实现 ArrayList 的迭代器

Question

我在向 ArrayList 添加迭代器支持时遇到问题 这是我为实现迭代器而创建的类

class MyArrayListIterator<E> implements Iterator<E> {
private E[] list = null;
private int currentIndex = 0;

@Override
public boolean hasNext() {
    if (currentIndex < list.length) {
        return true;
    } else {
        return false;
    }
}

@Override
public E next(){
    return list[currentIndex++];
}

} 这必须包括，我认为我做对了

MyArrayList 类型的“列表”
int 类型的“currentIndex”，初始为零

这是我的主要测试方法

public static void main(String[] args) throws Exception {
    MyList<String> names = new MyArrayList<>();
    
    names.add("Steve");
    names.add("Frank");
    names.add("Heather");
    names.add("Chris");
    names.add("Oliver");
    
      for (String string : names) {   // error at names Can only iterate over an array or an instance of java.lang.Iterable
            System.out.println(string);
        }
}

}

在我添加的 myArrayList 中，要求是通过添加 iterator() 方法使 MyArrayList 实现 Iterable 接口，该方法应返回 MyArrayListIterator 的实例。

public Iterator<E> iterator() {
    return new MyArrayListIterator();
}

请让我知道我做错了什么。

Answer 1

正如 cmets 中已经提到的，您的问题是您从 list 中的 MyArrayListIterator 读取而没有初始化它。这会导致NullPointerException。

您可以通过两种不同的方式解决此问题：

1. 将MyArrayListIterator 设为non-static nested（它们也称为inner classes）类MyArrayList。

   通过这样做，您可以访问外部类的所有字段，在本例中为 MyArrayList。比如看这段代码sn -p：

public class MyArrayList<E> implements MyList<E> {
  private Object[] list = new Object[10];
  private int size = 0;

  public Iterator<E> iterator(){
    return new MyArrayListIterator<>();
  }

  // more of your implementation...

  // This is the inner class
  private class MyArrayListIterator<T> implements Iterator<T> {
    private int currentIndex = 0;

    @Override
    public boolean hasNext() {
      // Since this is a inner class, we have access to the
      // "list" field defined by MyArrayList.
      return currentIndex < list.length;
    }

    @Override
    public T next() {
      // Since this is a inner class, we have access to the
      // "list" field defined by MyArrayList.
      return (T) list[currentIndex++];
    }
  }
}

1. 将MyArrayListIterator 设为static nested 类或单独的类。

   在这种情况下，您无权访问MyArrayList 中定义的字段，这意味着您必须自己提供这些字段。这可以使用构造函数来完成。

public class MyArrayList<E> implements MyList<E> {
  private Object[] list = new Object[10];
  private int size = 0;

  public Iterator<E> iterator(){
    return new MyArrayListIterator<>((E[])list);
  }

  // More of your list implementation

  // This is a static inner class, which means we do not have access to
  // the fields of the outer class.
  private static class MyArrayListIterator<T> implements Iterator<T> {
    private final T[] elements;
    private int currentIndex = 0;

    public MyArrayListIterator(T[] elements) {
      this.elements = elements;
    }

    @Override
    public boolean hasNext() {
      // Since we got the elements as constructor argument, they are not
      // null (assuming you did not call it with null as parameter).
      return currentIndex < elements.length;
    }

    @Override
    public T next() {
      // Since we got the elements as constructor argument, they are not
      // null (assuming you did not call it with null as parameter).
      return elements[currentIndex++];
    }
  }
}

有关嵌套类的更多信息，请参阅this tutorial by Oracle 嵌套类。