Java中的空指针错误？

Question

I'm pretty new to this forum thing. I'd always been pretty apprehensive about posting on forums but after two weeks of bashing my head against my code trying to make it work I'd finally given in. The code below is a critical part of some larger code but I think I've kind of pinpointed the problem section. I have a 2D array of object which are supposed to be initialized by default as soon as it is created. This, however, does not seem to be happening and the value remains null. Could this be the result of Java not being able to pass by reference? If so then why is it that the primitive 2D arrays are instantiated perfectly fine? Thank you in advance, Lasz.

    class ObjectThing
    {
        int someInt;
        double someDouble;

        public ObjectThing()
        {
            someInt = 0;
            someDouble = 0;
        }

        synchronized private void someIntIncrement ()
        {
           someInt++;
        }

        synchronized public void addSomeDouble (double sd)
        {
            someDouble += sd;
            someIntIncrement ();
        }

        synchronized public String toString ()
        {
            return someInt + "," + someDouble;
        }
  }


  class AnotherObject
  {
          String name;
          ObjectThing [][] someObjectMAtrix;
          double [][] someDoubleMAtrix01;
          double [][] someDoubleMAtrix02;   

          public AnotherObject()
          {
          }

          private void initDouble (double [][] mat)
          {
             for (double [] i: mat)
                for (double j: i)
                   j = 0;
          }

          private void initObject (ObjectThing [][] so)
          {
              for (ObjectThing [] i: so)
                 for (ObjectThing j: i)
                    j = new ObjectThing ();
          }         

          public void init (int r, int c)
          {
              someObjectMAtrix =  new ObjectThing [r][c];
              someDoubleMAtrix01 = new double [r][c];
              someDoubleMAtrix01 = new double [r][c];         
          initObject (someObjectMAtrix);
              initDouble (someDoubleMAtrix01);
              initDouble (someDoubleMAtrix02);
          }
   }

   class Driver
   {
       public static void main (String [] args)
       {
            initializeMethod();
       }

       public void initializeMethod ()
       {
            AnotherObject [] anotherObjectArray = new AnotherObject [1];
            for (AnotherObject i: anotherObjectArray)
            {
                i.init(72,72);
            }
       }
   } 

Answer 1

The problem is in your initializeMethod:

        AnotherObject [] anotherObjectArray = new AnotherObject [1];
        for (AnotherObject i: anotherObjectArray)
        {
            i.init(72,72);
        }

Going Object[] myArray = new Object[1] only allocates space for the array. It does not actually create these objects. Your code block should be

        AnotherObject [] anotherObjectArray = new AnotherObject [1];
        anotherObjectArray[0] = new AnotherObject();
        for (AnotherObject i: anotherObjectArray)
        {
            i.init(72,72);
        }

This is a bit clunky. I'm assuming you have some need for putting AnotherObject into arrays and you are just not showing it. If you only need one of those objects, then just create it directly. You should probably have a look at Lists

Answer 2

I believe this is by design. A primitive will get allocated on the stack, and always have some sort of value by default. An object gets allocated on the heap and will be passed around by reference. Those references will point to nothing by default until you initialize it.

Answer 3

You just initialized the array of AnotherObject objects...but there is no AnotherObject in the first field of the array... It is null as you get it out, because you didn't initialize it. In the initializeMethod you have to do the following:

AnotherObject [] anotherObjectArray = new AnotherObject [1];
 for (AnotherObject i: anotherObjectArray)
    {
        i = new AnotherObject();
        i.init(72,72);
    }

cheers!

Answer 4

The Java compiler will force you to initialise variables, primitives or reference types (like objects), or it will generate a compiler error and fail.

However, array initialisation will create an array of the desired size, with each element set to a default value. For primitives, this is something like 0 for numeric types, false for booleans. For reference types , the default is null . This is the source of your problem.

You correctly initialise the array, but do not populate it with valid values. So, it is not to do with pass-by-reference, but with proper array initialisation, and knowing the default values of a newly initialised array.

Specifically, you are using a for-each construct to loop over the array. This construct performs an operation on every object in a collection/array.

Therefore, this code:

AnotherObject [] anotherObjectArray = new AnotherObject [1];
for (AnotherObject i: anotherObjectArray){
    i.init(72,72);
}

is equivalent to this code:

AnotherObject [] anotherObjectArray = new AnotherObject [1];
for(int count=0; count <  anotherObjectArray ; count++){
    AnotherObject tempObj = anotherObjectArray[i]; // This is null, because anotherObjectArray[i] hasn't been initialised yet
    tempObj.init(72,72); // This is where we get the NullPointerException :(
}

Can you see the problem? You are attempting to perform an operation on null, hence the NullPointerException

I believe what you actually intended was this:

AnotherObject [] anotherObjectArray = new AnotherObject [1];
for(int count=0; count <  anotherObjectArray ; count++){
    AnotherObject tempObj = new AnotherObject(); // Here we create the new object
    tempObj.init(72,72); // Initialise the newly created object
    anotherObjectArray[i] = tempObj; // Now we store in the array :)
}

I hope I explained this clearly enough, but please ask for clarification if I haven't :)