()) just like any other method; or implicitly through println(). If an instance is passed to the println(anInstance) method, the toString() method of that instance will be invoked implicitly. For example, include the following toString() methods to the Circle class:
    // Return a description of this instance in the form of
    // Circle[radius=r,color=c]
    public String toString() {
       return "Circle[radius=" + radius + " color=" + color + "]";
    }
    Try calling toString() method explicitly, just like any other method:
    Circle c1 = new Circle(5.0);
    System.out.println(c1.toString());   // explicit call
    toString() is called implicitly when an instance is passed to println() method, for example,
    Circle c2 = new Circle(1.2);
    System.out.println(c2.toString());  // explicit call
    System.out.println(c2);             // println() calls toString() implicitly, same as above
    System.out.println("Operator '+' invokes toString() too: " + c2);  // '+' invokes toString() too

The final class diagram for the Circle class is as follows:

ExerciseOOP_Circle_Final.png

Ex: A Simplified Circle Class

ExerciseOOP_Circle_Simplified.png

Ex: The Rectangle Class

ExerciseOOP_Rectangle.png

Ex: The Employee Class

ExerciseOOP_Employee.png

Ex: The InvoiceItem Class

ExerciseOOP_InvoiceItem.png

Ex: The Account Class

ExerciseOOP_Account.png

Ex: The Date Class

ExerciseOOP_Date.png

Ex: The Time Class

ExerciseOOP_Time.png

Exercises on Composition

Ex: The Author and Book Classes (An Introduction to OOP Composition)

This first exercise shall lead you through all the concepts involved in OOP Composition.

ExerciseOOP_Author.png

A class called Author (as shown in the class diagram) is designed to model a book's author. It contains:

  • Three private instance variables: name (String), email (String), and gender (char of either 'm' or 'f');
  • One constructor to initialize the name, email and gender with the given values;
    public Author (String name, String email, char gender) {......}
    (There is no default constructor for Author, as there are no defaults for name, email and gender.)
  • public getters/setters: getName(), getEmail(), setEmail(), and getGender();
    (There are no setters for name and gender, as these attributes cannot be changed.)
  • A toString() method that returns "Author[name=?,email=?,gender=?]", e.g., "Author[name=Tan Ah Teck,email=ahTeck@somewhere.com,gender=m]".

Write the Author class. Also write a test driver called TestAuthor to test all the public methods, e.g.,

Author ahTeck = new Author("Tan Ah Teck", "ahteck@nowhere.com", 'm'); // Test the constructor
System.out.println(ahTeck);  // Test toString()
ahTeck.setEmail("paulTan@nowhere.com");  // Test setter
System.out.println("name is: " + ahTeck.getName());     // Test getter
System.out.println("eamil is: " + ahTeck.getEmail());   // Test getter
System.out.println("gender is: " + ahTeck.getGender()); // Test gExerciseOOP_MyPolynomial.pngetter

ExerciseOOP_Book.png

A class called Book is designed (as shown in the class diagram) to model a book written by one author. It contains:

  • Four private instance variables: name (String), author (of the class Author you have just created, assume that a book has one and only one author), price (double), and qty (int);
  • Two constructors:
    public Book (String name, Author author, double price) { ...... }
    public Book (String name, Author author, double price, int qty) { ...... }
  • public methods getName(), getAuthor(), getPrice(), setPrice(), getQty(), setQty().
  • A toString() that returns "Book[name=?,Author[name=?,email=?,gender=?],price=?,qty=?".  You should reuse Author’s toString().

Write the Book class (which uses the Author class written earlier). Also write a test driver called TestBook to test all the public methods in the class Book. Take Note that you have to construct an instance of Author before you can construct an instance of Book. E.g.,

// Construct an author instance
Author ahTeck = new Author("Tan Ah Teck", "ahteck@nowhere.com", 'm');
System.out.println(ahTeck);  // Author's toString()

Book dummyBook = new Book("Java for dummy", ahTeck, 19.95, 99);  // Test Book's Constructor
System.out.println(dummyBook);  // Test Book's toString()

// Test Getters and Setters
dummyBook.setPrice(29.95);
dummyBook.setQty(28);
System.out.println("name is: " + dummyBook.getName());
System.out.println("price is: " + dummyBook.getPrice());
System.out.println("qty is: " + dummyBook.getQty());
System.out.println("Author is: " + dummyBook.getAuthor());  // Author's toString()
System.out.println("Author's name is: " + dummyBook.getAuthor().getName());
System.out.println("Author's email is: " + dummyBook.getAuthor().getEmail());

// Use an anonymous instance of Author to construct a Book instance
Book anotherBook = new Book("more Java", 
      new Author("Paul Tan", "paul@somewhere.com", 'm'), 29.95);
System.out.println(anotherBook);  // toString()

Take note that both Book and Author classes have a variable called name. However, it can be differentiated via the referencing instance. For a Book instance says aBook, aBook.name refers to the name of the book; whereas for an Author's instance say auAuthor, anAuthor.name refers to the name of the author. There is no need (and not recommended) to call the variables bookName and authorName.

TRY:

  1. Printing the name and email of the author from a Book instance. (Hint: aBook.getAuthor().getName(), aBook.getAuthor().getEmail()).
  2. Introduce new methods called getAuthorName(), getAuthorEmail(), getAuthorGender() in the Book class to return the name, email and gender of the author of the book. For example,
    public String getAuthorName() {
       return author.getName();
          // cannot use author.name as name is private in Author class
    }

Exercise (Advanced): Book and Author Classes Again - An Array of Objects as an Instance Variable

ExerciseOOP_BookAuthorAgain.png

In the earlier exercise, a book is written by one and only one author. In reality, a book can be written by one or more author. Modify the Book class to support one or more authors by changing the instance variable authors to an Author array.

Notes:

  • The constructors take an array of Author (i.e., Author[]), instead of an Author instance. In this design, once a Book instance is constructor, you cannot add or remove author.
  • The toString() method shall return "Book[name=?,authors={Author[name=?,email=?,gender=?],......},price=?,qty=?]".

You are required to:

  1. Write the code for the Book class. You shall re-use the Author class written earlier.
  2. Write a test driver (called TestBook) to test the Book class.

