Justin's Words

Java Iterator and ListIterator

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本文包含的 ArrayList 用法可在上篇文章找到，先上代码：

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import java.util.ArrayList;
import java.util.Iterator;
import java.util.ListIterator;

public class IteratorExample {
    public static void main(String args[]) {

        ArrayList list = new ArrayList();

        list.add("One");
        list.add("Two");
        list.add("Three");
        list.add("Four");
        list.add("Five");

        System.out.println("Iterator using while loop: ");
        Iterator it1 = list.iterator();
        while (it1.hasNext()) {
            System.out.println("Next element: " + it1.next());
        }

        System.out.println("Iterator using for loop: ");
        for (Iterator it2 = list.iterator(); it2.hasNext();) {
            System.out.println("Next element: " + it2.next());
        }

        System.out.println("List iterator (forward iteration): ");
        ListIterator lit = list.listIterator();
        while (lit.hasNext()) {
            System.out.println("Next element: " + lit.next());

        }

        System.out.println("List iterator (backward iteration): ");
        lit = list.listIterator(list.size());
        while (lit.hasPrevious()) {
            System.out.println("Previous element: " + lit.previous());

        }

        lit.next();
        lit.add("One more element");
        System.out.println("Modified list after the insertion of the new element");

        System.out.println("Index of next element: " + lit.nextIndex());
        System.out.println("Index of previous element: " + lit.previousIndex());

        for (lit = list.listIterator(); lit.hasNext();) {
            System.out.println("Next element: " + lit.next());
        }

        lit.previous();
        lit.remove();

        System.out.println("Modified list after the removal of an element");
        for (lit = list.listIterator(); lit.hasNext();) {
            System.out.println("Next element: " + lit.next());
        }

    }
}

运行结果：

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Iterator using while loop:
Next element: One
Next element: Two
Next element: Three
Next element: Four
Next element: Five
Iterator using for loop:
Next element: One
Next element: Two
Next element: Three
Next element: Four
Next element: Five
List iterator (forward iteration):
Next element: One
Next element: Two
Next element: Three
Next element: Four
Next element: Five
List iterator (backward iteration):
Previous element: Five
Previous element: Four
Previous element: Three
Previous element: Two
Previous element: One
Modified list after the insertion of the new element
Index of next element: 2
Index of previous element: 1
Next element: One
Next element: One more element
Next element: Two
Next element: Three
Next element: Four
Next element: Five
Modified list after the removal of an element
Next element: One
Next element: One more element
Next element: Two
Next element: Three
Next element: Four

Iterator

Method

检查是否还有下一个元素和迭代到并获取下一个元素

检查是否还有下一个元素格式：

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boolean hasNext();

迭代下一个元素并返回它格式（不存在则抛出 NoSuchElementException 异常）：

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E next();

使用：

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System.out.println("Iterator using while loop: ");
Iterator it1 = list.iterator();
while (it1.hasNext()) {
    System.out.println("Next element: " + it1.next());
}

System.out.println("Iterator using for loop: ");
for (Iterator it2 = list.iterator(); it2.hasNext();) {
    System.out.println("Next element: " + it2.next());
}

移除当前元素

格式（不存在抛出 IllegalStateException 异常）：

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void remove();

用法：

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it.remove();

ListIterator

Method

查看是否有下一个元素和返回它

查看是否有下一个元素， 格式：

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boolean hasNext();

迭代到下一个元素并返回它， 格式：

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E next();

使用：

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System.out.println("List iterator (forward iteration): ");
ListIterator lit = list.listIterator();
while (lit.hasNext()) {
    System.out.println("Next element: " + lit.next());
}

查看是否有上一个元素并返回它

查看是否有上一个元素，格式：

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boolean hasPrevious();

迭代到上一个元素并返回它，格式：

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E previous();

使用：

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System.out.println("List iterator (backward iteration): ");
lit = list.listIterator(list.size());
while (lit.hasPrevious()) {
    System.out.println("Previous element: " + lit.previous());
}

给当前位置添加新元素

原来在该位置的元素索引加 1，一直类推到最后一个元素。

格式：

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void add(E e);

使用：

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lit.add("One more element");

获取前一位置的索引并迭代到前一位置

格式：

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int previousIndex();

使用：

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System.out.println("Index of previous element: " + lit.previousIndex());

获取后一位置的索引并迭代到后一位置

格式：

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int nextIndex();

使用：

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System.out.println("Index of next element: " + lit.nextIndex());

移除当前位置上的元素

格式：

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void remove();

使用：

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lit.remove();

替换当前位置上的元素

格式：

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void set(E e);

使用：

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lit.set("Modified element")

使用内置的迭代方法

格式：

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void forEachRemaining(Consumer<? super E> action)

使用：

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lit.forEachRemaining(n -> {
    System.out.println(n);
});