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 >

A Java Private Method

In this example I create a class called Square. It has a public method called display_area. This has to call a private method called calculate_area before it can display the result. Private methods are only accessible from within the class which contains them:

andrew@UBUNTU:~/Java$ cat Square.java
public class Square
  {
  double width;
  double area;
  public void display_area()
    {
    calculate_area();
    System.out.println("Area = " + area);
    }
  private void calculate_area()
    {
    area = width * width;
    }
  }
andrew@UBUNTU:~/Java$ javac Square.java
andrew@UBUNTU:~/Java$

I create a program to define a square and display its area (I have shown you a similar example already):

andrew@UBUNTU:~/Java$ cat SquareExample1.java
class SquareExample1
  {
  public static void main(String args[])
    {
    Square my_square = new Square();
    my_square.width = 5;
    my_square.display_area();
    }
  }
andrew@UBUNTU:~/Java$ javac SquareExample1.java
andrew@UBUNTU:~/Java$ java SquareExample1
Area = 25.0
andrew@UBUNTU:~/Java$

However, when I try to execute the calculate_area method directly from outside the class, Java returns a compilation error as the method concerned is private:

andrew@UBUNTU:~/Java$ cat SquareExample2.java
class SquareExample2
  {
  public static void main(String args[])
    {
    Square my_square = new Square();
    my_square.width = 5;
    my_square.calculate_area();
    }
  }
andrew@UBUNTU:~/Java$ javac SquareExample2.java
SquareExample2.java:7: calculate_area() has private access in Square
    my_square.calculate_area();
             ^
1 error
andrew@UBUNTU:~/Java$

Java static Variables

If a variable in a class is static, it has only one value, which is shared by all members of the class. You can see this in the example below, where I created a class called Tax with a static variable called VAT:

andrew@UBUNTU:~/Java$ cat Tax.java
public class Tax  
  {
  static double VAT = 10;
  }
andrew@UBUNTU:~/Java$ javac Tax.java
andrew@UBUNTU:~/Java$

Then I wrote a program to use the class. In this program, I created 2 members called member1 and member2 and showed that they both had the same VAT value, i,e, 10. I changed member1.VAT to 11 and showed that member2.VAT changed to 11 too, without a specific assignment. Doing it this way can make your code difficult to follow. An alternative is to modify the static variable by prefixing it with the name of the class. To demonstrate this, I changed Tax.VAT to 12 and checked member1.VAT and member2.VAT to see that they had been altered in the same way:

andrew@UBUNTU:~/Java$ cat Tax_test.java
public class Tax_test
  {
  public static void main(String args[])
    {
    Tax member1 = new Tax();
    System.out.println("member1.VAT = " + member1.VAT);
    Tax member2 = new Tax();
    System.out.println("member2.VAT = " + member2.VAT);
    member1.VAT = 11;
    System.out.println("member1.VAT = " + member1.VAT);
    System.out.println("member2.VAT = " + member2.VAT);
    Tax.VAT = 12;
    System.out.println("Tax.VAT = " + Tax.VAT);
    System.out.println("member1.VAT = " + member1.VAT);
    System.out.println("member2.VAT = " + member2.VAT);
    }
  }
andrew@UBUNTU:~/Java$ javac Tax_test.java
andrew@UBUNTU:~/Java$ java Tax_test
member1.VAT = 10.0
member2.VAT = 10.0
member1.VAT = 11.0
member2.VAT = 11.0
Tax.VAT = 12.0
member1.VAT = 12.0
member2.VAT = 12.0
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$

A Simple Example with a Class and 1 Object

As I am a retired COBOL programmer, I don’t know anything about Object Oriented programming so please forgive me if this example seems a bit elementary. Classes are templates for real-life objects. In the example below, a Rectangle class is created. Each object in this class (i.e. each rectangle) has a width and a height. These are called instance variables. An object called my_rectangle is created in the Rectangle class and its width and height are supplied. Finally, a variable called area is created, calculated and displayed:

 

Java > cat Rectangle.java

class Rectangle

  {

  double width;

  double height;

  }

class RectangleExample

  {

  public static void main(String args[])

    {

    Rectangle my_rectangle = new Rectangle();

    double area;

    my_rectangle.width = 3;

    my_rectangle.height = 4;

    area = my_rectangle.width * my_rectangle.height;

    System.out.println("Area = " + area);

    }

  }

Java > javac Rectangle.java

Java > java RectangleExample

Area = 12.0

Java >