Error when copying an array to another in Java

After googling for a while, I'm aware t开发者_JAVA百科hat there quite a few ways to copy an array to another in Java, namely using System.arraycopy.

However a few of my friends tried to use this:

boolean a[][] = new boolean[90][90];
boolean b[][] = new boolean[90][90];

/* after some computations */

a = b

This produces a rather non deterministic result, does anyone know what this actually does?

It's not non-deterministic at all.

a = b;

simply assigns the value of b to a. The value of b is a reference to the array - so now both variables contain references to the same array. The old value of a is irrelevant - and if it referred to an array which nothing else referred to, it will now be eligible for garbage collection.

Note that this isn't specific to arrays - it's the way all reference types work in Java.

Basically, you're not copying one array into another at all - you're copying the reference to an array into another variable. That's all.

Lines of Code and their meanings:

boolean a[][] = new boolean[90][90];

Allocate an array and assign its reference to a

boolean b[][] = new boolean[90][90];

Allocate a second array and assign its reference to b

a = b;

Assign bs value (second arrays reference) to a.

Before a=b;

a ---------> First arrays memory space

b ---------> Second arrays memory space

After a=b;

a ---           First arrays memory space
     ---
        ------>
b ------------> Second arrays memory space

Now you lost first array forever

Remember a=b means you are assigning value of b to a

    boolean a[][] = new boolean[90][90];
    boolean b[][] = new boolean[90][90];
    a[0][3]=false;
    b=a;
    a[0][5]=true;

    System.out.println(b[0][3]);
    System.out.println(b[0][5]);

The output is:-

false

true