generic class: what am i doing wrong?

[f. nikita thomas]

import java.util.*;
 
public class GenericStack<T>
{
   private Node first, previous;
 
   public void push(T item)
   {
previous = first;
first = new Node(item);
first.next = previous;
   }
   public T pop() throws EmptyStackException
   {
if(isEmpty())
    throw new RuntimeException("stack underflow");
T obj = (T)first.obj;
first = first.next;
return obj;
   }
   public boolean isEmpty()
   {
return first == null;
   }
   private class Node<T>
   {
private T obj;
private Node next;
 
private Node(T obj)
{
   this.obj = obj;
   next = null;
}
   }
}

warnings/errors:

javac -Xlint:unchecked GenericStack.java
GenericStack.java:10: warning: [unchecked] unchecked call to Node(T) as a member of the raw type GenericStack.Node
first = new Node(item);
^
GenericStack.java:17: warning: [unchecked] unchecked cast
found : java.lang.Object
required: T
T obj = (T)first.obj;
^
2 warnings


so what is the safe way of doing this without the cast? are there any resources for generics on the web? my textbook glazed over this like the plague. thanks.

nikita

[Garrett Rowe]

You're mixing generic Node<T>s and raw Nodes:

import java.util.*;
 
public class GenericStack<T>
{
   private Node<T> first, previous;
 
   public void push(T item)
   {
      previous = first;
      first = new Node<T>(item);
      first.next = previous;
   }
 
   public T pop() throws EmptyStackException
   {
      if(isEmpty())
          throw new RuntimeException("stack underflow");
      T obj = first.obj;
      first = first.next;
      return obj;
   }
 
   public boolean isEmpty()
   {
      return first == null;
   }
 
   private class Node<T>
   {
      private T obj;
      private Node<T> next;
 
      private Node(T obj)
      {
         this.obj = obj;
         next = null;
      }
   }
}

[f. nikita thomas]

thanks Garrett! any ideas on where i can find information on generics?

[Ulf Dittmer]

You could with the Sun Java Tutorial chapters on generics: here and then here