Java Constructors

If you have a class called, for example, Rectangle, you can add a method to it with the same name as the class. This method must have no return type, not even void. It is called a constructor. Here is an example:

Java > cat Rectangle.java

class Rectangle

  {

  double width;

  double height;

  Rectangle()

    {

    width = 2;

    height = 3;

    }

  }

Java > javac Rectangle.java

Java >

The purpose of a constructor is to give default values to a class member when it is created. You can see what I mean below, where an instance of Rectangle is created. It automatically has a width of 2 and a height of 3:

Java > cat RectangleExample.java

public class RectangleExample

  {

  public static void main(String args[])

    {

    Rectangle r1 = new Rectangle();

    System.out.println("width = " + r1.width);

    System.out.println("height = " + r1.height);

    }

  }

Java > javac RectangleExample.java

Java > java RectangleExample

width = 2.0

height = 3.0

Java >

Java Method Overloading

At this stage, I don’t know why you would want to do this, but Java allows method overloading. This means that you can have two or more methods with the same name:

Java > cat my_class.java

class my_class

  {

  public void multiply (int x, int y)

    {

    int a = x * y;

    System.out.println("a = " + a);

    }

  public void multiply (int x, int y, int z)

    {

    int b = x * y * z;

    System.out.println("b = " + b);

    }

  public void multiply (double x, double y)

    {

    double c = x * y;

    System.out.println("c = " + c);

    }

  }

Java > javac my_class.java

Java >

The only difference the outside world can see between these methods is in the number and type(s) of parameters they expect to receive. When you use an overloaded method, you let Java know which version to use by passing the parameters it requires:

Java > cat test_my_class.java

public class test_my_class

  {

  public static void main(String args[])

   {

    my_class my_class1 = new my_class();

    int i=2, j=3, k=4;

    double m=2.5, n=3;

    my_class1.multiply (i, j);

    my_class1.multiply (i, j, k);

    my_class1.multiply (m, n);

    }

  }

Java > javac test_my_class.java

Java > java test_my_class

a = 6

b = 24

c = 7.5

Java >

Primitive Data Types are Passed to Methods by Value in Java

I created a class called my_class with a method called times_two. The method accepts an integer and multiplies it by 2:

Java > cat my_class.java

public class my_class

  {

  public void times_two(int a)

    {

    a = a * 2;

    System.out.println("a = " + a);

    }

  }

Java > javac my_class.java

Java >

I created a program called test_my_class. This sets up an instance of my_class called b. It then creates an integer variable called x and gives it a values of 2. Next, it passes x as a parameter to the times_two method, which multiplies it by 2 to give 4. Finally, it displays the original value, which is still 2. This is because the value of x is passed to the method, not a reference to its location in memory.

Java > cat test_my_class.java

public class test_my_class

  {

  public static void main(String args[])

    {

    my_class b = new my_class();

    int x = 2;

    b.times_two(x);

    System.out.println("x = " + x);

    }

  }

Java > javac test_my_class.java

Java > java test_my_class

a = 4

x = 2

Java >

How to Pass Parameters to a Java Method

I created a simple class called two_numbers with a method called show_greater. The method accepts two integer parameters and displays the larger:

andrew@UBUNTU:~/Java$ cat two_numbers.java
public class two_numbers
  {
  public void show_greater(int arg1, int arg2)
    {
    int greater = arg1;
    if (arg2 > arg1) greater = arg2;
    System.out.println(greater);
    }
  }
andrew@UBUNTU:~/Java$ javac two_numbers.java
andrew@UBUNTU:~/Java$

Then I created  a program called two_numbers_test. This declares an object called compare in the two_numbers class. It then calls the show_greater method three times, comparing two integers each time to see which one is bigger:

andrew@UBUNTU:~/Java$ cat two_numbers_test.java
public class two_numbers_test
  {
  public static void main(String args[])
    {
    two_numbers compare = new two_numbers();
    compare.show_greater(0,1);
    compare.show_greater(2,1);
    compare.show_greater(3,3);
    }
  }
andrew@UBUNTU:~/Java$ javac two_numbers_test.java
andrew@UBUNTU:~/Java$ java two_numbers_test
1
2
3
andrew@UBUNTU:~/Java$

A Java Class with a Method

A Java class can have a method. This is a piece of code which can be used on members of the class. In the example below, a class called Square is created. The Square class has a method called display_area, which calculates and displays the area of the square: 

andrew@UBUNTU:~/Java$ cat Square.java
public class Square
  {
  double width;
  double area;
  public void display_area()
    {
    area = width * width;
    System.out.println("Area = " + area);
    }
  }
class SquareExample
  {
  public static void main(String args[])
    {
    Square my_square = new Square();
    my_square.width = 5;
    my_square.display_area();
    }
  }
andrew@UBUNTU:~/Java$ javac Square.java
andrew@UBUNTU:~/Java$ java SquareExample
Area = 25.0
andrew@UBUNTU:~/Java$