Question

我对此作业有轻微问题：我想完成LinkedStack实现并确保peek，isEmpty和size方法按Stack接口中的定义实现。我相信我已经使大部分工作正常（虽然还没有测试过）但是我遇到了ToString方法的障碍

这是讲师提供的启动

public interface Stack<T> {
/**
* Adds the specified element to the top of this stack.
* @param element element to be pushed onto the stack
*/
public void push(T element);
/**
 * Removes and returns the top element from this stack.
 * @return the element removed from the stack
 */
public T pop();
/**
 * Returns without removing the top element of this stack.
 * @return the element on top of the stack
 */
public T peek();
/**
* Returns true if this stack contains no elements.
* @return true if the stack is empty
*/
public boolean isEmpty();
/**
* Returns the number of elements in this stack.
* @return the number of elements in the stack
*/
public int size();
/**
 * Returns a string representation of this stack.
 * @return a string representation of the stack
 */
public String toString();}

这是我的LinkedStack类的代码：

public class LinkedStack<T> implements Stack<T> {
private Node head; //the head node
private int size; // number of items


private class Node {
    T item;
    Node next;
}


public LinkedStack() {
    head = null;
    size = 0;

}

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


public T pop() {
    T element = head.item;
    head = head.next;
    size--;

    return element;
}


public void push(T element) {
    Node oldHead = head;
    head = new Node();
    head.item = element;
    head.next = oldHead;
    size++;
}

public int size() { return size; }


public T peek() {
    if (isEmpty()) throw new NoSuchElementException("Error: Stack underflow");
    return head.item;
}

public String toString() {
    StringBuilder string = new StringBuilder();
    for (T stack : this) {
        string.append(stack + " ");
    }
    return string.toString();
}}

从此我得到了错误 for-each不适用于表达类型 required：array或java.lang.Iterable 发现：edu.csuniv.isiahjohnson.LinkedStack

我是否需要堆栈项的Iterator对象，或者这只适用于LinkedList类？

Answer 1

如果你想在java中使用每个循环，你的对象需要实现Iterable接口。

在java中，这段代码：

for (T stack : this) {
   ...
}

是一个语法词汇：

for(Iterator<T> iter = this.iterator(); iter.hasNext(); ) {
    T item = iter.next();
    ...
}

很明显，你需要iterator()来做到这一点，因此 - 实施Iterable

Answer 2

这是一个解决方案。它没有实现Iterable，但它只是迭代节点。我也删除了最后一个空格。您可以使用自{8}以来存在的StringBuilder而不是StringJoiner。

@Override
public String toString() {
  final StringBuilder string = new StringBuilder();
  Node node = this.head;
  while (node != null) {
    string.append(node.item).append(' ');
    node = node.next;
  }
  // remove last space:
  if (string.length() > 0)
    string.setLength(string.length() - 1);
  return string.toString();
}

Java LinkedStack实现中ToString方法的问题

2 个答案: