Question

How do I pass a primitive data type by reference? For example, how do I make an int passed to a function modifiable.

Answer

Java passes all primitive data types by value. This means that a copy is made, so that it cannot be modified. When passing Java objects, you're passing an object reference, which makes it possible to modify the object's member variables. If you want to pass a primitive data type by reference, you need to wrap it in an object.

The easiest of all is to pass it as an array (or even a Vector). Your array only needs to contain a single element, but wrapping it in an array means it can be changed by a function. Here's a simple example of it in action.

public static void increment(int[] array, int amount)
{
   array[0] = array[0] + amount;
}

public static void main(String args[])
{
   int[] myInt = { 1 };

   increment (myInt, 5);

   System.out.println ("Array contents : " + myInt[0]);
}

Of course, if you're modifying the contents of parameters passed to a method, you really should try to avoid this behavior. It increases the complexity of code, and really should be avoided. The preferred way is to return a value from a method, rather than modifying parameter values directly.

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