Java getting nullpointerexception

Question

mainMethod.java

public class mainMethod{
    public animalsData[] animals;

    public mainMethod(){
        animals[this.animals.length + 1] = new animalsData("CAT", "4");

    }

    public static void main(String[] args) {
        mainMethod run = new mainMethod();
    }
}

animalsData.java

public class animalsData{
    String name, l;

    public animalsData(String name, String l) {
        super();
        this.name= name;
        this.l= l;
    }
}

I hava this problem: Exception in thread "main" java.lang.NullPointerException

animals has not been assigned a value. Now that you know what the problem is, please delete and/or help me close this question. (P.S. there are many such related questions. Try searching first for "ideas" about what could be wrong.) — user166390
– user166390, Commented Aug 31, 2012 at 17:42
this.animals is null so it's not possible to acces length property — Dario
– Dario, Commented Aug 31, 2012 at 17:42
Also, in addition, arr[arr.length + 1] is never valid in Java because arrays have a fixed length. — user166390
– user166390, Commented Aug 31, 2012 at 17:44

Colin D · Accepted Answer · 2012-08-31 17:42:22Z

6

You are never initializing your animals array in your mainMethod class.

In your public mainMethod() method, you need to do animals = new animalsData[INITIAL SIZE];

If you want it to grow automatically, you should use a List. Even then you would not use this.animals.length + 1 as its index. you would simply do List.add()

answered Aug 31, 2012 at 17:42

Colin D

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2 Comments

Roddy of the Frozen Peas Over a year ago

it's an array not an object, actually, so he's going about it totally wrong. :-/

Roddy of the Frozen Peas Over a year ago

@Gray - per the spec, yes. But the OP is obviously new to the language and since you instantiate arrays and objects in different fashions (new Object() versus new int[5]), it's best to stay simple.

jdevelop · Accepted Answer · 2012-08-31 17:42:52Z

4

public List<animalsData> animals = new ArrayList<animalsData>()

public mainMethod(){
    animals.add(new animalsData("CAT", "4"));
}

learn more about Lists in Java, arrays are not expandable by default. And can not expand beyond it's dimension if you add an item after last item of the array.

answered Aug 31, 2012 at 17:42

jdevelop

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2 Comments

Colin D Over a year ago

It is good that you provide an alternate solution, but you should mention what is actually causing the errors he is encountering.

jdevelop Over a year ago

I've seen another answers explaining this, why to reinvent the wheel? :)

Kumar Vivek Mitra · Accepted Answer · 2012-08-31 17:49:25Z

1. Array must be initialized when its declared.

Eg:

public animalsData[] animals; 


    public static void main(String[] args) {
        mainMethod run = new mainMethod();
        System.out.println(run.animals.length);
    }

The above code gives a NullPointerException, cause animals which is an Object Reference Array Variable is Null and is not assigned the Array Object on the heap.

public animalsData[] animals = new animalsData[10]; 


        public static void main(String[] args) {
            mainMethod run = new mainMethod();
            System.out.println(run.animals.length);
        }

The above code works fine....as the Array is now initialized.

I would recommend you to use java.util.Collections instead of arrays, they are very flexible.

List<animalsData> animals = new ArrayList<animalsData>();

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