关于Map表循环过程删除操作出现的java.util.ConcurrentModificationException 异常

使用Iterate

	List list = new ArrayList();
	list.add("a");list.add("b");list.add("c");
	
	//for结构 自动生成
	for (Iterator iterator = list.iterator(); iterator.hasNext();) {
		String s = (String) iterator.next();
		if(s.equals("b")){
			iterator.remove();
		}
	}
	
	System.out.println(list.size());

以下代码会抛出异常 

package tristan;

import java.util.HashMap;
import java.util.Map;

public class Test {

	public static void main(String[] args) {
		Map<String,String> map = new HashMap<String, String>();
		map.put("1", "a1");
		map.put("2", "a2");
		map.put("3", "a3");
		map.put("4", "a4");
		map.put("5", "a5");
		
		for(String key : map.keySet()){
			if("a2".equals(map.get(key))){
				map.remove(key);
			}
		}
		System.out.println(map.size());
	}
}

应该深拷贝一个tmpMap，在迭代的过程中，对tmpMap进行remove操作
之后将map 赋值为 tmpMap.

package tristan;

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.util.HashMap;
import java.util.Map;

public class Test {

	public static void main(String[] args) throws Exception {
		Map<String,String> map = new HashMap<String, String>();
		map.put("1", "a1");
		map.put("2", "a2");
		map.put("3", "a3");
		map.put("4", "a4");
		map.put("5", "a5");
		
		Map<String,String> tmpMap = (Map<String, String>) clone(map);
		
		for(String key : map.keySet()){
			if("a2".equals(map.get(key))){
				tmpMap.remove(key);
			}
		}
		
		map = tmpMap;
		
		System.out.println(map.size());
	}
	
	public static Object clone(Object obj) throws Exception {
		ByteArrayOutputStream byteOut = new ByteArrayOutputStream();
		ObjectOutputStream outStream = new ObjectOutputStream(byteOut);
		outStream.writeObject(obj);
		ByteArrayInputStream byteIn = new ByteArrayInputStream(byteOut.toByteArray());
		ObjectInputStream inStream = new ObjectInputStream(byteIn);
		return inStream.readObject();
	}
}

但是这种写法可能有未知的问题， 就像四国中 灭掉一家时出现的问题一样。
最好是用下列方法

package tristan;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class Test {

	public static void main(String[] args) throws Exception {
		Map<String,String> map = new HashMap<String, String>();
		map.put("1", "a1");
		map.put("2", "a2");
		map.put("3", "a3");
		map.put("4", "a4");
		map.put("5", "a5");
		
		List<String> delList = new ArrayList<String>();
		
		for(String key : map.keySet()){
			if("a2".equals(map.get(key))){
				delList.add(key);
			}
		}
		
		for(String key : delList){
			map.remove(key);
		}
		
		System.out.println(map.size());
	}
	
}

谢谢latent同学的建议，最好还是采用Iterate来做，List Set也适用

import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;

public class Test {
	public static void main(String[] args) throws Exception {
		testMap();
	}

	public static void testSet() {
		Set<String> set = new HashSet<String>();
		set.add("a1");
		set.add("a2");
		set.add("a3");
		set.add("a4");
		set.add("a5");

		Iterator<String> it = set.iterator();
		while (it.hasNext()) {
			String v = it.next();
			if ("a2".equals(v)) {
				it.remove();
			}
		}

		for (String v : set) {
			System.out.println(v);
		}
	}

	public static void testList() {
		List<String> list = new ArrayList<String>();
		list.add("a1");
		list.add("a2");
		list.add("a3");
		list.add("a4");
		list.add("a5");

		Iterator<String> it = list.iterator();
		while (it.hasNext()) {
			String v = it.next();
			if ("a2".equals(v)) {
				it.remove();
			}
		}

		for (String v : list) {
			System.out.println(v);
		}
	}

	public static void testMap() {
		Map<String, String> map = new HashMap<String, String>();
		map.put("1", "a1");
		map.put("2", "a2");
		map.put("3", "a3");
		map.put("4", "a4");
		map.put("5", "a5");

/*		Set<Entry<String, String>> set = map.entrySet();  
        Iterator<Entry<String, String>> it = set.iterator();  
        while (it.hasNext()) {  
            String v = it.next().getValue();  
            if (v.equals("a2")) {  
                it.remove();  
            }  
        } */ 
		
		Set<String> set = map.keySet();
		Iterator<String> it = set.iterator();
		while (it.hasNext()) {
			String k = it.next();
			if (map.get(k).equals("a2")) {
				it.remove();
			}
		}

		for (Map.Entry<String, String> e : map.entrySet()) {
			System.out.println(e.getValue());
		}

	}
}