Question

运行javac ResizingArrayStack.java

时出现错误消息

$ javac ResizingArrayStack.java 
ResizingArrayStack.java:39: cannot find symbol
symbol  : class item
location: class ResizingArrayStack<Item>
  private class ReverseArrayIterator implements Iterator<item> {
                                                         ^
ResizingArrayStack.java:53: non-static class Item cannot be referenced from a static context
    ResizingArrayStack<Item> s;
                       ^
ResizingArrayStack.java:54: non-static class Item cannot be referenced from a static context
    s = new ResizingArrayStack<Item>();
                               ^
Note: ResizingArrayStack.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
3 errors

代码

import java.util.Iterator;
public class ResizingArrayStack<Item> implements Iterable<Item>
{
  private Item[] a = (Item[]) new Object[1];
  private int N = 0;

  public boolean isEmpty() {
    return N == 0;
  }

  public int size() {
    return N;
  }

  private void resize(int max) {
    Item[] temp = (Item[]) new Object[max];
    for (int i = 0; i < N; i++) {
      temp[i] = a[i];
    }
    a = temp;
  }

  public void push(Item item) {
    if (N == a.length) resize(2*a.length);
    a[N++] = item;
  }

  public Item pop() {
    Item item = a[--N];
    a[N] = null;
    if (N > 0 && N == a.length/4) resize(a.length/2);
    return item;
  }

  public Iterator<Item> iterator() {
    return new ReverseArrayIterator(); 
  }

  private class ReverseArrayIterator implements Iterator<item> {
    private int i = N;
    public boolean hasNext() {
      return i > 0;
    }
    public Item next() {
      return a[--i];
    }
    public void remove() {

    }
  }

  public static void main(String[] args) {
    ResizingArrayStack<Item> s;
    s = new ResizingArrayStack<Item>();
    while (!StdIn.isEmpty()) {
      String item = StdIn.readString();
      if (!item.equals("-")) {
        s.push(item);
      }
      else if (!s.isEmpty()) {
        StdOut.print(s.pop() + " ");
      }

      StdOut.println("(" + s.size() + " left on stack");
    }
  }
}

知道编译错误发生的原因吗？

Answer 1

一些事情。首先，你似乎有一个错字

 private class ReverseArrayIterator implements Iterator<item> { 
                                                        ^

其次，在

中使用Item

public class ResizingArrayStack<Item> implements Iterable<Item>

声明一个名为Item的新类型变量。它相当于

public class ResizingArrayStack<T> implements Iterable<T>

如果您打算将其用作Iterable的类型参数，请按照

使用它

public class ResizingArrayStack implements Iterable<Item>

第三，我不知道这里使用的Item类型

ResizingArrayStack<Item> s;

被声明，但似乎是在内部类中。内部类需要外部类的实例。因此无法从static上下文访问它。

Answer 2

你真的需要处理你的Java，很明显你来自不同的语言。以下是固定代码，可帮助您进一步理解。这不是完整的答案，但它应该足以解释您的代码有什么问题。

如果您要将其转换为Item，则无需使用Object的通用。

public class ResizingArrayStack implements Iterable<Object>
{
    private Object[] a = new Object[1];
    private int N = 0;

    public boolean isEmpty() {
        return N == 0;
    }

    public int size() {
        return N;
    }

    private void resize(int max) {
        Object[] temp = new Object[max];
        for (int i = 0; i < N; i++) {
            temp[i] = a[i];
        }
        a = temp;
    }

    public void push(Object item) {
        if (N == a.length) resize(2*a.length);
        a[N++] = item;
    }

    public Object pop() {
        Object item = a[--N];
        a[N] = null;
        if (N > 0 && N == a.length/4) resize(a.length/2);
        return item;
    }

    public Iterator<Object> iterator() {
        return new ReverseArrayIterator();
    }

    private class ReverseArrayIterator implements Iterator<Object> {
        private int i = N;
        public boolean hasNext() {
            return i > 0;
        }
        public Object next() {
            return a[--i];
        }
        public void remove() {

        }
    }

    public static void main(String[] args) {
        ResizingArrayStack s;
        s = new ResizingArrayStack();
        for (;;) {
            try {
                Scanner scanner = new Scanner(System.in);
                String item = scanner.nextLine();
                if (scanner.nextLine() != null) {
                    if (!item.equals("-")) {
                        s.push(item);
                    }
                    else if (!s.isEmpty()) {
                        System.out.println(s.pop() + " ");
                    }
                }

                Thread.sleep(500);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
}

由于“非静态类无法从静态上下文引用”而无法编译

2 个答案: