Java ListIterator – Java中的列表迭代器

1 年 ago

清, 宇

5 minutes

我们知道Java有四个游标：Enumeration，Iterator，ListIterator和Spliterator。我们已经在之前的帖子中讨论了Enumeration和Iterator游标。在阅读本帖子之前，请先阅读我的上一篇帖子：Java Iterator。在本文中，我们将讨论第三个Java游标：ListIterator。

Java的ListIterator

和Iterator一样，ListIterator是Java中的一个迭代器，用于逐个访问实现了List接口的对象中的元素。

It is available since Java 1.2.
It extends Iterator interface.
It is useful only for List implemented classes.
Unlike Iterator, It supports all four operations: CRUD (CREATE, READ, UPDATE and DELETE).
Unlike Iterator, It supports both Forward Direction and Backward Direction iterations.
It is a Bi-directional Iterator.
It has no current element; its cursor position always lies between the element that would be returned by a call to previous() and the element that would be returned by a call to next().

请解释集合API中的CRUD操作是什么？

CREATE: Adding new elements to Collection object.
READ: Retrieving elements from Collection object.
UPDATE: Updating or setting existing elements in Collection object.
DELETE: Removing elements from Collection object.

Java ListIterator 类的类图

Java ListIterator 方法

Java ListIterator 接口具有以下方法。

void add(E e): Inserts the specified element into the list.
boolean hasNext(): Returns true if this list iterator has more elements when traversing the list in the forward direction.
boolean hasPrevious(): Returns true if this list iterator has more elements when traversing the list in the reverse direction.
E next(): Returns the next element in the list and advances the cursor position.
int nextIndex(): Returns the index of the element that would be returned by a subsequent call to next().
E previous(): Returns the previous element in the list and moves the cursor position backwards.
int previousIndex(): Returns the index of the element that would be returned by a subsequent call to previous().
void remove(): Removes from the list the last element that was returned by next() or previous().
void set(E e): Replaces the last element returned by next() or previous() with the specified element.

在接下来的部分中，我们将一一探讨这些方法，并举出有用的例子。

Java ListIterator 基本示例

在本节中，我们将讨论一些带有示例的ListIterator方法。首先，我们需要了解如何获取此迭代器对象。怎么获取ListIterator呢？

ListIterator<E> listIterator()

这将返回一个列表迭代器，用于遍历此列表中的各个元素。例如：

import java.util.*;

public class ListIteratorDemo 
{
  public static void main(String[] args) 
  {
	List<String&gt names = new LinkedList<>();
	names.add("Rams");
	names.add("Posa");
	names.add("Chinni");
		
	// Getting ListIterator
	ListIterator<String&gt namesIterator = names.listIterator();
	
	// Traversing elements
	while(namesIterator.hasNext()){
	   System.out.println(namesIterator.next());			
	}	

	// Enhanced for loop creates Internal Iterator here.
	for(String name: names){
	   System.out.println(name);			
	}	
  }
}

产出：

Rams
Posa
Chinni

双向迭代示例：ListIterator

在本节中，我们将探索ListIterator的方法如何用于执行正向迭代和反向迭代。

import java.util.*;

public class BiDirectinalListIteratorDemo 
{
	public static void main(String[] args) 
	{
		List<String&gt names = new LinkedListt<>();
		names.add("Rams");
		names.add("Posa");
		names.add("Chinni");
		
		// Getting ListIterator
		ListIterator<String&gt listIterator = names.listIterator();
		
		// Traversing elements
		System.out.println("Forward Direction Iteration:");
		while(listIterator.hasNext()){
			System.out.println(listIterator.next());			
		}	
		
		// Traversing elements, the iterator is at the end at this point
		System.out.println("Backward Direction Iteration:");
		while(listIterator.hasPrevious()){
			System.out.println(listIterator.previous());			
		}
	}
}

输出：

Forward Direction Iteration:
Rams
Posa
Chinni
Backward Direction Iteration:
Chinni
Posa
Rams

Java 迭代器的类型

正如我们所知，Java有四个游标：Enumeration、Iterator、ListIterator和Spliterator。我们可以将它们分为两种主要类型，如下所示：

Uni-Directional Iterators
They are Cursors which supports only Forward Direction iterations. For instance, Enumeration, Iterator, etc. are Uni-Directional Iterators.- Bi-Directional Iterators
They are Cursors which supports Both Forward Direction and Backward Direction iterations. For instance, ListIterator is Bi-Directional Iterator.

Java ListIterator的内部工作原理是什么？

正如我们所知，Java的ListIterator既可以在正向方向上工作，也可以在反向方向上工作。它是一个双向迭代器。为了支持这个功能，它有两组方法。

Forward Direction Iteration methods
We need to use the following methods to support Forward Direction Iteration:

Backward Direction Iteration methods
We need to use the following methods to support Backward Direction Iteration:

在我之前的帖子中，我们已经讨论过如何在向前的方向上内部运作的迭代器，即“Java迭代器的内部工作原理”部分。即使ListIterator也是以相同的方式工作。如果您想查看我之前的帖子，请点击这里：Java迭代器。在这一部分，我们将讨论ListIterator在向后方向上的工作原理。让我们拿以下的LinkedList对象来理解这个功能。

List<String> names = new LinkedList<>();
names.add("E-1");
names.add("E-2");
names.add("E-3");
.
.
.
names.add("E-n");

现在在LinkedList上创建一个ListIterator对象，如下所示：

ListIterator<String> namesIterator = names.listLterator();

namesIterator.hasNext();
namesIterator.next();

namesIterator.hasPrevious();
namesIterator.previous();

namesIterator.hasPrevious();

ListIterator的优势

与迭代器不同，列表迭代器具有以下优点：

Like Iterator, it supports READ and DELETE operations.
It supports CREATE and UPDATE operations too.
That means, it supports CRUD operations: CREATE, READ, UPDATE and DELETE operations.
It supports both Forward direction and Backward direction iteration. That means it’s a Bi-Directional Java Cursor.
Method names are simple and easy to use them.

ListIterator的限制

与迭代器相比，Java的ListIterator具有许多优点。然而，它仍然有以下一些限制。

It is an Iterator only List implementation classes.
Unlike Iterator, it is not applicable for whole Collection API.
It is not a Universal Java Cursor.
Compare to Spliterator, it does NOT support Parallel iteration of elements.
Compare to Spliterator, it does NOT support better performance to iterate large volume of data.

迭代器和列表迭代器之间的相似之处

在这一部分中，我们将讨论Java两个游标之间的相似之处：迭代器和列表迭代器。

Bother are introduced in Java 1.2.
Both are Iterators used to iterate Collection or List elements.
Both supports READ and DELETE operations.
Both supports Forward Direction iteration.
Both are not Legacy interfaces.

迭代器和列表迭代器之间的区别

在这一部分中，我们将讨论Java两种迭代器：Iterator和ListIterator之间的差异。

Iterator	ListIterator
Introduced in Java 1.2.	Introduced in Java 1.2.
It is an Iterator for whole Collection API.	It is an Iterator for only List implemented classes.
It is an Universal Iterator.	It is NOT an Universal Iterator.
It supports only Forward Direction Iteration.	It supports both Forward and Backward Direction iterations.
It’s a Uni-Directional Iterator.	It’s a Bi-Directional Iterator.
It supports only READ and DELETE operations.	It supports all CRUD operations.
We can get Iterator by using iterator() method.	We can ListIterator object using listIterator() method.

这就是关于Java中ListIterator的全部内容。希望这些Java ListIterator的理论和例子能帮助你开始ListIterator编程。参考文献：ListIterator API文档。