Re: about the Object reference

You can check if an object is of a certain type using 'instanceof'.

if (o instanceof String) {
    String s = (String) o;
}

however it would be better if you designed the Stack with generics.

/**
 * A stack
 * @param <T> the generic type
 */
public interface Stack<T> {
    /**
     * Push an object onto the stack
     * @param t the object
     */
    void push(T t);

    /**
     * @return pop an object from the stack
     */
    T pop();
}

A simple implementation used a linked list:

import java.util.LinkedList;

/**
 * Implementation of Stack
 */
public class StackImpl<T> implements Stack<T> {
    private LinkedList<T> list = new LinkedList<T>();

    /**
     * {@inheritDoc}
     */
    public T pop() {
        return list.removeLast();
    }

    /**
     * {@inheritDoc}
     */
    public void push(T t) {
        list.addLast(t);
    }
}

Example usage:

public class StackTest {
    public static void main(String[] args) {
        Stack<String> stack = new StackImpl<String>();
        stack.push("hi");
        stack.push("joe");
        System.out.println(stack.pop());
        System.out.println(stack.pop());
    }
}

Cheers,
Martin.

"jtl.zheng" <jtl.zheng@gmail.com> wrote in message
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