Hints:

// Declare and allocate an array of Authors
Author[] authors = new Author[2];
authors[0] = new Author("Tan Ah Teck", "AhTeck@somewhere.com", 'm');
authors[1] = new Author("Paul Tan", "Paul@nowhere.com", 'm');

// Declare and allocate a Book instance
Book javaDummy = new Book("Java for Dummy", authors, 19.99, 99);
System.out.println(javaDummy);  // toString()

Ex: The MyPoint Class

ExerciseOOP_MyPoint.png

A class called MyPoint, which models a 2D point with x and y coordinates, is designed as shown in the class diagram. It contains:

  • Two instance variables x (int) and y (int).
  • A default (or "no-argument" or "no-arg") constructor that construct a point at the default location of (0, 0).
  • A overloaded constructor that constructs a point with the given x and y coordinates.
  • Getter and setter for the instance variables x and y.
  • A method setXY() to set both x and y.
  • A method getXY() which returns the x and y in a 2-element int array.
  • A toString() method that returns a string description of the instance in the format "(x, y)".
  • A method called distance(int x, int y) that returns the distance from this point to another point at the given (x, y) coordinates, e.g.,
    MyPoint p1 = new MyPoint(3, 4);
    System.out.println(p1.distance(5, 6));
  • An overloaded distance(MyPoint another) that returns the distance from this point to the given MyPoint instance (called another), e.g.,
    MyPoint p1 = new MyPoint(3, 4);
    MyPoint p2 = new MyPoint(5, 6);
    System.out.println(p1.distance(p2));
  • Another overloaded distance() method that returns the distance from this point to the origin (0,0), e.g.,
    MyPoint p1 = new MyPoint(3, 4);
    System.out.println(p1.distance());

You are required to:

  1. Write the code for the class MyPoint. Also write a test program (called TestMyPoint) to test all the methods defined in the class.
    Hints:
    // Overloading method distance()
    // This version takes two ints as arguments
    public double distance(int x, int y) {
       int xDiff = this.x – x;
       int yDiff = ......
       return Math.sqrt(xDiff*xDiff + yDiff*yDiff);
    }
      
    // This version takes a MyPoint instance as argument
    public double distance(MyPoint another) {
       int xDiff = this.x – another.x;
       .......
    }
    // Test program to test all constructors and public methods
    MyPoint p1 = new MyPoint();  // Test constructor
    System.out.println(p1);      // Test toString()
    p1.setX(8);   // Test setters
    p1.setY(6);
    System.out.println("x is: " + p1.getX());  // Test getters
    System.out.println("y is: " + p1.getY());
    p1.setXY(3, 0);   // Test setXY()
    System.out.println(p1.getXY()[0]);  // Test getXY()
    System.out.println(p1.getXY()[1]);
    System.out.println(p1);
    
    MyPoint p2 = new MyPoint(0, 4);  // Test another constructor
    System.out.println(p2);
    // Testing the overloaded methods distance()
    System.out.println(p1.distance(p2));    // which version?
    System.out.println(p2.distance(p1));    // which version?
    System.out.println(p1.distance(5, 6));  // which version?
    System.out.println(p1.distance());      // which version?
    
  2. Write a program that allocates 10 points in an array of MyPoint, and initializes to (1, 1), (2, 2), ... (10, 10).
    Hints: You need to allocate the array, as well as each of the 10 MyPoint instances.  In other words, you need to issue 11 new, 1 for the array and 10 for the MyPoint instances.
    MyPoint[] points = new MyPoint[10];  // Declare and allocate an array of MyPoint
    for (int i = 0; i < points.length; i++) {
       points[i] = new MyPoint(...);    // Allocate each of MyPoint instances
    }
    // use a loop to print all the points
    

Notes: Point is such a common entity that JDK certainly provided for in all flavors.

Ex: The MyCircle and MyPoint Classes

ExerciseOOP_MyCircle.png

A class called MyCircle, which models a circle with a center (x,y) and a radius, is designed as shown in the class diagram. The MyCircle class uses an instance of MyPoint class (created in the previous exercise) as its center.

The class contains:

  • Two private instance variables: center (an instance of MyPoint) and radius (int).
  • A constructor that constructs a circle with the given center's (x, y) and radius.
  • An overloaded constructor that constructs a MyCircle given a MyPoint instance as center, and radius.
  • A default constructor that construct a circle with center at (0,0) and radius of 1.
  • Various getters and setters.
  • A toString() method that returns a string description of this instance in the format "MyCircle[radius=r,center=(x,y)]". You shall reuse the toString() of MyPoint.
  • getArea() and getCircumference() methods that return the area and circumference of this circle in double.
  • A distance(MyCircle another) method that returns the distance of the centers from this instance and the given MyCircle instance.  You should use MyPoint’s distance() method to compute this distance.

Write the MyCircle class. Also write a test driver (called TestMyCircle) to test all the public methods defined in the class.

Hints:

// Constructors
public MyCircle(int x, int y, int radius) {
   // Need to construct an instance of MyPoint for the variable center
   center = new MyPoint(x, y);
   this.radius = radius;
}
public MyCircle(MyPoint center, int radius) {
   // An instance of MyPoint already constructed by caller; simply assign.
   this.center = center;
   ......
}
public MyCircle() {
   center = new MyPoint(.....);  // construct MyPoint instance
   this.radius = ......
}

// Returns the x-coordinate of the center of this MyCircle
public int getCenterX() {
   return center.getX();   // cannot use center.x and x is private in MyPoint
}

// Returns the distance of the center for this MyCircle and another MyCircle
public double distance(MyCircle another) {
   return center.distance(another.center); // use distance() of MyPoint
}

Ex: The MyTriangle and MyPoint Classes

ExerciseOOP_MyTriangle.png

A class called MyTriangle, which models a triangle with 3 vertices, is designed as shown. The MyTriangle class uses three MyPoint instances (created in the earlier exercise) as its three vertices.

It contains:

  • Three private instance variables v1, v2, v3 (instances of MyPoint), for the three vertices.
  • A constructor that constructs a MyTriangle with three set of coordinates, v1=(x1, y1), v2=(x2, y2), v3=(x3, y3).
  • An overloaded constructor that constructs a MyTriangle given three instances of MyPoint.
  • A toString() method that returns a string description of the instance in the format "MyTriangle[v1=(x1,y1),v2=(x2,y2),v3=(x3,y3)]".
  • A getPerimeter() method that returns the length of the perimeter in double. You should use the distance() method of MyPoint to compute the perimeter.
  • A method printType(), which prints "equilateral" if all the three sides are equal, "isosceles" if any two of the three sides are equal, or "scalene" if the three sides are different.

Write the MyTriangle class. Also write a test driver (called TestMyTriangle) to test all the public methods defined in the class.

Ex: The MyRectangle and MyPoint Classes

Design a MyRectangle class which is composed of two MyPoint instances as its top-left and bottom-right corners. Draw the class diagrams, write the codes, and write the test drivers.

Ex: The Customer and Invoice classes

ExerciseOOP_Customer.png

The Customer class models a customer is design as shown in the class diagram. Write the codes for the Customer class and a test driver to test all the public methods.

ExerciseOOP_Invoice.png

The Invoice class, design as shown in the class diagram, composes a Customer instance (written earlier) as its member. Write the codes for the Invoice class and a test driver to test all the public methods.

Ex: The Customer and Account classes

ExerciseOOP_CustomerAccount.png

The Customer class models a customer is design as shown in the class diagram. Write the codes for the Customer class and a test driver to test all the public methods.

ExerciseOOP_AccountCustomer.png

The Account class models a bank account, design as shown in the class diagram, composes a Customer instance (written earlier) as its member. Write the codes for the Account class and a test driver to test all the public methods.

