我如何在 Iterator<E> 的 "next" 方法中使用 return？

Question

在 Iterator 中实现的方法 "next" 有一个错误。我不能 return 'e' 值，因为 Java 说它是对象类型。我不明白为什么。因为在 Elem 中 class 如果我给例如 Point 对象，它必须 return 它。但我不太了解泛型，也许有人可以解释一下。谢谢）

package Tutorial3;

import java.util.Iterator;


public class MyLinkedList<E> implements Iterable<E> {
    Elem<E> head;
    Elem<E> tail;

    public MyLinkedList() {
        head = null;
        tail = null;
    }

    public void add(E e) {
        Elem<E> newElem = new Elem<E>(e);
        if (head == null) {
            tail.setNext(newElem);
            head = newElem;
            tail = newElem;
        } else {
            tail.setNext(newElem);
            tail = newElem;
        }
    }

    public void addToHead(E e) {
        Elem<E> newElem = new Elem<E>(e);
        newElem.setNext(head);
        head = newElem;
    }

    public void addToTail(E e) {
        add(e);
    }

    public void removeFirstValue() {
        head = head.getNext();
    }

    public void removeLastValue() {
        Elem<E> cursor;
        cursor = head;
        while (cursor.getNext() != tail) {
            cursor = cursor.getNext();
        }
        tail = cursor;
        tail.setNext(null);
    }

    public E get(int index) {
        if ((index + 1) > size()) {
            throw new IndexOutOfBoundsException("Index: " + index + " Size: " + size());
        } else {
            Elem<E> cursor;
            cursor = head;
            int i = 0;
            if (i == index) {
                return cursor.getE();
            } else {
                while (i != index) {
                    cursor = cursor.getNext();
                    i++;
                }
                return cursor.getE();
            }
        }
    }

    public int size() {
        Elem<E> cursor;
        cursor = head;
        int size = 0;
        while (cursor != null) {
            size++;
            cursor = cursor.getNext();
        }
        return size;
    }

    @Override
    public Iterator<E> iterator() {
        return new MyLinkedListIterator<E>();
    }

    private class MyLinkedListIterator<T> implements Iterator<T> {
        private Elem elem = head;

        @Override
        public boolean hasNext() {
            return elem.getNext() != null;
        }

        @Override
        public T next() {
            return elem.getNext().getE();  //here is mistake: can't return e
                                           //required : T, Found: Object
        }
    }
}

package Tutorial3;

public class Elem<E> {
    private E e;
    private Elem<E> next;

    public Elem(E e) {
        this.e = e;
    }

    public Elem(E e, Elem<E> next) {
        this.e = e;
        this.next = next;
    }

    public E getE() {
        return e;
    }

    public void setE(E e) {
        this.e = e;
    }

    public Elem<E> getNext() {
        return next;
    }

    public void setNext(Elem<E> next) {
        this.next = next;
    }
}

Answer 1

应该是

private class MyLinkedListIterator implements Iterator<E> {
    private Elem<E> elem = head;

    @Override
    public boolean hasNext() {
        return elem.getNext() != null;
    }

    @Override
    public E next() {
        return elem.getNext().getE();
    }
}

您的 MyLinkedListIterator 类型应该从外部 MyLinkedList<E> 类型继承 E，而不是声明它自己的元素类型 T，然后您通过使用原始 Elem 类型而不是 Elem<E>.

我如何在 Iterator<E> 的 "next" 方法中使用 return？

How I have to return in "next" method of Iterator<E>?

java

generics

iterator

iterable