Java Argument Passing Mechanism (call by value)

When a primitive type is passed to a method, it is done by use of call-by-value approach. In case of objects what is actually passed is an object reference. As a point of interest, an object reference is also passed by using call-by-value approach. However, since the value being passed refers to an object, the copy of that value will still refer to the same object that its corresponding argument does.

Example: The following example illustrates that Java uses call-by-value when passing primitive data types.

1 class Args1

2 { void swap(int a, int b)

3 { int t= a;

4 a = b; b = t;

5 }

6 public static void main(String args[])

7 { int x = 5, y = 7;

8 System.out.println("x = " + x + " y = " + y);

9 Args1 obj = new Args1(); obj.swap(x,y);

10 System.out.println("x = " + x + " y = " + y);

11 }

12 }

Output:

x = 5 y = 7

x = 5 y = 7

Example: The following example also illustrates that Java uses call-by-value when passing primitive data types.

1 class Args2

2 { static void swap(int a, int b)

3 { int t= a;

4 a = b; b = t;

5 }

6 public static void main(String args[])

7 { int x = 5, y = 7;

8 System.out.println("x = " + x + " y = " + y);

9 //Args2.swap(x,y);

10 swap(x,y);

11 System.out.println("x = " + x + " y = " + y);

12 }

13 }

Output:

x = 5 y = 7

x = 5 y = 7

Example: The following example illustrates that Java uses call-by-value even when passing reference types.

1 class Args3

2 { int a,b;

3 Args3(int x, int y)

4 { a = x;

5 b = y;

6 }

7 static void swap(Args3 ob1, Args3 ob2)

8 { Args3 t;

9 t = ob1;

10 ob1 = ob2;

11 ob2 = t;

12 }

13 public static void main(String args[])

14 { Args3  obj1 = new Args3(5,7);

15 Args3  obj2 = new Args3(4,6);

16 System.out.print("obj1.a = " + obj1.a + " obj1.b = " + obj1.b);

17 System.out.println(" obj2.a = " + obj2.a + " obj2.b = " + obj2.b);

18 swap(obj1,obj2);

19 System.out.print("obj1.a = " + obj1.a + " obj1.b = " + obj1.b);

20 System.out.println(" obj2.a = " + obj2.a + " obj2.b = " + obj2.b);

21 }

22 }

Output:

obj1.a = 5 obj1.b = 7 obj2.a = 4 obj2.b = 6

obj1.a = 5 obj1.b = 7 obj2.a = 4 obj2.b = 6

Example: The following example illustrates that although Java uses call-by-value when passing reference types but we can still modify the object referred to by the argument inside the called method, as what is actually passed is a reference.

1 class Args4

2 { int a,b;

3 Args4(int x, int y)

4 { a = x;

5 b = y;

6 }

7 static void swap(Args4 ob1, Args4 ob2)

8 { int x,y;

9 x = ob1.a; y = ob1.b; ob1.a = ob2.a; ob1.b = ob2.b;

10 ob2.a = x; ob2.b = y;

11 }

12 public static void main(String args[])

13 { Args4  obj1 = new Args4(5,7);

14 Args4  obj2 = new Args4(4,6);

15 System.out.print("obj1.a = " + obj1.a + " obj1.b = " + obj1.b);

16 System.out.println(" obj2.a = " + obj2.a + " obj2.b = " + obj2.b);

17 swap(obj1,obj2);

18 System.out.print("obj1.a = " + obj1.a + " obj1.b = " + obj1.b);

19 System.out.println(" obj2.a = " + obj2.a + " obj2.b = " + obj2.b);

20 }

21 }

Output:

obj1.a = 5 obj1.b = 7 obj2.a = 4 obj2.b = 6

obj1.a = 4 obj1.b = 6 obj2.a = 5 obj2.b = 7