More Exercises on Classes

Ex: The MyComplex class

ExerciseOOP_MyComplex.png

A class called MyComplex, which models complex numbers x+yi, is designed as shown in the class diagram. It contains:

  • Two instance variable named real (double) and imag (double) which stores the real and imaginary parts of the complex number, respectively.
  • A constructor that creates a MyComplex instance with the given real and imaginary values.
  • A default constructor that create a MyComplex at 0.0 + 0.0i.
  • Getters and setters for instance variables real and imag.
  • A method setValue() to set the value of the complex number.
  • A toString() that returns "(x + yi)" where x and y are the real and imaginary parts, respectively.
  • Methods isReal() and isImaginary() that returns true if this complex number is real or imaginary, respectively.
    Hints:
    return (imag == 0);
  • A method equals(double real, double imag) that returns true if this complex number is equal to the given complex number (real, imag).
    Hints:
    return (this.real == real && this.imag == imag);
  • An overloaded equals(MyComplex another) that returns true if this complex number is equal to the given MyComplex instance another.
    Hints:
    return (this.real == another.real && this.imag == another.imag);
  • A method magnitude() that returns the magnitude of this complex number.
    magnitude(x+yi) = Math.sqrt(x*x + y*y)
  • Methods argument() that returns the argument of this complex number in radians (double).
    arg(x+yi) = Math.atan2(y, x) (in radians)
    Note: The Math library has two arc-tangent methods, Math.atan(double) and Math.atan2(double, double). We commonly use the Math.atan2(y, x) instead of Math.atan(y/x) to avoid division by zero. Read the documentation of Math class in package java.lang.
  • Methods add(MyComplex right) and subtract(MyComplex right) that adds and subtract the given MyComplex instance (called right), into/from this instance and returns this instance.
    (a + bi) + (c + di) = (a+c) + (b+d)i
    (a + bi) - (c + di) = (a-c) + (b-d)i
    Hints:
    return this;  // return "this" instance
  • Methods addNew(MyComplex right) and subtractNew(MyComplex right) that adds and subtract this instance with the given MyComplex instance called right, and returns a new MyComplex instance containing the result.
    Hint:
    // construct a new instance and return the constructed instance
    return new MyComplex(..., ...);
  • Methods multiply(MyComplex right) and divide(MyComplex right) that multiplies and divides this instance with the given MyComplex instance right, and keeps the result in this instance, and returns this instance.
    (a + bi) * (c + di) = (ac - bd) + (ad + bc)i
    (a + bi) / (c + di) = [(a + bi) * (c – di)] / (c*c + d*d)
  • A method conjugate() that operates on this instance and returns this instance containing the complex conjugate.
    conjugate(x+yi) = x - yi

You are required to:

  1. Write the MyComplex class.
  2. Write a test driver to test all the public methods defined in the class.
  3. Write an application called MyComplexApp that uses the MyComplex class. The application shall prompt the user for two complex numbers, print their values, check for real, imaginary and equality, and carry out all the arithmetic operations.
    Enter complex number 1 (real and imaginary part): 1.1 2.2
    Enter complex number 2 (real and imaginary part): 3.3 4.4
      
    Number 1 is: (1.1 + 2.2i)
    (1.1 + 2.2i) is NOT a pure real number
    (1.1 + 2.2i) is NOT a pure imaginary number
      
    Number 2 is: (3.3 + 4.4i)
    (3.3 + 4.4i) is NOT a pure real number
    (3.3 + 4.4i) is NOT a pure imaginary number
       
    (1.1 + 2.2i) is NOT equal to (3.3 + 4.4i)
    (1.1 + 2.2i) + (3.3 + 4.4i) = (4.4 + 6.6000000000000005i)
    (1.1 + 2.2i) - (3.3 + 4.4i) = (-2.1999999999999997 + -2.2i)

Take note that there are a few flaws in the design of this class, which was introduced solely for teaching purpose:

  • Comparing doubles in equal() using "==" may produce unexpected outcome. For example, (2.2+4.4)==6.6 returns false. It is common to define a small threshold called EPSILON (set to about 10^-8) for comparing floating point numbers.
  • The method addNew(), subtractNew() produce new instances, whereas add(), subtract(), multiply(), divide() and conjugate() modify this instance. There is inconsistency in the design (introduced for teaching purpose).

Also take note that methods such as add() returns an instance of MyComplex. Hence, you can place the result inside a System.out.println() (which implicitly invoke the toString()). You can also chain the operations, e.g., c1.add(c2).add(c3) (same as (c1.add(c2)).add(c3)), or c1.add(c2).subtract(c3).

Ex: The MyPolynomial Class

ExerciseOOP_MyPolynomial.png

A class called MyPolynomial, which models polynomials of degree-n (see equation), is designed as shown in the class diagram.

ExerciseOOP_MyPolynomialEqn.png

It contains:

  • An instance variable named coeffs, which stores the coefficients of the n-degree polynomial in a double array of size n+1, where c0 is kept at index 0.
  • A constructor MyPolynomial(coeffs:double...) that takes a variable number of doubles to initialize the coeffs array, where the first argument corresponds to c0.
    The three dots is known as varargs (variable number of arguments), which is a new feature introduced in JDK 1.5. It accepts an array or a sequence of comma-separated arguments. The compiler automatically packs the comma-separated arguments in an array. The three dots can only be used for the last argument of the method.
    Hints:
    public class MyPolynomial {
       private double[] coeffs;
       public MyPolynomial(double... coeffs) {  // varargs
          this.coeffs = coeffs;                 // varargs is treated as array
       }
       ......
    }
       
    // Test program
    // Can invoke with a variable number of arguments
    MyPolynomial p1 = new MyPolynomial(1.1, 2.2, 3.3);
    MyPolynomial p1 = new MyPolynomial(1.1, 2.2, 3.3, 4.4, 5.5);
    // Can also invoke with an array
    Double coeffs = {1.2, 3.4, 5.6, 7.8}
    MyPolynomial p2 = new MyPolynomial(coeffs);
  • A method getDegree() that returns the degree of this polynomial.
  • A method toString() that returns "cnx^n+cn-1x^(n-1)+...+c1x+c0".
  • A method evaluate(double x) that evaluate the polynomial for the given x, by substituting the given x into the polynomial expression.
  • Methods add() and multiply() that adds and multiplies this polynomial with the given MyPolynomial instance another, and returns this instance that contains the result.

Write the MyPolynomial class. Also write a test driver (called TestMyPolynomial) to test all the public methods defined in the class.

Question: Do you need to keep the degree of the polynomial as an instance variable in the MyPolynomial class in Java? How about C/C++? Why?

Ex: Using JDK's BigInteger Class

Recall that primitive integer type byte, short, int and long represent 8-, 16-, 32-, and 64-bit signed integers, respectively. You cannot use them for integers bigger than 64 bits. Java API provides a class called BigInteger in a package called java.math. Study the API of the BigInteger class (Java API ⇒ From "Packages", choose "java.math" " From "classes", choose "BigInteger" " Study the constructors (choose "CONSTR") on how to construct a BigInteger instance, and the public methods available (choose "METHOD"). Look for methods for adding and multiplying two BigIntegers.

