Question

I have a class. In this class I declare a a private variable private Agent agent;. In my class I have

private Thread controller = new Thread() {
...
}

Within the above private thread I call getParameter which is a private method of the considered class. Within the getParameter I call one of the methods of the agent. As a result I get a NullPointerException. So, I conclude that agent is not visible from the getParameter.

Why is that? Can it be that the reason of that is that the getParameter is within the private Thread? And, if it is the case, how the described problem can be resolved?

ADDED

I realized that I need to be more specific. My code is organized like that:

public class GameWindow {
   ...
   private Agent agent;
   ...
   private Thread controller = new Thread() {
        public void run() {
           ...
           Agent agent = new Agent();
           ...
           parameter = getParameter();
        }
  }
  ...
  private String getParameter() {   
     ...
     agent.someMethod();
     ...
  }
}

ADDED 2
In the GameWindow I have:

public void startWindow() {
    SwingUtilities.invokeLater(new Runnable() {
        public void run() {
            controller.start();
        }
    });
}

Answer 1

The NullPointerException has nothing to do with visibility. You are probably calling the getParameter method of a null object. Your agent variable is declared, but not initialiazed. You may want to code something like:

private Agent agent;
private Thread controller = new Thread() {
    agent = new Agent();
    // ...
    agent.getParameter(); // should not throw a NullPointerException
}

UPDATE AFTER CODE WAS ADDED

Your code has two definitions of agent. The first one in the class:

private Agent agent;

And the second one inside your run() method:

Agent agent = new Agent();

Your getParameter() method does not know the agent defined inside the run() method. It only knows the agent member of the class, which was not initialized. Your problem will be solved when you remove the second definition of agent:

public void run() {
       ...
       // Agent agent = new Agent();
       agent = new Agent(); // works fine: you are initiliazing the class member
       ...
       parameter = getParameter();
 }

OLD UPDATE:
The NullPointerException is thrown inside the getParameter() method. So I understand your code is like the following:

private Something getParameter() {
    agent.someMethod(); // exception thrown here
}

If this is your code, the problem remains the same: agent is not initialized. You must initialize it before calling any of its methods inside the getParameter() method.