java: newbie on indexing object arrays

Question

I wish to create a java method that returns an array of type: ABCout, where class ABCout is defined as:

public class ABCout {
    public int numOut;
    public double[] myArray;
}

and the java method is:

public ABCout[] GetABC( double myInput ) throws Exception {
   ABCout[] userABC = new ABCout[3];

   userABC[0].numOut = 10;
   userABC[0].myArray = new double[1];
   userABC[0].myArray[0] = myInput;
   /* here I only fill the 0'th element, but once working I will fill the others */

   return userABC;
}

but I'm getting the error: java.lang.NullPointerException : null

Anyone see what I'm doing wrong?

MByD · Accepted Answer · 2012-03-21 20:26:39Z

4

You initialized the array, but not the objects in it. You probably need to do:

ABCout[] userABC = new ABCout[3];
for (int i = 0; i < userABC.length; ++i) {
    userABC[i] = new ABCout();
}

Also, you need to instantiate myArray:

public class ABCout {
    public int numOut;
    public double[] myArray;

    public ABCout() {
        myArray = new double[10];
    }
}

answered Mar 21, 2012 at 20:26

MByD

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1 Comment

ggkmath Over a year ago

Thanks. I'd prefer to instantiate outside of the class definition because the length will depend on myInput (received in method: GetABC), although not illustrated this way above for simplicity.

Steven · Accepted Answer · 2012-03-21 20:27:46Z

2

You need to instantiate the ABCout object you store in the array.

public ABCout[] GetABC( double myInput ) throws Exception {
   ABCout[] userABC = new ABCout[3];

   userABC[0] = new ABCout(); // instantiate

   userABC[0].numOut = 10;
   userABC[0].myArray = new double[1];
   userABC[0].myArray[0] = myInput;
   /* here I only fill the 0'th element, but once working I will fill the others */

   return userABC;
}

answered Mar 21, 2012 at 20:27

Steven

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Comments

Chris · Accepted Answer · 2012-03-21 20:28:03Z

2

You're declaring an array of ABCout, but you're trying to access the first element of that array before assigning it.

public ABCout[] GetABC( double myInput ) throws Exception {
    ABCout[] userABC = new ABCout[3];

    userABC[0] = new ABCout();
    userABC[0].numOut = 10;
    userABC[0].myArray = new double[1];
    userABC[0].myArray[0] = myInput;
    return userABC;
}

answered Mar 21, 2012 at 20:28

Chris

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Comments

fmucar · Accepted Answer · 2012-03-21 20:27:45Z

0

//add this
userABC[0] = new ABCount();

 userABC[0].numOut = 10;
   userABC[0].myArray = new double[1];
   userABC[0].myArray[0] = myInput;

Although you definetly need to study some object oriented coding before doing + you should read some basic java tutorial as well

answered Mar 21, 2012 at 20:27

fmucar

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Comments

Travis J · Accepted Answer · 2012-03-21 20:36:56Z

0

As others have stated, there was no instantiation of the 0th object and therefore you had a runtime error. You could also do this:

public ABCout[] GetABC( double myInput ) throws Exception {
 ABCout[] userABC = new ABCout[3];

 ABCout current = new ABCout();
 current.numOut = 10;
 current.myArray = new double[1];
 current.myArray[0] = myInput;

 userABC[0] = current;

 return userABC;
}

answered Mar 21, 2012 at 20:36

Travis J

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