Write a program called TestBigInteger that:

  1. adds "11111111111111111111111111111111111111111111111111111111111111" to "22222222222222222222222222222222222222222222222222" and prints the result.
  2. multiplies the above two number and prints the result.

Hints:

import java.math.BigInteger
public class TestBigInteger {
   public static void main(String[] args) {
      BigInteger i1 = new BigInteger(...);
      BigInteger i2 = new BigInteger(...);
      System.out.println(i1.add(i2));
      .......
   }
}

Ex: The MyTime Class

ExerciseOOP_MyTime.png

A class called MyTime, which models a time instance, is designed as shown in the class diagram.

It contains the following private instance variables:

  • hour: between 0 to 23.
  • minute: between 0 to 59.
  • Second: between 0 to 59.

You are required to perform input validation.

It contains the following public methods:

  • setTime(int hour, int minute, int second): It shall check if the given hour, minute and second are valid before setting the instance variables.
    (Advanced: Otherwise, it shall throw an IllegalArgumentException with the message "Invalid hour, minute, or second!".)
  • Setters setHour(int hour), setMinute(int minute), setSecond(int second): It shall check if the parameters are valid, similar to the above.
  • Getters getHour(), getMinute(), getSecond().
  • toString(): returns "HH:MM:SS".
  • nextSecond(): Update this instance to the next second and return this instance. Take note that the nextSecond() of 23:59:59 is 00:00:00.
  • nextMinute(), nextHour(), previousSecond(), previousMinute(), previousHour(): similar to the above.

Write the code for the MyTime class. Also write a test driver (called TestMyTime) to test all the public methods defined in the MyTime class.

Ex: The MyDate Class

ExerciseOOP_MyDate.png

A class called MyDate, which models a date instance, is defined as shown in the class diagram.

The MyDate class contains the following private instance variables:

  • year (int): Between 1 to 9999.
  • month (int): Between 1 (Jan) to 12 (Dec).
  • day (int): Between 1 to 28|29|30|31, where the last day depends on the month and whether it is a leap year for Feb (28|29).

It also contains the following private static variables (drawn with underlined in the class diagram):

  • strMonths (String[]), strDays (String[]), and dayInMonths (int[]): static variables, initialized as shown, which are used in the methods.

The MyDate class has the following public static methods (drawn with underlined in the class diagram):

  • isLeapYear(int year): returns true if the given year is a leap year. A year is a leap year if it is divisible by 4 but not by 100, or it is divisible by 400.
  • isValidDate(int year, int month, int day): returns true if the given year, month, and day constitute a valid date. Assume that year is between 1 and 9999, month is between 1 (Jan) to 12 (Dec) and day shall be between 1 and 28|29|30|31 depending on the month and whether it is a leap year on Feb.
  • getDayOfWeek(int year, int month, int day): returns the day of the week, where 0 for Sun, 1 for Mon, ..., 6 for Sat, for the given date. Assume that the date is valid. Read the earlier exercise on how to determine the day of the week (or Wiki "Determination of the day of the week").

The MyDate class has one constructor, which takes 3 parameters: year, month and day. It shall invoke setDate() method (to be described later) to set the instance variables.

The MyDate class has the following public methods:

  • setDate(int year, int month, int day): It shall invoke the static method isValidDate() to verify that the given year, month and day constitute a valid date.
    (Advanced: Otherwise, it shall throw an IllegalArgumentException with the message "Invalid year, month, or day!".)
  • setYear(int year): It shall verify that the given year is between 1 and 9999.
    (Advanced: Otherwise, it shall throw an IllegalArgumentException with the message "Invalid year!".)
  • setMonth(int month): It shall verify that the given month is between 1 and 12.
    (Advanced: Otherwise, it shall throw an IllegalArgumentException with the message "Invalid month!".)
  • setDay(int day): It shall verify that the given day is between 1 and dayMax, where dayMax depends on the month and whether it is a leap year for Feb.
    (Advanced: Otherwise, it shall throw an IllegalArgumentException with the message "Invalid month!".)
  • getYear(), getMonth(), getDay(): return the value for the year, month and day, respectively.
  • toString(): returns a date string in the format "xxxday d mmm yyyy", e.g., "Tuesday 14 Feb 2012".
  • nextDay(): update this instance to the next day and return this instance. Take note that nextDay() for 31 Dec 2000 shall be 1 Jan 2001.
  • nextMonth(): update this instance to the next month and return this instance. Take note that nextMonth() for 31 Oct 2012 shall be 30 Nov 2012.
  • nextYear(): update this instance to the next year and return this instance. Take note that nextYear() for 29 Feb 2012 shall be 28 Feb 2013.
    (Advanced: throw an IllegalStateException with the message "Year out of range!" if year > 9999.)
  • previousDay(), previousMonth(), previousYear(): similar to the above.

Write the code for the MyDate class.

Use the following test statements to test the MyDate class:

MyDate d1 = new MyDate(2012, 2, 28);
System.out.println(d1);             // Tuesday 28 Feb 2012
System.out.println(d1.nextDay());   // Wednesday 29 Feb 2012
System.out.println(d1.nextDay());   // Thursday 1 Mar 2012
System.out.println(d1.nextMonth()); // Sunday 1 Apr 2012
System.out.println(d1.nextYear());  // Monday 1 Apr 2013

MyDate d2 = new MyDate(2012, 1, 2);
System.out.println(d2);                 // Monday 2 Jan 2012
System.out.println(d2.previousDay());   // Sunday 1 Jan 2012
System.out.println(d2.previousDay());   // Saturday 31 Dec 2011
System.out.println(d2.previousMonth()); // Wednesday 30 Nov 2011
System.out.println(d2.previousYear());  // Tuesday 30 Nov 2010

MyDate d3 = new MyDate(2012, 2, 29);
System.out.println(d3.previousYear());  // Monday 28 Feb 2011

// MyDate d4 = new MyDate(2099, 11, 31); // Invalid year, month, or day!
// MyDate d5 = new MyDate(2011, 2, 29);  // Invalid year, month, or day!

Write a test program that tests the nextDay() in a loop, by printing the dates from 28 Dec 2011 to 2 Mar 2012.

Ex: Bouncing Balls - Ball and Container Classes

ExerciseOOP_Ball.png

A class called Ball is designed as shown in the class diagram.

The Ball class contains the following private instance variables:

  • x, y and radius, which represent the ball's center (x, y) co-ordinates and the radius, respectively.
  • xDelta (Δx) and yDelta (Δy), which represent the displacement (movement) per step, in the x and y direction respectively.

