Array of Object Causes NullPointerException Java

Question

I do not know why but this line of code gives a NullPointerException. I searched creating array of an user obj. in Java, again but, I could not find any problem.

private State[] states;
.    
.
.
private void initializeStates( )
        {
            states = new State[ stateNames.length ]; // Throws NullPointerException

        for( int i = 0; i < stateNames.length; i++ )
        {
            states[i] = new State();
            states[i].setName( stateNames[i] );
            states[i].setColor( stateColors.getColor() );
        }
    } // End of initializeStates()

And here is the class:

public class State
{
    private String stateName;
    private String color;
    private boolean isStartState;
    private boolean isFinalState;

        State()
        {
            stateName = new String(); // Can this line cause nullPtrException?
            color = new String(); // Can this line cause nullPtrException?
            isStartState = false;
            isFinalState = false;
        }

        public void setName( String name ){ stateName = name; }
        public void setColor( String clr ) {   color = clr; }
        public void makeStart( ) {  isStartState = true; }
        public void makeFinal( ) {  isFinalState = true; }

    }

Can you post the stacktrace and point to the line of the exception? — Jens Egholm
– Jens Egholm, Commented Apr 24, 2013 at 22:37

Jon Skeet · Accepted Answer · 2013-04-24 22:36:22Z

5

If this line throws the exception:

states = new State[ stateNames.length ]; // Throws NullPointerException

... then the problem is that stateNames is null. You're not making it as far as any of the rest of your code.

You haven't shown where stateNames is declared or initialized, but that's where you should look next.

As an aside, this:

stateName = new String();

... is inefficient for no reason. Just use an empty string literal:

stateName = "";

(Or better, take the appropriate initial values as constructor parameters.)

answered Apr 24, 2013 at 22:36

Jon Skeet

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