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发表时间：2010-03-01
最后修改：2010-03-02
/**
* 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
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发表时间：2010-06-01
这是java对iterator设定的一种机制！！
list2.remove(i);//wrong |
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