The Ball class contains the following public methods:

  • A constructor which accepts x, y, radius, speed, and direction as arguments. For user friendliness, user specifies speed (in pixels per step) and direction (in degrees in the range of (-180°, 180°]). For the internal operations, the speed and direction are to be converted to (Δx, Δy) in the internal representation. Note that the y-axis of the Java graphics coordinate system is inverted, i.e., the origin (0, 0) is located at the top-left corner.
    ExerciseOOP_BallCoord.png
    Δx = d × cos(θ)
    Δy = -d × sin(θ)
  • Getter and setter for all the instance variables.
  • A method move() which move the ball by one step.
    x += Δx
    y += Δy
  • reflectHorizontal() which reflects the ball horizontally (i.e., hitting a vertical wall)
    Δx = -Δx
    Δy no changes
  • reflectVertical() (the ball hits a horizontal wall).
    Δx no changes
    Δy = -Δy
  • toString() which prints the message "Ball at (x, y) of velocity (Δx, Δy)".

Write the Ball class. Also write a test program to test all the methods defined in the class.


ExerciseOOP_Container.png

A class called Container, which represents the enclosing box for the ball, is designed as shown in the class diagram. It contains:

  • Instance variables (x1, y1) and (x2, y2) which denote the top-left and bottom-right corners of the rectangular box.
  • A constructor which accepts (x, y) of the top-left corner, width and height as argument, and converts them into the internal representation (i.e., x2=x1+width-1). Width and height is used in the argument for safer operation (there is no need to check the validity of x2>x1 etc.).
  • A toString() method that returns "Container at (x1,y1) to (x2, y2)".
  • A boolean method called collidesWith(Ball), which check if the given Ball is outside the bounds of the container box. If so, it invokes the Ball's reflectHorizontal() and/or reflectVertical() to change the movement direction of the ball, and returns true.
    public boolean collidesWith(Ball ball) {
       if (ball.getX() - ball.getRadius() <= this.x1 ||
           ball.getX() - ball.getRadius() >= this.x2) {
          ball.reflectHorizontal();
          return true;
       }
       ......
    }

Use the following statements to test your program:

Ball ball = new Ball(50, 50, 5, 10, 30);
Container box = new Container(0, 0, 100, 100);
for (int step = 0; step < 100; ++step) {
   ball.move();
   box.collidesWith(ball);
   System.out.println(ball); // manual check the position of the ball
}

Ex: The Ball and Player Classes

ExerciseOOP_BallPlayer.png

The Ball class, which models the ball in a soccer game, is designed as shown in the class diagram. Write the codes for the Ball class and a test driver to test all the public methods.

ExerciseOOP_PlayerBall.png

The Player class, which models the players in a soccer game, is designed as shown in the class diagram. The Player interacts with the Ball (written earlier). Write the codes for the Player class and a test driver to test all the public methods. Make your assumption for the kick().

Can you write a very simple soccer game with 2 teams of players and a ball, inside a soccer field?

Exercises on Inheritance

Ex: The Circle and Cylinder Classes

This exercise shall guide you through the important concepts in inheritance.

ExerciseOOP_CircleCylinder.png

In this exercise, a subclass called Cylinder is derived from the superclass Circle as shown in the class diagram (where an an arrow pointing up from the subclass to its superclass). Study how the subclass Cylinder invokes the superclass' constructors (via super() and super(radius)) and inherits the variables and methods from the superclass Circle.

You can reuse the Circle class that you have created in the previous exercise. Make sure that you keep "Circle.class" in the same directory.

public class Cylinder extends Circle {  // Save as "Cylinder.java"
   private double height;  // private variable
   
   // Constructor with default color, radius and height
   public Cylinder() {
      super();        // call superclass no-arg constructor Circle()
      height = 1.0; 
   }
   // Constructor with default radius, color but given height
   public Cylinder(double height) {
      super();        // call superclass no-arg constructor Circle()
      this.height = height;
   }
   // Constructor with default color, but given radius, height
   public Cylinder(double radius, double height) {
      super(radius);  // call superclass constructor Circle(r)
      this.height = height;
   }
   
   // A public method for retrieving the height
   public double getHeight() {
      return height; 
   }
  
   // A public method for computing the volume of cylinder
   //  use superclass method getArea() to get the base area
   public double getVolume() {
      return getArea()*height; 
   }
}

Write a test program (says TestCylinder) to test the Cylinder class created, as follow:

public class TestCylinder {  // save as "TestCylinder.java"
   public static void main (String[] args) {
      // Declare and allocate a new instance of cylinder
      //   with default color, radius, and height
      Cylinder c1 = new Cylinder();
      System.out.println("Cylinder:"
            + " radius=" + c1.getRadius()
            + " height=" + c1.getHeight()
            + " base area=" + c1.getArea()
            + " volume=" + c1.getVolume());
   
      // Declare and allocate a new instance of cylinder
      //   specifying height, with default color and radius
      Cylinder c2 = new Cylinder(10.0);
      System.out.println("Cylinder:"
            + " radius=" + c2.getRadius()
            + " height=" + c2.getHeight()
            + " base area=" + c2.getArea()
            + " volume=" + c2.getVolume());
   
      // Declare and allocate a new instance of cylinder
      //   specifying radius and height, with default color
      Cylinder c3 = new Cylinder(2.0, 10.0);
      System.out.println("Cylinder:"
            + " radius=" + c3.getRadius()
            + " height=" + c3.getHeight()
            + " base area=" + c3.getArea()
            + " volume=" + c3.getVolume());
   }
}

Method Overriding and "Super": The subclass Cylinder inherits getArea() method from its superclass Circle. Try overriding the getArea() method in the subclass Cylinder to compute the surface area (=2π×radius×height + 2×base-area) of the cylinder instead of base area. That is, if getArea() is called by a Circle instance, it returns the area. If getArea() is called by a Cylinder instance, it returns the surface area of the cylinder.

If you override the getArea() in the subclass Cylinder, the getVolume() no longer works. This is because the getVolume() uses the overridden getArea() method found in the same class. (Java runtime will search the superclass only if it cannot locate the method in this class). Fix the getVolume().

Hints: After overridding the getArea() in subclass Cylinder, you can choose to invoke the getArea() of the superclass Circle by calling super.getArea().

TRY:

Provide a toString() method to the Cylinder class, which overrides the toString() inherited from the superclass Circle, e.g.,

@Override
public String toString() {      // in Cylinder class
   return "Cylinder: subclass of " + super.toString()  // use Circle's toString()
          + " height=" + height;
}

Try out the toString() method in TestCylinder.

Note: @Override is known as annotation (introduced in JDK 1.5), which asks compiler to check whether there is such a method in the superclass to be overridden. This helps greatly if you misspell the name of the toString(). If @Override is not used and toString() is misspelled as ToString(), it will be treated as a new method in the subclass, instead of overriding the superclass. If @Override is used, the compiler will signal an error. @Override annotation is optional, but certainly nice to have.

