NullPointerException when creating OBJECT ARRAYS and using them

Question

I am new in java and I've got a problem while working with arrays of objects. My main program is like this:

package bicycledemo;

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

        // Create two different 
        // Bicycle objects with an array
        Bicycle[] bike = new Bicycle[2];

        bike[0].cadence=50; //line 10, where the NullPointerException prompts out
        bike[0].gear=2;

        bike[1].cadence=10;
        bike[1].gear=3;

        System.out.println("gear: "+bike[0].gear+"\n"+"cadence: "+bike[0].cadence+"\n");
        System.out.println("gear: "+bike[1].gear+"\n"+"cadence: "+bike[1].cadence+"\n");


        System.out.println("\b");
    }
}

and the Bicycle class is this one:

package bicycledemo;

public class Bicycle {
    public Bicycle() {

    }

    public int cadence;
    public int gear;

}

When I run the program, the output error is:

Exception in thread "main" java.lang.NullPointerException
        at bicycledemo.BicycleDemo.main(BicycleDemo.java:10)
Java Result: 1

I suppose that what happens is that the object bike is not correctly created, but I don't see why.

Thank you very much for your help! I am really desperate to resolve this!

@TedHopp: No, the code really doesn't look fine at all.

Jon Skeet
– Jon Skeet

2013-02-24 18:27:21 +00:00
Commented Feb 24, 2013 at 18:27 — Jon Skeet
– Jon Skeet, Commented Feb 24, 2013 at 18:27

Jon Skeet · Accepted Answer · 2013-02-24 18:25:53Z

4

This line:

Bicycle[] bike = new Bicycle[2];

creates an array. It has two elements, and both elements are initially null references. You haven't created any Bicycle objects. Then in the next statement:

bike[0].cadence=50;

... you're trying to dereference that null value. You need to initialize the elements of the array before you use them as objects:

// Set the first element of the array to refer to a newly created object
bike[0] = new Bicycle();
// Then this will be fine
bike[0].cadence = 50;

If you don't understand exactly what's going on here, it's really important that you study it more closely. Understanding the difference between references and objects (and understanding that variables and arrays only ever contain references or primitive values, never objects) is fundamental to Java. Until you "get" that, you'll run into all kinds of difficulties.

Note that you could also change your Bicycle class to include a constructor taking the cadence and gear as parameters:

public Bicycle(int cadence, int gear) {
    this.cadence = cadence;
    this.gear = gear;
}

Then you could change your initialization into:

bike[0] = new Bicycle(50, 2);
bike[1] = new Bicycle(10, 3);

Or even as part of the array creation:

Bicycle[] bike = {
    new Bicycle(50, 2),
    new Bicycle(10, 3)
};

answered Feb 24, 2013 at 18:25

Jon Skeet

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1 Comment

davidws Over a year ago

Thank you very much! I was so confused because I was comparing with C++ and this doesn't happen, so I couldn't see the answer!

mockaroodev · Accepted Answer · 2013-02-24 18:27:17Z

2

You have to set each index in the array to a new instance before you can start accessing its properties.

Bicycle[] bike = new Bicycle[2];

bike[0] = new Bicycle(); // add this

bike[0].cadence=50;
...

answered Feb 24, 2013 at 18:27

mockaroodev

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Comments

PermGenError · Accepted Answer · 2013-02-24 18:31:10Z

2

you never initialized your array elements, Thus as your array element is an Object they get initialized to their default value null. thus when you try to invoke the element at an index it throw NPE.

    Bicycle[] bike = new Bicycle[2];// initializes the array with size 2
                                       but the elements hold default
                                        values(null in this case)
   bike[0] = new Bicycle();// initializing the array element at index 0
   bike[1] = new Bicycle();// initializing the array element at index 1

Of course the better design is also to have 2-args Bicycle constructor and initialize your attributes in it.or to have setter and getters for your attributes.

  public Bicycle(int cadence, int gear){
        this.cadence = cadence;
        this.gear = gear;
   }

Then,

        bike[0] = new Bicycle(2, 2);
        bike[1] = new Bicycle(3, 5);

edited Feb 24, 2013 at 18:31

answered Feb 24, 2013 at 18:25

PermGenError

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1 Comment

davidws Over a year ago

Thank you very much! I was so confused because I was comparing with C++ and this doesn't happen, so I couldn't see the answer!

JB Nizet · Accepted Answer · 2013-02-24 18:36:05Z

2

You need to populate your array first:

Bicycle[] bike = new Bicycle[2];
bike[0] = new Bicycle();
bike[1] = new Bicycle();

Otherwise, you create an array which just contains null elements.

edited Feb 24, 2013 at 18:36

answered Feb 24, 2013 at 18:25

JB Nizet

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Comments

Alexandr · Accepted Answer · 2013-02-24 18:30:38Z

0

You created an array of objects and access them without initializing.

bike = [null, null] - your result

answered Feb 24, 2013 at 18:30

Alexandr

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