Custom class in an array?

[bugmenot]

I was wondering if it was possible to put a custom class in an array. So that way you could do like:

//something.java
class something {
boolean visible;
int x,y;
}

//blah.java
//...
something[4].x = 40;
something[4].y = 30;
something[4].visible = true;
//...

[Fubarable]

You bet it's possible, and it's done often. Why not give it a try and see what happens?

[bugmenot]

Well this is what I tried:

/*
* Properties for each brick
*/

class brick {
boolean visible;
int x,y,width,height;
}

/**
* BreakOut
*/

import java.awt.*;
import java.applet.*;

public class BreakOut extends Applet {

brick block[] = new brick[20];

public void init() {
block[3].visible = true;
block[4].x = 60;
System.out.println(block[4].x);
}

public void paint(Graphics g) {

}
}

And it's returning:

--------------------Configuration: BreakOut - JDK version 1.6.0_13 <Default> - <Default>--------------------
java.lang.NullPointerException
at BreakOut.init(BreakOut.java:14)
at sun.applet.AppletPanel.run(AppletPanel.java:424)
at java.lang.Thread.run(Thread.java:619)

Process completed.

[Fubarable]

Ah, you are now learning the difference between an array of primitive, say an int array, and an array of objects, say a brick array. With the primitive array, all you have to do is declare the array and use it -- it's ready immediately for your use.

With an object array on the other hand, you have to declare the array first, and then you have to initialize the objects in the array before you can use them. Think of creating an object array as if you were creating a parking lot. You have to fill it with cars first before you can use a car. So for this code:

try changing your init to:

  public void init()
  {
    // initialize every brick in the block array
    for (int i = 0; i < block.length; i++)
    {
      block[i] = new brick();
    }
    block[3].visible = true;
    block[4].x = 60;
    System.out.println(block[4].x);
  }

and then see if it works

[bugmenot]

Perfect, thanks a lot.

[Fubarable]

You're quite welcome!