Ex: Superclass Person and its subclasses

ExerciseOOP_PersonAndSubclasses.png

Ex: Point2D and Point3D

ExerciseOOP_Point2DPoint3D.png

Ex: Point and MovablePoint

ExerciseOOP_PointMovablePoint.png

Ex: Superclass Shape and its subclasses Circle, Rectangle and Square

ExerciseOOP_ShapeAndSubclasses.png

Write a superclass called Shape (as shown in the class diagram), which contains:

  • Two instance variables color (String) and filled (boolean).
  • Two constructors: a no-arg (no-argument) constructor that initializes the color to "green" and filled to true, and a constructor that initializes the color and filled to the given values.
  • Getter and setter for all the instance variables. By convention, the getter for a boolean variable xxx is called isXXX() (instead of getXxx() for all the other types).
  • A toString() method that returns "A Shape with color of xxx and filled/Not filled".

Write a test program to test all the methods defined in Shape.

Write two subclasses of Shape called Circle and Rectangle, as shown in the class diagram.

The Circle class contains:

  • An instance variable radius (double).
  • Three constructors as shown. The no-arg constructor initializes the radius to 1.0.
  • Getter and setter for the instance variable radius.
  • Methods getArea() and getPerimeter().
  • Override the toString() method inherited, to return "A Circle with radius=xxx, which is a subclass of yyy", where yyy is the output of the toString() method from the superclass.

The Rectangle class contains:

  • Two instance variables width (double) and length (double).
  • Three constructors as shown. The no-arg constructor initializes the width and length to 1.0.
  • Getter and setter for all the instance variables.
  • Methods getArea() and getPerimeter().
  • Override the toString() method inherited, to return "A Rectangle with width=xxx and length=zzz, which is a subclass of yyy", where yyy is the output of the toString() method from the superclass.

Write a class called Square, as a subclass of Rectangle. Convince yourself that Square can be modeled as a subclass of Rectangle. Square has no instance variable, but inherits the instance variables width and length from its superclass Rectangle.

  • Provide the appropriate constructors (as shown in the class diagram). Hint:
    public Square(double side) {
       super(side, side);  // Call superclass Rectangle(double, double)
    }
  • Override the toString() method to return "A Square with side=xxx, which is a subclass of yyy", where yyy is the output of the toString() method from the superclass.
  • Do you need to override the getArea() and getPerimeter()? Try them out.
  • Override the setLength() and setWidth() to change both the width and length, so as to maintain the square geometry.

Exercises on Composition vs Inheritance

They are two ways to reuse a class in your applications: composition and inheritance.

Ex: The Point and Line Classes

Let us begin with composition with the statement "a line composes of two points".

Complete the definition of the following two classes: Point and Line. The class Line composes 2 instances of class Point, representing the beginning and ending points of the line. Also write test classes for Point and Line (says TestPoint and TestLine).

public class Point {
   // Private variables
   private int x;    // x co-ordinate
   private int y;    // y co-ordinate
   
   // Constructor
   public Point (int x, int y) {......}
   
   // Public methods
   public String toString() {
      return "Point: (" + x + "," + y + ")";
   }
   
   public int getX() {......}
   public int getY() {......}
   public void setX(int x) {......}
   public void setY(int y) {......}
   public void setXY(int x, int y) {......}
}
public class TestPoint {
   public static void main(String[] args) {
      Point p1 = new Point(10, 20);   // Construct a Point
      System.out.println(p1);
      // Try setting p1 to (100, 10).
      ......
   }
}
public class Line {
   // A line composes of two points (as instance variables)
   private Point begin;    // beginning point
   private Point end;      // ending point
   
   // Constructors
   public Line (Point begin, Point end) {  // caller to construct the Points
      this.begin = begin;
      ......
   }
   public Line (int beginX, int beginY, int endX, int endY) {
      begin = new Point(beginX, beginY);   // construct the Points here
      ......
   }
   
   // Public methods
   public String toString() { ...... }
   
   public Point getBegin() { ...... }
   public Point getEnd() { ...... }
   public void setBegin(......) { ...... }
   public void setEnd(......) { ...... }
   
   public int getBeginX() { ...... }
   public int getBeginY() { ...... }
   public int getEndX() { ...... }
   public int getEndY() { ...... }
   
   public void setBeginX(......) { ...... }
   public void setBeginY(......) { ...... }
   public void setBeginXY(......) { ...... }
   public void setEndX(......) { ...... }
   public void setEndY(......) { ...... }
   public void setEndXY(......) { ...... }
   
   public int getLength() { ...... } // Length of the line
                                     // Math.sqrt(xDiff*xDiff + yDiff*yDiff)
   public double getGradient() { ...... } // Gradient in radians
                                          // Math.atan2(yDiff, xDiff)
}
public class TestLine {
   public static void main(String[] args) {
      Line l1 = new Line(0, 0, 3, 4);
      System.out.println(l1);
   
      Point p1 = new Point(...);
      Point p2 = new Point(...);
      Line l2 = new Line(p1, p2);
      System.out.println(l2);
      ...
   }
}

The class diagram for composition is as follows (where a diamond-hollow-head arrow pointing to its constituents):

ExerciseOOP_PointLineComp.png

Instead of composition, we can design a Line class using inheritance. Instead of "a line composes of two points", we can say that "a line is a point extended by another point", as shown in the following class diagram:

ExerciseOOP_PointLineInherit.png

Let's re-design the Line class (called LineSub) as a subclass of class Point. LineSub inherits the starting point from its superclass Point, and adds an ending point. Complete the class definition. Write a testing class called TestLineSub to test LineSub.

public class LineSub extends Point {
   // A line needs two points: begin and end.
   // The begin point is inherited from its superclass Point.
   // Private variables
   Point end;               // Ending point
   
   // Constructors
   public LineSub (int beginX, int beginY, int endX, int endY) {
      super(beginX, beginY);             // construct the begin Point
      this.end = new Point(endX, endY);  // construct the end Point
   }
   public LineSub (Point begin, Point end) {  // caller to construct the Points
      super(begin.getX(), begin.getY());      // need to reconstruct the begin Point
      this.end = end;
   }
   
   // Public methods
   // Inherits methods getX() and getY() from superclass Point
   public String toString() { ... }
   
   public Point getBegin() { ... }
   public Point getEnd() { ... }
   public void setBegin(...) { ... }
   public void setEnd(...) { ... }
   
   public int getBeginX() { ... }
   public int getBeginY() { ... }
   public int getEndX() { ... }
   public int getEndY() { ... }
   
   public void setBeginX(...) { ... }
   public void setBeginY(...) { ... }
   public void setBeginXY(...) { ... }
   public void setEndX(...) { ... }
   public void setEndY(...) { ... }
   public void setEndXY(...) { ... }
   
   public int getLength() { ... }       // Length of the line
   public double getGradient() { ... }  // Gradient in radians
}

Summary: There are two approaches that you can design a line, composition or inheritance. "A line composes two points" or "A line is a point extended with another point"”. Compare the Line and LineSub designs: Line uses composition and LineSub uses inheritance. Which design is better?

