Question

I am working on some project in java.
Here I’m stuck at this problem and not able to figure out where I am going wrong.

I have made two classes: Test and Child.

When I run the code I am get a NullPointerException.

package com.test;
public class Test {

    child newchild = new child();
    public static void main(String[] args) {
        new Test().method();
    }
    void method() {
        String[] b;
        b = newchild.main();
        int i = 0;
        while (i < b.length) {
            System.out.println(b[i]);
        }
    }
}

---

package com.test;
public class child {
    public String[] main() {
        String[] a = null;
        a[0] = "This";
        a[1] = "is";
        a[2] = "not";
        a[3] = "working";
        return a;
    }
}

Answer 1

Here’s the problem:

String[] a = null;
a[0]="This";

You’re immediately trying to dereference a, which is null, in order to set an element in it. You need to initialize the array:

String[] a = new String[4];
a[0]="This";

If you don’t know how many elements your collection should have before you start populating it (and often even if you do) I would suggest using a List of some sort. For example:

List<String> a = new ArrayList<String>();
a.add("This");
a.add("is");
a.add("not");
a.add("working");
return a;

Note that you have another problem here:

int i=0;
while(i<b.length)
    System.out.println(b[i]);

You’re never changing i, so it will always be 0 – if you get into the while loop at all, you will never get out of it. You want something like this:

for (int i = 0; i < b.length; i++)
{
    System.out.println(b[i]);
}

Or better:

for (String value : b)
{
    System.out.println(value);
}