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