Ex: The Circle and Cylinder Classes Using Composition

ExerciseOOP_CircleCylinderComp.png

Try rewriting the Circle-Cylinder of the previous exercise using composition (as shown in the class diagram) instead of inheritance. That is, "a cylinder is composed of a base circle and a height".

public class Cylinder {
   private Circle base;   // Base circle, an instance of Circle class
   private double height;
   
   // Constructor with default color, radius and height
   public Cylinder() {
      base = new Circle(); // Call the constructor to construct the Circle
      height = 1.0; 
   }
   ......
}

Which design (inheritance or composition) is better?

Exercises on Polymorphism, Abstract Classes and Interfaces

Ex: Abstract Superclass Shape and Its Concrete Subclasses

Rewrite the superclass Shape and its subclasses Circle, Rectangle and Square, as shown in the class diagram.

ExerciseOOP_ShapeAbstract.png

In this exercise, Shape shall be defined as an abstract class, which contains:

  • Two protected instance variables color(String) and filled(boolean). The protected variables can be accessed by its subclasses and classes in the same package. They are denoted with a '#' sign in the class diagram.
  • Getter and setter for all the instance variables, and toString().
  • Two abstract methods getArea() and getPerimeter() (shown in italics in the class diagram).

The subclasses Circle and Rectangle shall override the abstract methods getArea() and getPerimeter() and provide the proper implementation. They also override the toString().

Write a test class to test these statements involving polymorphism and explain the outputs. Some statements may trigger compilation errors. Explain the errors, if any.

Shape s1 = new Circle(5.5, "RED", false);  // Upcast Circle to Shape
System.out.println(s1);                    // which version?
System.out.println(s1.getArea());          // which version?
System.out.println(s1.getPerimeter());     // which version?
System.out.println(s1.getColor());
System.out.println(s1.isFilled());
System.out.println(s1.getRadius());
   
Circle c1 = (Circle)s1;                   // Downcast back to Circle
System.out.println(c1);
System.out.println(c1.getArea());
System.out.println(c1.getPerimeter());
System.out.println(c1.getColor());
System.out.println(c1.isFilled());
System.out.println(c1.getRadius());
   
Shape s2 = new Shape();
   
Shape s3 = new Rectangle(1.0, 2.0, "RED", false);   // Upcast
System.out.println(s3);
System.out.println(s3.getArea());
System.out.println(s3.getPerimeter());
System.out.println(s3.getColor());
System.out.println(s3.getLength());
   
Rectangle r1 = (Rectangle)s3;   // downcast
System.out.println(r1);
System.out.println(r1.getArea());
System.out.println(r1.getColor());
System.out.println(r1.getLength());
   
Shape s4 = new Square(6.6);     // Upcast
System.out.println(s4);
System.out.println(s4.getArea());
System.out.println(s4.getColor());
System.out.println(s4.getSide());
  
// Take note that we downcast Shape s4 to Rectangle, 
//  which is a superclass of Square, instead of Square
Rectangle r2 = (Rectangle)s4;
System.out.println(r2);
System.out.println(r2.getArea());
System.out.println(r2.getColor());
System.out.println(r2.getSide());
System.out.println(r2.getLength());
   
// Downcast Rectangle r2 to Square
Square sq1 = (Square)r2;
System.out.println(sq1);
System.out.println(sq1.getArea());
System.out.println(sq1.getColor());
System.out.println(sq1.getSide());
System.out.println(sq1.getLength());

What is the usage of the abstract method and abstract class?

Ex: Polymorphism

Examine the following codes and draw the class diagram.

abstract public class Animal {
   abstract public void greeting();
}
public class Cat extends Animal {
   @Override
   public void greeting() {
      System.out.println("Meow!");
   }
}
public class Dog extends Animal {
   @Override
   public void greeting() {
      System.out.println("Woof!");
   }
   
   public void greeting(Dog another) {
      System.out.println("Woooooooooof!");
   }
}
public class BigDog extends Dog {
   @Override
   public void greeting() {
      System.out.println("Woow!");
   }
   
   @Override
   public void greeting(Dog another) {
      System.out.println("Woooooowwwww!");
   }
}

Explain the outputs (or error) for the following test program.

public class TestAnimal {
   public static void main(String[] args) {
      // Using the subclasses
      Cat cat1 = new Cat();
      cat1.greeting();
      Dog dog1 = new Dog();
      dog1.greeting();
      BigDog bigDog1 = new BigDog();
      bigDog1.greeting();
       
      // Using Polymorphism
      Animal animal1 = new Cat();
      animal1.greeting();
      Animal animal2 = new Dog();
      animal2.greeting();
      Animal animal3 = new BigDog();
      animal3.greeting();
      Animal animal4 = new Animal();
      
      // Downcast
      Dog dog2 = (Dog)animal2;
      BigDog bigDog2 = (BigDog)animal3;
      Dog dog3 = (Dog)animal3;
      Cat cat2 = (Cat)animal2;
      dog2.greeting(dog3);
      dog3.greeting(dog2);
      dog2.greeting(bigDog2);
      bigDog2.greeting(dog2);
      bigDog2.greeting(bigDog1);
   }
}

Ex: Interface Movable and its implementations MovablePoint and MovableCircle

Suppose that we have a set of objects with some common behaviors: they could move up, down, left or right. The exact behaviors (such as how to move and how far to move) depend on the objects themselves. One common way to model these common behaviors is to define an interface called Movable, with abstract methods moveUp(), moveDown(), moveLeft() and moveRight(). The classes that implement the Movable interface will provide actual implementation to these abstract methods.

Let's write two concrete classes - MovablePoint and MovableCircle - that implement the Movable interface.

ExerciseOOP_Movable.png

The code for the interface Movable is straight forward.

public interface Movable {  // saved as "Movable.java"
   public void moveUp();
   ......
}

For the MovablePoint class, declare the instance variable x, y, xSpeed and ySpeed with package access as shown with '~' in the class diagram (i.e., classes in the same package can access these variables directly). For the MovableCircle class, use a MovablePoint to represent its center (which contains four variable x, y, xSpeed and ySpeed). In other words, the MovableCircle composes a MovablePoint, and its radius.

public class MovablePoint implements Movable { // saved as "MovablePoint.java"
   // instance variables
   int x, y, xSpeed, ySpeed;     // package access
   
   // Constructor
   public MovablePoint(int x, int y, int xSpeed, int ySpeed) {
      this.x = x;
      ......
   }
   ......
   
   // Implement abstract methods declared in the interface Movable
   @Override
   public void moveUp() {
      y -= ySpeed;   // y-axis pointing down for 2D graphics
   }
   ......
}
public class MovableCircle implements Movable { // saved as "MovableCircle.java"
   // instance variables
   private MovablePoint center;   // can use center.x, center.y directly
                                  //  because they are package accessible
   private int radius;
   
   // Constructor
   public MovableCircle(int x, int y, int xSpeed, int ySpeed, int radius) {
      // Call the MovablePoint's constructor to allocate the center instance.
      center = new MovablePoint(x, y, xSpeed, ySpeed);
      ......
   }
   ......
   
