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December 2001



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Nullifying Those Pointer Exceptions

Posted by Matt Gerrans on December 08, 2001 at 6:27 PM

Oops - I should have used the pre keyword, not code!

> I am coming from a C++ backround but haven't programmed in a few years. I am repeatedly making the same mistake and I think its due to a general error in how I'm trying to solve things. I keep getting nullpointer exception when I try to create an array of class objects. This also happens when I try to use methods from another class. I have read Java in a Nutshell. I'm not sure I can make my problem any clearer right now because I've been staring at it and fiddling with it so long I don't know what is going on anymore. Can anyone point me to a simplest case scenario of object oriented class use in java? Thanks and sorry to just jump in with a general question like this.

Coming from a C++ background, you may be used to doing something like this:

const int itemCount = 10;
MyObject objects[itemCount ];


MyObject *objects = new MyObjects[itemCount ];

in both these cases, you've created an array and 10 of your objects, which are then pointed to by the array.

However, in Java, when you do this:

final static int itemCount = 10;
MyObject [] objects = new MyObject[itemCount];

all you have done is create an array with 10 slots. You have not created any object. This is more like creating an array of pointers (or references) in C++. You still need to put things in the array:

for(int i=0; i < itemCount; i++ )
objects[i] = new MyObject();

You would have the same problem in C++ if you did this:

MyObject * * objects = new MyObject *[itemCount];
objects[0].MyMethod(); // Crash!
delete [] objects;

- mfg


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