关于java stack压栈出错

栈是后进先出，只要记住这一点，就不难实现。下面是我的实现：

/**
 * StackByArray：数组方式实现栈，实现栈的初始化，入栈出栈操作
 * 
 * @author zeng.xiangdong 1770534116@qq.com
 * @version V1.0 2014-8-24 下午10:00:03
 */
public class StackByArray {

    // 栈的长度
    private int length;
    // 栈
    private Object[] array;
    // 栈顶的下标
    private int topIndex = -1;

    /**
     * StackByArry：构造方法
     * 
     * @param length 初始化栈的长度
     */
    public StackByArray(int length) {
        this.length = length;
        array = new Object[length];
    }

    /**
     * push:入栈
     * 
     * @param obj
     * @throws Exception
     */
    private void push(Object obj) throws Exception {
        if(isFullStack()) {
            throw new Exception("栈已满");
        }
        array[++topIndex] = obj;
    }

    /**
     * offer: 出栈，取出栈顶元素
     * 
     * @return obj
     * @throws Exception
     */
    private Object offer() throws Exception {
        if(isEmptyStack()) {
            throw new Exception("栈已空");
        }

        Object topObject = array[topIndex];
        array[topIndex] = null;
        topIndex--;
        return topObject;
    }

    /**
     * isEmptyStack:判断栈是否为空
     * 
     * @return
     */
    private boolean isEmptyStack() {
        return (topIndex == -1);
    }

    /**
     * isFullStack:判断栈是否已满
     * 
     * @return
     */
    private boolean isFullStack() {
        return topIndex == (length - 1);
    }

    /**
     * getLength:取得栈内的数据长度
     * 
     * @return length
     */
    private int getLength() {
        return topIndex + 1;
    }

    /**
     * outPut:栈内容输出
     */
    private void outPut() {
        for(Object obj : array) {
            if(obj != null) {
                System.out.print(obj + ", ");
            }
        }

        System.out.println();
    }

    /**
     * main:(这里用一句话描述这个方法的作用)
     * 
     * @param args
     */
    public static void main(String[] args) throws Exception {
        StackByArray stack = new StackByArray(10);

        for(int i = 1; i <= 8; i++) {
            stack.push(i);
            stack.outPut();
        }

        System.out.println("---------------");
        System.out.println(stack.offer());
        stack.outPut();
        System.out.println("---------------");
        System.out.println(stack.offer());
        stack.outPut();

        System.out.println(stack.getLength());
    }
}