使用一个数组实现两个栈

public class TwoStack {
    private Object[] stack;
    //栈1在数组中的下标
    private int size1;
    //栈2在数组中的下标
    private int size2;
    //初始化数组的大小
    public TwoStack(int length){
        stack=new Object[length];
        size1=0;
        size2=length-1;
    }
    //栈1的元素的个数
    public int size1(){
        return size1;
    }
    //栈2的元素的个数
    public int size2(){
        return  capacity()-1-size2;
    }
    //栈1是否为空
    public boolean isEmpty1(){
        return size1==0;
    }
    //栈2是否为空
    public boolean isEmpty2(){
        return size2==(capacity()-1);
    }
    //数组的大小
    public int  capacity(){
        return stack.length;
    }
    //数组自动扩展大小
    public void ensureCapacity(){
        if((size1()+size2())>=capacity()){
            Object[] newStack=new Object[capacity()*2];
            int k=newStack.length-1;
            for (int i = 0; i <size1(); i++) {
                newStack[i]=stack[i];
            }
            for(int j=capacity()-1;j>=0;j--){
                newStack[k]=stack[j];
                k--;
            }
            size2=newStack.length-(capacity()-size2);
            stack=newStack;
        }
    }
    //栈1入栈
    public void push1(Object obj){
        ensureCapacity();
        stack[size1]=obj;
        size1++;
    }
    //栈2入栈
    public void push2(Object obj){
        ensureCapacity();
        stack[size2]=obj;
        size2--;
    }
    //栈1出栈
    public Object pop1(){
        if(isEmpty1()){
            throw new ArrayIndexOutOfBoundsException("栈1不能为空");
        }
        Object obj=stack[--size1];
        stack[size1]=null;
        return obj;
    }
    //栈2出栈
    public Object pop2(){
        if(isEmpty2()){
            throw new ArrayIndexOutOfBoundsException("栈2不能为空");
        }
        Object obj=stack[++size2];
        stack[size2]=null;
        return obj;
    }
    }