   // Implement abstract methods declared in the interface Movable
   @Override
   public void moveUp() {
      center.y -= center.ySpeed;
   }
   ......
}

Write a test program and try out these statements:

Movable m1 = new MovablePoint(5, 6, 10, 15);     // upcast
System.out.println(m1);
m1.moveLeft();
System.out.println(m1);
   
Movable m2 = new MovableCircle(1, 2, 3, 4, 20);  // upcast
System.out.println(m2);
m2.moveRight();
System.out.println(m2);

Write a new class called MovableRectangle, which composes two MovablePoints (representing the top-left and bottom-right corners) and implementing the Movable Interface. Make sure that the two points has the same speed.

ExerciseOOP_MovableRectangle.png

What is the difference between an interface and an abstract class?

Ex: Interfaces GeometricObject and Resizable

ExerciseOOP_GeometricObject.png
  1. Write the interface called GeometricObject, which declares two abstract methods: getParameter() and getArea(), as specified in the class diagram.
    Hints:
    public interface GeometricObject {
       public double getPerimeter();
       ......
    }
  2. Write the implementation class Circle, with a protected variable radius, which implements the interface GeometricObject.
    Hints:
    public class Circle implements GeometricObject {
       // Private variable
       ......
    
       // Constructor
       ......
    
       // Implement methods defined in the interface GeometricObject
       @Override
       public double getPerimeter() { ...... }
    
       ......
    }
  3. Write a test program called TestCircle to test the methods defined in Circle.
  4. The class ResizableCircle is defined as a subclass of the class Circle, which also implements an interface called Resizable, as shown in class diagram. The interface Resizable declares an abstract method resize(), which modifies the dimension (such as radius) by the given percentage. Write the interface Resizable and the class ResizableCircle.
    Hints:
    public interface Resizable {
       public double resize(...);
    }
    public class ResizableCircle extends Circle implements Resizeable {
     
       // Constructor
       public ResizableCircle(double radius) {
          super(...);
       }
     
       // Implement methods defined in the interface Resizable
       @Override
       public double resize(int percent) { ...... }
    }
  5. Write a test program called TestResizableCircle to test the methods defined in ResizableCircle.

More Exercises on OOP

Ex: The Discount System

You are asked to write a discount system for a beauty saloon, which provides services and sells beauty products. It offers 3 types of memberships: Premium, Gold and Silver. Premium, gold and silver members receive a discount of 20%, 15%, and 10%, respectively, for all services provided. Customers without membership receive no discount. All members receives a flat 10% discount on products purchased (this might change in future). Your system shall consist of three classes: Customer, Discount and Visit, as shown in the class diagram. It shall compute the total bill if a customer purchases $x of products and $y of services, for a visit. Also write a test program to exercise all the classes.

ExerciseOOP_Discount.png

The class DiscountRate contains only static variables and methods (underlined in the class diagram).

Ex: Polyline of Points with ArrayList

ExerciseOOP_Polyline.png

A polyline is a line with segments formed by points. Let's use the ArrayList (dynamically allocated array) to keep the points, but upcast to List in the instance variable. (Take note that array is of fixed-length, and you need to set the initial length).

public class Point {
   private int x;
   private int y;
   public Point(int x, int y) { ...... }
   public int getX() { ...... }
   public int getY() { ...... }
   public void setX(int x) { ...... }
   public void setY(int y) { ...... }
   public int[] getXY() { ...... }
   public void setXY(int x, int y) { ...... }
   public String toString() { ...... }
   public double distance(Point another) { ...... }
}
import java.util.*;
public class PolyLine {
   private List<Point> points;   // List of Point instances

   // Constructors   
   public PolyLine() {  // default constructor
      points = new ArrayList<Point>();  // implement with ArrayList
   }
   public PolyLine(List<Point> points) {
      this.points = points;
   }
 
   // Append a point (x, y) to the end of this polyline
   public void appendPoint(int x, int y) {
      Point newPoint = new Point(x, y);
      points.add(newPoint);
   }
 
   // Append a point instance to the end of this polyline
   public void appendPoint(Point point) {
      points.add(point);
   }
 
   // Return {(x1,y1)(x2,y2)(x3,y3)....}
   public String toString() {
      // Use a StringBuilder to efficiently build the return String
      StringBuilder sb = new StringBuilder("{");
      for (Point aPoint : points) {
          sb.append(aPoint.toString());
      }
      sb.append("}");
      return sb.toString();
   }

   // Return the total length of this polyline
   public double getLength() { ...... }
}
/* 
 * A Test Driver for the PolyLine class.
 */
import java.util.*;
public class TestPolyLine {
   public static void main(String[] args) {
      // Test default constructor and toString()
      PolyLine l1 = new PolyLine();
      System.out.println(l1);  // {}

      // Test appendPoint()
      l1.appendPoint(new Point(1, 2));
      l1.appendPoint(3, 4);
      l1.appendPoint(5, 6);
      System.out.println(l1);  // {(1,2)(3,4)(5,6)}

      // Test constructor 2
      List<Point> points = new ArrayList<Point>();
      points.add(new Point(11, 12));
      points.add(new Point(13, 14));
      PolyLine l2 = new PolyLine(points);
      System.out.println(l2);  // {(11,12)(13,14)}
   }
}

Exercises on Data Structures

Ex: MyIntStack

A stack is a first-in-last-out queue. Write a program called MyIntStack, which uses an array to store the contents, restricted to int.

OOP_MyIntStack.java

Write a test program.

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public class MyIntStack {
   private int[] contents;
   private int tos;  // Top of the stack
 
   // constructors
   public MyIntStack(int capacity) {
      contents = new int[capacity];
      tos = -1;
   }
 
   public void push(int element) {
      contents[++tos] = element;
   }
 
   public int pop() {
      return contents[tos--];
   }
 
   public int peek() {
      return contents[tos];
   }
 
   public boolean isEmpty() {
      return tos < 0;
   }
 
   public boolean isFull() {
      return tos == contents.length - 1;
   }
}

Try:

  1. Modify the push() method to throw an IllegalStateException if the stack is full.
  2. Modify the push() to return true if the operation is successful, or false otherwise.
  3. Modify the push() to increase the capacity by reallocating another array, if the stack is full.

Exercise (Nodes, Link Lists, Trees, Graphs):

[TODO]

  • Study the existing open source codes, including JDK.
  • Specialized algorithms, such as shortest path.

Exercise (Maps):

[TODO]

  • Representation of map data.
  • Specialized algorithms, such as shortest path.

Exercise (Matrix Operations for 3D Graphics):

[TODO]

  • Study the existing open source codes, including JDK's 2D Graphics and JOGL's 3D Graphics.
  • Efficient and specialized codes for 3D Graphics (4D matrices). Handle various primitive types such as int, float and double efficiently.