優先考慮泛型

import java.util.Objects;

/**
 * Created by kaizige on 2017/7/3.
 */
public class Stack {
    private Object[] eles;
    private int size=0;
    private static  final int CAPATY=10;
    public Stack(){
        eles=new Object[CAPATY];
    }
    public void push(Object o){
        eles[size++]=o;
    }
    public Object pop(){
        if(size==0){
            throw new RuntimeException("沒有元素");
        }
        Object o=eles[--size];
        eles[size]=null;
        return o;
    }

    public static void main(String[] args) {
        Stack s=new Stack();
        s.push("hello");
        System.out.println(s.pop());
    }
}

/**
 * Created by kaizige on 2017/7/3.
 */
public class Stack2<E> {
    private E[] eles;
    private int size=0;
    private static  final int CAPATY=10;
    @SuppressWarnings("unchecked")
    public Stack2(){
        eles=(E[])new Object[CAPATY];
    }
    public void push(E o){
        eles[size++]=o;
    }
    public E pop(){
        if(size==0){
            throw new RuntimeException("沒有元素");
        }
        E o=eles[--size];
        eles[size]=null;
        return o;
    }
    public static void main(String[] args) {
        Stack2<String> s=new Stack2<String>();
        s.push("hello");
        System.out.println(s.pop().getClass().getCanonicalName());
    }
}

/**
 * Created by kaizige on 2017/7/3.
 */
public class Stack3<E> {
    private Object[] eles;
    private int size=0;
    private static  final int CAPATY=10;
    @SuppressWarnings("unchecdked")
    public Stack3(){
        eles=new Object[CAPATY];
    }
    public void push(E o){
        eles[size++]=o;
    }
    public E pop(){
        if(size==0){
            throw new RuntimeException("沒有元素");
        }
        E o=(E) eles[--size];
        eles[size]=null;
        return o;
    }
    public static void main(String[] args) {
        Stack2<Integer> s=new Stack2<Integer>();
        s.push(2);
        System.out.println(s.pop().getClass().getCanonicalName());
    }
}