ArrayIndexOutOfBoundsException from array initialized with field value

Question

When I try to push an element into a Java array where I take the array size from a constructor argument, it throws an ArrayIndexOutOfBoundsException exception. However, when I set the size while declaring the array adding an element works.

Here is my code:

public class Stack {
    public int size;

    public Stack(int size)
    {
        this.size = size;
    }

    public int[] arr = new int[size];

    public int top = -1;

    // Methods
    public void push(int value)
    {
        top++;
        arr[top] = value;
    }
}

The following throws the exception:

new Stack(10).push(123);

Possible duplicate of Default constructor vs. inline field initialization — Logan
– Logan, Commented Jul 23, 2018 at 20:17
You need to put this line into constructor's body: arr = new int[size]; — Ivan
– Ivan, Commented Jul 23, 2018 at 20:20
When an instance of Stacki is created the member variable size is initialized by default with the value 0 and in consequence the array arr is initialized with size == 0. When calling push the first time there is the attempt to set element 0 of arr. But arrhas no elements as it's size is 0. — LuCio
– LuCio, Commented Jul 23, 2018 at 20:31
@Ivan yeah it works ty but can you explain why we need to do that ? — A.Usama
– A.Usama, Commented Jul 23, 2018 at 20:31
Ahmed Usama, @LuCio is completely right, see my answer about it in order to see a detailed explanation (pay attention to the comments in the code) — lealceldeiro
– lealceldeiro, Commented Jul 23, 2018 at 20:43

lealceldeiro · Accepted Answer · 2019-09-18 13:41:18Z

7

You need to initialize the array when the value of this.size is the "correct" one. Initially the value of this.size is 0 (zero) (see Initial Values of Variables) so this is not the time of initializing the array; you must "wait" in order to know what's the size the array will have. Where that size is provided? In the class constructor.

So it is inside the constructor where you have to initialize the array (with the provided size).

For example, see below commented code (yours):

public class Stack {
    public int size ;                   // size = 0 at this time
    public Stack(int size)
    {
        this.size = size;
    }
    public int[] arr = new int[size];  // still size = 0 at this time!
                                       // so you're creating an array of size zero
                                       // (you won't be able to "put" any value in it)
    public int top = -1;

    //Methods
    public void push(int value)
    {
        top++;             // after this line `top` is 0
        arr[top] = value;  // in an array of zero size you are trying to set in its
                           // "zero" position the value of `value`
                           // it's something like this:
                           // imagine this array (with no more room)[], can you put values?,
                           // of course not
    }
}

So, in order to fix this you need to change your code like this:

public class Stack {

    public int size;
    public int[] arr;         // declare the array variable, but do not initialize it yet
    public int top = -1;

    public Stack(int size) {
        this.size = size;
        arr = new int[size];  // when we know in advance which will be the size of the array,
                              // then initialize it with that size
    }

    //Methods
    public void push(int value) {
        top++;
        arr[top] = value;
    }
}

edited Sep 18, 2019 at 13:41

answered Jul 23, 2018 at 20:31

lealceldeiro

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6 Comments

A.Usama Over a year ago

In this Block , how size still 0 and i just take the value from main ? public Stack(int size) { this.size = size; //the value must be taken and assigned to size } public int[] arr = new int[size]; //How still 0 here

LuCio Over a year ago

Further reading: specification of default values in the JLS

lealceldeiro Over a year ago

Thanks @LuCio!, can I add that info to the answer?

LuCio Over a year ago

Sure. Go ahead!

lealceldeiro Over a year ago

@AhmedUsama in your initial code size has a default value (see LuCio link), so if you don't initialize the array inside the controller with the size provided as parameter the array length will be that default (initial) value

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