ArrayList中删除null元素效率比较

package test;

import java.util.ArrayList;

import java.util.Collections;

import java.util.Iterator;

import java.util.LinkedList;

import java.util.List;

public class TestList {

    static final int size = 1000000;

    public static void main(String[] args) {

        List<Integer> linkedList = new LinkedList<Integer>();

        List<Integer> arrayList = new ArrayList<Integer>();

        long startTime;

        long endTime;

        Iterator<Integer> iterator;

        //添加数据

        for (int i = 0; i < size; i++) {

            int value = (int) (Math.random() * 1000);

            if (value == 0) {

                linkedList.add(null);

            } else {

                linkedList.add(value);

            }

        }

        arrayList.addAll(linkedList);

        //采用倒叙方法移除空元素

        arrayList.clear();

        arrayList.addAll(linkedList);

        startTime = System.currentTimeMillis();

        for (int i = arrayList.size() - 1; i >= 0; i--) {

            if (arrayList.get(i) == null) {

                arrayList.remove(i);

            }

        }

        endTime = System.currentTimeMillis();

        System.out.println("倒叙方法移除ArrayList中null 耗时" + (endTime - startTime) + "ms, 剩余数据" + arrayList.size() + "个");

        arrayList.clear();

        arrayList.addAll(linkedList);

        int len = arrayList.size();

        startTime = System.currentTimeMillis();

        for (int i = 0; i < len; i++) {

            if (arrayList.get(i) == null) {

                arrayList.remove(i);

                i--;

                len--;

            }

        }

        endTime = System.currentTimeMillis();

        System.out.println("正序方法移除ArrayList中null 耗时" + (endTime - startTime) + "ms, 剩余数据" + arrayList.size() + "个");

        arrayList.clear();

        arrayList.addAll(linkedList);

        startTime = System.currentTimeMillis();

        arrayList.removeAll(Collections.singleton(null));

        endTime = System.currentTimeMillis();

        System.out.println("Collections方法移除ArrayList中null 耗时" + (endTime - startTime) + "ms, 剩余数据" + arrayList.size() + "个");

        arrayList.clear();

        arrayList.addAll(linkedList);

        startTime = System.currentTimeMillis();

        iterator = arrayList.iterator();

        while (iterator.hasNext()) {

            Integer valueInteger = iterator.next();

            if (valueInteger == null) {

                iterator.remove();

            }

        }

        endTime = System.currentTimeMillis();

        System.out.println("迭代器方法移除ArrayList中null 耗时" + (endTime - startTime) + "ms, 剩余数据" + arrayList.size() + "个");

        startTime = System.currentTimeMillis();

        iterator = linkedList.iterator();

        while (iterator.hasNext()) {

            Integer valueInteger = iterator.next();

            if (valueInteger == null) {

                iterator.remove();

            }

        }

        endTime = System.currentTimeMillis();

        System.out.println("迭代器方法移除LinkedList中null 耗时" + (endTime - startTime) + "ms, 剩余数据" + arrayList.size() + "个");

    }

}

结果（跟List中null元素个数有关）：

倒叙方法移除ArrayList中null 耗时126ms, 剩余数据998946个

正序方法移除ArrayList中null 耗时122ms, 剩余数据998946个

Collections方法移除ArrayList中null 耗时13ms, 剩余数据998946个

迭代器方法移除ArrayList中null 耗时131ms, 剩余数据998946个

迭代器方法移除LinkedList中null 耗时12ms, 剩余数据998946个