Monday, 10 April 2017

What does a Java array look like in memory?

Arrays in Java store one of two things: either primitive values (int, char, ...) or references (a.k.a pointers).

When an object is creating by using "new", memory is allocated on the heap and a reference is returned. This is also true for arrays, since arrays are objects.


1. Single-dimension Array


int arr[] = new int[3];

The int[] arr is just the reference to the array of 3 integer. If you create an array with 10 integer, it is the same - an array is allocated and a reference is returned.

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2. Two-dimensional Array


How about 2-dimensional array? Actually, we can only have one dimensional arrays in Java. 2D arrays are basically just one dimensional arrays of one dimensional arrays.

int[ ][ ] arr = new int[3][ ];
arr[0] = new int[3];
arr[1] = new int[5];
arr[2] = new int[4];

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Multi-dimensional arrays use the name rules.

3. Where are they located in memory?


Arrays are also objects in Java, so how an object looks like in memory applies to an array.

As we know that JVM runtime data areas include heap, JVM stack, and others. For a simple example as follows, let's see where the array and its reference are stored.

class A {
int x;
int y;
}

...

public void m1() {
int i = 0;
m2();
}

public void m2() {
A a = new A();
}

...

With the above declaration, let's invoke m1() and see what happens:
  1. When m1 is invoked, a new frame (Frame-1) is pushed into the stack, and local variable i is also created in Frame-1.
  2. Then m2 is invoked inside of m1, another new frame (Frame-2) is pushed into the stack. In m2, an object of class A is created in the heap and reference variable is put in Frame-2. Now, at this point, the stack and heap looks like the following:

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Arrays are treated the same way like objects, so how array locates in memory is straight-forward.