数据结构之基于Java的顺序栈实现

public class StackArray implements Stack {
    private final int LEN = 4;  //数组的默认大小
    private Object[] elements;  //数据元素数组
    private int top;            //栈顶指针
    public StackArray() {
        top = -1;
        elements = new Object[LEN];
    }
    public int getSize() {
        return top+1;
    }//返回堆栈的大小
    public boolean isEmpty() {
        return top<0;
    }//判断堆栈是否为空
    public void push(Object e) {
        if (getSize()>=elements.length) expandSpace();
        elements[++top] = e;
    }//数据元素e入栈
    private void expandSpace(){
        Object[] a = new Object[elements.length*2];
        for (int i=0; i<elements.length; i++)
            a[i] = elements[i];
        elements = a;
    }
    public Object pop() throws StackEmptyException {
        if (getSize()<1)
            throw new StackEmptyException("错误，堆栈为空。");
        Object obj = elements[top];
        elements[top--] = null;
        return obj;
    }//栈顶元素出栈
    public Object peek() throws StackEmptyException {
        if (getSize()<1)
            throw new StackEmptyException("错误，堆栈为空。");
        return elements[top];
    }//取栈顶元素
}
public interface Stack {
    //返回堆栈的大小
    public int getSize();
    //判断堆栈是否为空
    public boolean isEmpty();
    //数据元素e入栈
    public void push(Object e);
    //栈顶元素出栈
    public Object pop() throws StackEmptyException;
    //取栈顶元素
    public Object peek() throws StackEmptyException;
}
package dsa.exception;
//堆栈为空时出栈或取栈顶元素抛出此异常
public class StackEmptyException extends RuntimeException{
    public StackEmptyException(String err) {
        super(err);
    }   
}