Method returning one index of an array throws NullPointer

Question

When working on a program for school, I created a method which returned the index of an array based on the value you gave the method. The method throws a NullPointerException but in every index of the array has already been instantiated. To show this, I created a test program which does the same thing.

public class Test {
private static Number[] numbers = new Number[16];

public static void main(String[] args) {
    new Test();
}

public Test() {
    for (Number n : numbers) {
        n = new Number(0);
    }
    System.out.println(get(0).value);
}

public Number get(int index) {
    return numbers[index];
}

class Number {
    public int value;

    public Number(int value) {
        this.value = value;
        }
    }
}

When run this should print the value of the Integer "value", in this case of the object in the first position of the array.

I am sure there is a simple explanation to this, but I am unaware of the problem.

Alexis C. · Accepted Answer · 2014-04-25 18:02:07Z

4

You have to use the for-each loop for read-only passes. Currently, in your constructor, the elements in your array still remain uninitialized and hence the error.

So change it to a normal for loop.

for(int i = 0; i < numbers.length; i++){
    numbers[i] = new Number(0);
}

edited Apr 25, 2014 at 18:02

answered Apr 25, 2014 at 17:55

Alexis C.

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2 Comments

piechesse Over a year ago

Thank you immensely! I had no idea the for each loop did not act exactly like the regular for loop.

Alexis C. Over a year ago

@piechesse Yes, you can read the link I provided, particulary at the end ;)

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