java - 返回一个迭代器是什么意思？ java

Question

我必须编写一个实现 Iterable 接口(interface)的类。我对返回迭代器对象的含义感到困惑。迭代器只是遍历列表的元素，那么我如何将其作为对象返回呢？我会返回一个能够迭代的列表还是什么？当迭代器所做的只是遍历或更改其他对象中的数据时，它怎么可能是一个对象？

Answer 1

这是一个非常简单的列表示例。它将列表表示为链接元素。 迭代器对象被创建为一个匿名内部类，将当前元素作为状态。每次调用 iterator() 都会创建一个新的迭代器对象。

import java.util.Iterator;

public class SimplisticList<T> implements Iterable<T> {

  /*
   * A list element encapsulates a data value and a reference to the next
   * element.
   */
  private static class Element<T> {
    private T data;
    private Element<T> next;

    Element(T data) {
      this.data = data;
      next = null;
    }

    public T getData() {
      return data;
    }

    public Element<T> getNext() {
      return next;
    }

    public void setNext(Element<T> next) {
      this.next = next;
    }

  }

  // We only need a reference to the head of the list.
  private Element<T> first = null;

  // The list is empty if there is no first element.
  public boolean isEmpty() {
    return first == null;
  }

  // Adding a new list element.
  // For an empty list we only have to set the head.
  // Otherwise we have to find the last element to add the new element.
  public void add(T data) {
    if(isEmpty()) {
      first = new Element<T>(data);
    } else {
      Element<T> current = first;
      while(current.getNext() != null) {
        current = current.getNext();
      }
      current.setNext(new Element<T>(data));
    }
  }

  @Override
  public Iterator<T> iterator() {
    // Create an anonymous implementation of Iterator<T>.
    // We need to store the current list element and initialize it with the
    // head of the list.
    // We don't implement the remove() method here. 
    return new Iterator<T>() {
      private Element<T> current = first;

      @Override
      public boolean hasNext() {
        return current != null;
      }

      @Override
      public T next() {
        T result = null;
        if(current != null) {
          result = current.getData();
          current = current.getNext();
        }
        return result;
      }

      @Override
      public void remove() {
        // To be done ...
        throw new UnsupportedOperationException();
      }
    };
  }

}