标签：iterator遍历   arraylist遍历   linkedlist遍历   get和next的比较   java   

经常使用ArrayList遍历，尝试总结一下for配合get()的遍历和Iterator配合next()遍历的区别，进入Java的JDK源码中进行深度剖析一下

这里参考一下http://bbs.csdn.net/topics/250025827论坛的测试程序：

import java.util.Iterator;
import java.util.List;
import java.util.ArrayList;
import java.util.LinkedList;

/**
 * IteratorTest
 * 
 * @author SageZk
 */
public class IteratorTest {

	public static long testForloops(List<String> list) {
		long start = 0L;
		long end = 0L;
		start = System.nanoTime();
		
		for (int i = list.size() - 1; i >= 0; --i) {
			list.get(i);
		}
		end = System.nanoTime();
		return end - start;
	}

	public static long testIterator(List<String> list) {
		long start = 0L, end = 0L;
		start = System.nanoTime();
		Iterator<String> it = list.iterator();
		while (it.hasNext()) {
			it.next();
		}
		end = System.nanoTime();
		return end - start;
	}

	public static void main(String[] args) {
		// 测试列表长度
		final int LEN = 10000;

		// 初始化测试用数据
		List<String> arraylist = new ArrayList<String>();
		List<String> linkedlist = new LinkedList<String>();
		for (int i = 0; i < LEN; ++i) {
			String s = Integer.toString(i, 2);
			arraylist.add(s);
			linkedlist.add(s);
		}

		// 打印测试结果
		final String FORMAT = "%1$-16s%2$-16s%3$16d\n";
		System.out.println("List\t\tType\t\tTime(nanoseconds)");
		System.out.println("-------------------------------------------------");
		System.out.printf(FORMAT, "ArrayList", "for", testForloops(arraylist));
		System.out.printf(FORMAT, "ArrayList", "Iterator",
				testIterator(arraylist));
		System.out
				.printf(FORMAT, "LinkedList", "for", testForloops(linkedlist));
		System.out.printf(FORMAT, "LinkedList", "Iterator",
				testIterator(linkedlist));
	}

}

ListTypeTime(nanoseconds)
-------------------------------------------------
ArrayList       for                       607064
ArrayList       Iterator                  809922
LinkedList      for                    154326455
LinkedList      Iterator                  958140

根据这里的区别，我跟踪了一下源码。

ArrayList的Iterator和for-get两种遍历

next

public E next() {
    checkForComodification();
    int i = cursor;
    if (i >= size)
	throw new NoSuchElementException();
    Object[] elementData = ArrayList.this.elementData;
    if (i >= elementData.length)
	throw new ConcurrentModificationException();
    cursor = i + 1;
    return (E) elementData[lastRet = i];
}

get

private transient Object[] elementData;

public E get(int index) {
	rangeCheck(index);
	return elementData(index);
}

E elementData(int index) {
	return (E) elementData[index];
}

LinkedList的Iterator和for-get两种遍历

next

public E next() {
    checkForComodification();
    if (!hasNext())
	throw new NoSuchElementException();

    lastReturned = next;
    next = next.next;
    nextIndex++;
    return lastReturned.item;
}

private Node<E> next;

private static class Node<E> {
	E item;
	Node<E> next;
	Node<E> prev;

	Node(Node<E> prev, E element, Node<E> next) {
	    this.item = element;
	    this.next = next;
	    this.prev = prev;
	}
}

get

public E get(int index) {
	checkElementIndex(index);
	return node(index).item;
}

总结一下

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源码剖析Iterator接口遍历和直接for-get组合遍历的区别

标签：iterator遍历   arraylist遍历   linkedlist遍历   get和next的比较   java   

原文地址：http://blog.csdn.net/dreamintheworld/article/details/39203917