initializing an array from objects makes NullPointerException

Question

I want to set some data in array from objects, here is class A, it cause an NullPointerException. Why? how to solve this? what is the problem with my initializing?

class A {

    int a;

    public void setA(int a) {
        this.a = a;
    }

public class ObjArry {
public static void main(String[] args) {

    A[] ObjectArray = new A[5];

    ObjectArray[0].setA(10); //Exception occurs in this line
    ObjectArray[1].setA(9);
    ObjectArray[2].setA(8);
    ObjectArray[3].setA(7);
    ObjectArray[4].setA(6);
}

score 2 · Accepted Answer · answered Mar 09 '19 at 13:52

2

An exception will occur on all other lines, too. By default, the newly initialized array new A[5] is empty, i.e., all its fields are null. You first have to do something like:

ObjectArray[0] = new A();
ObjectArray[1] = new A();
ObjectArray[2] = new A();
ObjectArray[3] = new A();
ObjectArray[4] = new A();

answered Mar 09 '19 at 13:52

F. Knorr

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Or, better, provide a constructor with a parameter, so you can construct and set the value in one go. – Andy Turner Mar 09 '19 at 14:11

Turing85 · Answer 2 · 2019-03-09T23:55:28.300

JLS §10.6 states that

[...] each component of the (newly created¹) array is initialized to its default value (§4.12.5).

_{(¹The comment was added by me.)}

Looking at §4.12.5, we find that

For all reference types (§4.3), the default value is null.

In consequence,

A[] ObjectArray = new A[5];

creates an array of length 5 and each cell contains a reference to null.

So the answer is: It happens because the JLS says so.

initializing an array from objects makes NullPointerException

2 Answers2