Question

I am putting together a simple simulated network in java, where i can add computers, servers, and connect two objects with ethernet ports. This is where the null pointer exception is being thrown, when i call “this.etherPort.addElement(t);”

import java.util.Vector;

public class Server extends Computer{

     public Vector<Ethernet> etherPort; 

     public void addPort(Ethernet t)
  {
   this.etherPort.addElement(t);
  }
}

This code is run when i make a new Ethernet object using this code:

public class Ethernet {

public Computer terminal1, terminal2;
public int volume;
public Ethernet(Computer term, Server term2)
{
this.terminal1 = term;
this.terminal2 = (Computer) term2;
if(term != null)
{
term.addPort(this);
}
if(term2 != null)
{
term2.addPort(this);
}
}
}


Answer 1

You need to instanciate your etherPort member :

public class Server extends Computer{

     public Vector<Ethernet> etherPort = new Vector<Ethernet>(); 

     public void addPort(Ethernet t)
     {
        this.etherPort.addElement(t);
     }
}

You should make sure that addPort() is not overriding a method called from your Computer constructor, though. Given the context, I assume it’s safe (i.e. Computer has no addPort() method).

As stated below in a comment, it’s generally better to use interfaces that don’t constrain the containers implementations : you’d better declare etherPort as a

List<Ethernet> 

instead of a

Vector<Ethernet>

and use etherPort.add(element) instead of the Vector-specific addElement method.