单线程情况下遍历ArrayList的ConcurrentModificationException的问题

    看到一段程序在单线程方式下运行还抛出ConcurrentModificationException，大概看了下是在以for-each方式遍历ArrayList时，循环内部又对集合作修改引起，换种方式遍历即可解决。
   所以在使用JDK集合类的Iterator时，还要对其实现原理有大概了解。由于一直都没有了解集合遍历，标记下><
code:

@Test
	public void testConcurrentModificationException() throws Exception {
		try {
			List<String> arr = new ArrayList<String>();
			arr.add("1");arr.add("2");arr.add("3");
			
			/* 不会抛出ConcurrentModificationException,
			 * 可在单线程情况下使用此遍历方式
			 */
			for(int i=0; i<arr.size(); i++) {
				arr.add("4");
				if(i==1)
					break;
			}
			System.out.println(arr);
			
			/* 抛出ConcurrentModificationException，
			 * 因为for-each内部以Iterator方遍历集合, 
			 * 由ArrayList创建的Iterator是以快速失败的方式处理遍历过程中的并发修改问题 
			 * @see: java.util.AbstractList.Itr.next()
			 */
			for(String i : arr) {
				arr.add("4");
				if(i.equals("2"))
					break;
			}
			System.out.println(arr);
		} catch(Exception e) {
			e.printStackTrace();
			Assert.fail();
		}
	}

运行结果

[1, 2, 3, 4, 4]
java.util.ConcurrentModificationException
	at java.util.AbstractList$Itr.checkForComodification(AbstractList.java:372)
	at java.util.AbstractList$Itr.next(AbstractList.java:343)
	at com.ys.kj.service.impl.UpgradeServiceTest.testConcurrentModificationException(UpgradeServiceTest.java:57)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:45)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:42)
	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:20)
	at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:28)
	at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:30)
	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:263)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:68)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:47)
	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:231)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:60)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:229)
	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:50)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:222)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:300)
	at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
	at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)