Question

I have a constructor like as follows:

public Agent(){      this.name = 'John';     this.id = 9;     this.setTopWorldAgent(this, 'Top_World_Agent', true);  } 

I’m getting a null pointer exception here in the method call. It appears to be because I’m using ‘this’ as an argument in the setTopWorldAgent method. By removing this method call everything appears fine. Why does this happen? Has anyone else experienced this?

Answer 1

You can pass this to methods, but setTopWorldAgent() cannot be abstract. You can’t make a virtual call in the constructor.

In the constructor of an object, you can call methods defined in that object or base classes, but you cannot expect to call something that will be provided by a derived class, because parts of the derived class are not constructed yet. I would have expected some kind of compiler error if setTopWorldAgent() was abstract.

In Java, you can get surprising behavior with the contructor and derived classes — here is an example

http://en.wikipedia.org/wiki/Virtual_functions#Java_3

If you are used to C# or C++, you might think it’s safe to call virtual functions and not be calling overridden ones. In Java, the virtual call is made even though the derived class is not fully constructed.

If this isn’t what’s happening, then presumably, all of the parts of this that setTopWorldAgent() needs are initialized — if not, it’s probably one of the members of this that needs to be initialized.

Edit: thought this was C#