每次執行到s.push(item)這里就錯了..剛開始學java并不是很懂..package test;import java.util.Scanner;public class FixedCapacityStackOfStrings {    private String[] a;    private int N;    public FixedCapacityStackOfStrings(int cap)    { a = new String[cap];}    public boolean isEmpty() { return N==0;}    public int size() {return N;}    public void push(String item)    { a[N++] = item;}    public String pop()    { return a[--N];}    public static void main(String[] args)    {        FixedCapacityStackOfStrings s;        s = new FixedCapacityStackOfStrings(100);        Scanner scan =new Scanner(System.in);        System.out.println("請輸出");        while(scan.hasNext())        {            String item = scan.next();            if(!item.equals("-"))                s.push(item);            else if(!s.isEmpty()) System.out.println(s.pop()+" ");                    }        System.out.println("結束");        scan.close();        System.out.println("("+s.size()+" left on stack)");    }}