Java: Variables are Always Passed by Copy

Published on 14 March 2015

I am learning Java. One of the important concepts to understand is whether arguments of a function is passing by copy or reference. Passing by copy means when a variable is passed to a function, a copy is made. Passing by reference means when a variable is passed to a function, the code in the function operates on the original variable.

In Java, variables are always passed by copy. Here are three scenarios:

Case 1: Passing Primitives

void incrementValue(int inFunction){
inFunction ++;
System.out.printIn(“In function: “ + inFunction);
}

int original = 10;
System.out.printIn(“Original before: “ + original); incrementValue(original);
System.out.printIn(“Original after: “ + original);

And the result is:

Original before: 10
In Function: 11
Original after: 10

The original value didn’t change.

Case 2: Passing primitives wrapped in objects

void incrementValue(int[] inFunction){
inFunction[0] ++;
System.out.printIn(“In function: “ + inFunction[0]);
}

int[] arOriginal = {10, 20, 30};
System.out.println(“Original before: “ + original[0]); incrementValue(original);
System.out.println(“Original after: “ + original[0]);

And the result is:

Original before: 10
In function: 11
Original after: 11

The original value did change! This is because complex object variables are references. A reference variable points to a location in memory. When a variable is passed to a function, a new refernce is always created. Both references point to the original objects or values.

int[] origial = {10, 20, 30}

original[0] –> 10 <– inFunction[0]
  20  
  30  

Both array elements point to the same memory location

Case 3: Passing Strings

void changeString(String inFunction){
inFunction = “New!”;
System.out.println(“In function: “ + inFunction);
}

String original = “Original!”;
System.out.println(“Original before: “ + original); changeString(original);
System.out.println(“Original after: “ + original);

The result is:

Original before: Original!
In function: New!
Original after: Original!

Remember, strings are immutable. A copy of the entire String is created when passed to a function.


Originally published at victorleungtw.com on March 9, 2015.