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Are parameters passed by reference or passed by value in method invocation?

In programming, there are two ways to pass arguments to a method, pass-by-value and pass-by-reference:

  • When you have a pass-by-value parameter, a copy of the argument is stored into the memory location allocated for the formal parameter. In this case, any changes made to the formal parameter inside the method will not affect the value of the argument back in the calling method.
  • When a parameter is pass-by-reference, the memory address of the argument is passed to the method, making the formal parameter an alias for the argument. This means that changes made to the formal parameter inside the method will be reflected in the value of the argument when control is returned to the calling function.

Technically, all parameters in Java are pass-by-value.

All primitives are pass-by-value, period. When a primitive value is passed into a method, a copy of the primitive is made. The copy is what is actually manipulated in the method. So, the value of the copy can be changed within the method, but the original value remains unchanged.

For example:

public class TestPassPrimitive {
static void doSomething(int m) {
m = m + 2;
System.out.println("The new value is " + m + ".");
}
public static void main(String[] args) {
int m = 5;
System.out.println("Before doSomething, m is " + m + ".");
doSomething(m);
System.out.println("After doSomething, m is " + m + ".");
}
}

The output result is

Before doSomething, m is 5.
The new value is 7.
After doSomething, m is 5.

Objects, however, work a bit differently. When you pass a Java object or array as a parameter, an object reference or array reference is passed into a method. The method can manipulate the attributes of the object that is referenced by the reference (formal parameter).

This reference is passed-by-value. What does this mean exactly?

  • Any direct changes made to the reference (formal parameter) inside the method will be reflected in the value of the object back in the calling method;
  • If you were to reassigns a new instantiation of an object or array to the reference (formal parameter) within the method, the reassignment only affects the copy reference (formal parameter), not the original reference passed by the caller. After that, any direct changes made to the reference (formal parameter) inside the method will be reflected the new reassigned object or array.

The subtleties of passing parameters will become clearer as you write more and more methods. Experimentation is the best form of learning. For example:

class TestReferenceParameter{
public static void main (String args[]) {
String s1 = "one";
String s2 = doSomething(s1);
System.out.println(s1 + " " + s2);
}


static String doSomething(String s1) {
s1 = s1 + " two";
System.out.println(s1 + " ");
return "three";
}
}

The output result is

one two 
one three


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