SINGLE-DIMENSIONAL ARRAYS - Introduction to Java Programming: Comprehensive Version - page 259

Note
Arrays are objects in Java (objects are introduced in Chapter 9). The JVM stores the
objects in an area of memory called the heap , which is used for dynamic memory
allocation.
Listing 7.3 gives another program that shows the difference between passing a primitive data
type value and an array reference variable to a method.
The program contains two methods for swapping elements in an array. The first method,
named swap , fails to swap two int arguments. The second method, named swapFirst-
TwoInArray , successfully swaps the first two elements in the array argument.

heap

L ISTING 7.3

TestPassArray.java
1 public class TestPassArray {
2 /** Main method */

3 public static void main(String[] args) {

4 int [] a = { 1 , 2 };

5
6 // Swap elements using the swap method
7 System.out.println( "Before invoking swap" );
8 System.out.println( "array is {" + a[ 0 ] + ", " + a[ 1 ] + "}" );
9 swap(a[ 0 ], a[ 1 ]);
10 System.out.println( "After invoking swap" );
11 System.out.println( "array is {" + a[ 0 ] + ", " + a[ 1 ] + "}" );
12
13 // Swap elements using the swapFirstTwoInArray method
14 System.out.println( "Before invoking swapFirstTwoInArray" );
15 System.out.println( "array is {" + a[ 0 ] + ", " + a[ 1 ] + "}" );
16 swapFirstTwoInArray(a);
17 System.out.println( "After invoking swapFirstTwoInArray" );
18 System.out.println( "array is {" + a[ 0 ] + ", " + a[ 1 ] + "}" );
19 }
20
21 /** Swap two variables */
22 public static void swap( int n1, int n2) {
23 int temp = n1;
24 n1 = n2;
25 n2 = temp;
26 }
27
28 /** Swap the first two elements in the array */
29 public static void swapFirstTwoInArray( int [] array) {
30 int temp = array[ 0 ];
31 array[ 0 ] = array[ 1 ];
32 array[ 1 ] = temp;
33 }
34 }

false swap

swap array elements

Before invoking swap
array is {1, 2}
After invoking swap
array is {1, 2}
Before invoking swapFirstTwoInArray
array is {1, 2}
After invoking swapFirstTwoInArray
array is {2, 1}