Is the base object in the world when you call the 'setMessage(String)' method? It is really difficult to understand what you are doing. 'ABC' and 'base' and 'm' are vague descriptors for classes and fields. What behavior is each supposed to have? Also, what exactly is this method supposed to do? It looks like it could either add or remove the 'm' actor to or from the world.

I guess you use the method from a constructor. You can't use methods like 'getX()' there, because the object is not placed in the world yet when the constructor code is executed. You can use the 'addedToWorld' method which is executed automatically right after the object is placed in the world. I also wonder what you want to do there actually. You set a Text to an object (reference 'm') and if this object exists, you delete it. By the way, you don't need to get a reference to your world when using regular world methods, you can just use 'getWorld().addObject(...)'. (I am wondering when the Greenfoot team will increase the size of the "Posting code? read this!" link below the text area...)

//Full Class public class Full extends Actor { Message m = new Message(); // Message is used to print message in world. public Full() { GreenfootImage image = getImage() ; image.scale( 150, 180 ) ; } protected void setMessage( String msg ) { int x, y ; m.setText( msg ); x=getX(); y=getY(); World world = getWorld(); if ( m.getWorld() != null ) { world.removeObject( m ) ; return; } world.addObject(m, X,Y ) ; } public class Half extends Full { public void update(Coin coin){ inspect(coin); } public void inspect( Coin coin ) { System.out.println( "Coin: " + coin.getClass().getName() ) ; setMessage(coin.getClass().getName()) ; } } /// I am calling Half using below code snippet: Half half = new Half(); half.update(coin); Getting null pointer exception in base class i.e Full class at line: X=getX(); I want to display a message in my world.

With this, the first line creates a new Half object and the second calls 'update' on that object. 'update' in turn calls 'setMessage' on the Half object which will require the Half object be in the world (because it uses 'getX()' and 'getY()' ). But this new Half object was never added into the world; so, an exception is thrown.

I cannot say exactly what you should do or how you should add the Half object into the world. It depends on what class this snippet is in (what type -- Actor or World or other). Then, if it is an Actor, is that actor in the world or not. It might help to show the entire class where this snippet appears. And please use the 'code' link below the 'Post a reply' box to insert (copy/paste) your code into your posts.

The problem is you have too many Message objects being created. Each time you instantiate (create an object from) the Alien class, the Inspector class or the GumballMachine class, you are, in turn, creating a Message object (see line 5 and line 93 -- because Inspector extends Alien, an Inspector object is considered an Alien object and one is create there also -- I am not sure if you want the one to extend the other, anyway; if the inspector is not an alien, then Inspector should not extend Alien).

Inspector is Alien. And i need all these Message object to print different messages on ever click and event in my world. I am getting Null Pointer exception while calling setMessage function from Inspector class. Please help me in resolving this

Maybe you should actually add the 'inspector' (line 6) to your world. You are trying to get the position of it in the world although it was never added. If it is added in your world class, then the 'inspector' in line 6 doesn't refer to the same object.

Well put!