请教JAVA面试题

面试题：编写实例，要求利用数组实现栈，当栈空间满时，栈空间增加一倍。
本人在线等，希望各位大侠帮帮忙，小弟这里谢过各位啦！

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Java code:

import java.util.Arrays;

public class ArrStack{

private int size;

private Object[] elementData;

public ArrStack(){
this(10);
}

public ArrStack(int length){
elementData = new Object[length];
}

public int size(){
return this.size;
}

public boolean isEmpty(){
return this.size == 0;
}

public boolean isFull(){
return this.size == elementData.length;
}

public void put(Object obj){
if(isFull()){
int newSize = this.size*2;
System.out.println("stack capcity is "+newSize);
elementData = Arrays.copyOf(elementData,newSize);
}
elementData[size] = obj;
this.size ++ ;
}

public Object pop(){
if(isEmpty()){
//添加自己的处理
return "stack is empty !";
}
Object obj = elementData[--size] ;
elementData[size] = null;
return obj;
}

public static void main(String[] args){
ArrStack stack = new ArrStack();
int elementCount = 12;
for(int i = 0 ; i < el