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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); } } }

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3 Answers 3

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10

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.

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Comments

3

You didn't initialize the vector. Should be:

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

Should probably be private final List<Ethernet> etherPorts = new ArrayList<Ethernet>(); The final would have caught the error.
2

etherPort is null. You are apparently never initializing it with an actual Vector. I think you want:

public class Server extends Computer{

     public Vector<Ethernet> etherPort; 

     public Server()
     {
        etherPort = new Vector<Ethernet>();
     }

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