for iterator ArrayList遍历效率 测试

 

/**
 * 测试 对于ArrayList的iterator遍历和for遍历的效率问题
 * 注： 这个测试结果对其他容器类不一定成立 比如LinkedList
 *  
 */
public class IteratorTest {

    static int size = 1000000;
    static List<Object> list1 = new ArrayList<Object>();
    static List<Object> list2 = new ArrayList<Object>();
    static {
        for (int i = 0; i < size; i++) {
            list1.add(new Object());
            list2.add(new Object());
        }
    }

    @Test
    public void iteratorTest() {
        long start = System.currentTimeMillis();
        for (Iterator<Object> iterator = list1.iterator(); iterator.hasNext();) {
            Object entry = (Object) iterator.next();
            // System.out.print(entry);
        }
        System.out.println("\niterator " + (System.currentTimeMillis() - start) + " ms");
    }

    @Test(dependsOnMethods = "iteratorTest")
    public void forTest() {
        long start = System.currentTimeMillis();
        for (int i = 0, size = list2.size(); i < size; i++) {//这里是一次性求出size() ，假如在遍历的过程中list的内容有增减的话，遍历逻辑会出错
            Object entry = list2.get(i);
            // System.out.print(entry);
        }
        System.out.println("\nfor " + (System.currentTimeMillis() - start) + " ms");
    }

    @Test(groups="alone",invocationCount=10)    //testng 测试10次
    public void compareTest() {
        long start = System.currentTimeMillis();
        for (Iterator<Object> iterator = list1.iterator(); iterator.hasNext();) {
            Object entry = (Object) iterator.next();
            // System.out.print(entry);
        }
        long time=System.currentTimeMillis()-start;
        start = System.currentTimeMillis();
        for (int i = 0, size = list2.size(); i < size; i++) {
            Object entry = list2.get(i);
            // System.out.print(entry);
        }
        long time2=System.currentTimeMillis() - start;
        System.out.println("iterator "+time +" ms, for "+time2+" ms");
    }
}

 

输出

iterator 32 ms

for 15 ms
iterator 32 ms, for 15 ms
iterator 31 ms, for 0 ms
iterator 31 ms, for 0 ms
iterator 32 ms, for 15 ms
iterator 31 ms, for 16 ms
iterator 31 ms, for 16 ms
iterator 31 ms, for 0 ms
iterator 31 ms, for 0 ms
iterator 31 ms, for 16 ms
iterator 31 ms, for 16 ms

 

 

遍历中的remove测试

 @Test
    public void iteratorTest() {
        long start = System.currentTimeMillis();
        for (Iterator<Object> iterator = list1.iterator(); iterator.hasNext();) {
            Object entry = (Object) iterator.next();
            System.out.println(entry);
            iterator.remove();//right
        }
        System.out.println("\niterator " + (System.currentTimeMillis() - start) + " ms");
    }

    @Test(expectedExceptions = IndexOutOfBoundsException.class)
    public void forTest() {
        long start = System.currentTimeMillis();
        for (int i = 0, size = list2.size(); i < size; i++) {
            Object entry = list2.get(i);
            System.out.println(entry);
            list2.remove(i);//wrong
        }
        System.out.println("\nfor " + (System.currentTimeMillis() - start) + " ms");
    }

 

输出

0
1
2
3
4
5
6
7
8
9

iterator 0 ms
0
2
4
6
8

 

 

 

public static void main(String[] args) {
		List a =new ArrayList();
		for (int i = 0; i < 10; i++) {
			a.add(i);
			System.out.println(i);
		}
		System.out.println("-------------");
		int j =0;
		for (Iterator iterator = a.iterator(); iterator.hasNext();) {
			Object object = (Object) iterator.next();
			if(j%2==0){
				iterator.remove();
			}
			j++;
		}
		for (Object object : a) {
			System.out.println(object);
		}
	}



0
1
2
3
4
5
6
7
8
9
-------------
1
3
5
7
9

 

 

 

 

 

 

 

 

 

 

评论

1 楼 lz12366 2010-06-01  

这是java对iterator设定的一种机制！！

list2.remove(i);//wrong