hashMap遍历java.util.ConcurrentModificationException

我不是女神ヾ 2024-03-26 12:45 62阅读 0赞

#### **一、Map遍历4种方式：** ####

**1、迭代器**

// 通过iterator遍历所有的value,但是不能遍历key
    Iterator<Map.Entry<String, String>> iterator = map.entrySet().iterator();
    while (iterator.hasNext()){
    	Map.Entry<String, String> next = iterator.next();
    	System.out.println("key="+next.getKey()+"value="+next.getValue());
    }

**2、keyset()迭代**

// 通过map.keyset进行遍历key和value
    for (String key:map.keySet()){
    	System.out.println("key=  "+key+"   and value=  "+map.get(key));
    }

**3、entrySet()迭代**

// Map.entrySet
    Set<Map.Entry<String, String>> entries = map.entrySet();
    for (Map.Entry<String, String>entry:entries){
    	String value = entry.getValue();
    	String key = entry.getKey();
    	System.out.println("key="+key+"value="+value);
    }

**4、java8**

// forEach
    map.forEach((key,value) -> {
        
    });

#### **二、替换已经存在键的值** ####

Map<String,String> map = new HashMap<String,String>();
    map.put("name","ccshen");
    map.put("age","30");
    map.forEach((key,value) ->{
        if("age".equeals(key)){
            map.put(key,"29");
        } 
    });

**报错：由于HashMap不安全**

Exception in thread "main" java.util.ConcurrentModificationException
        at java.util.HashMap$HashIterator.nextEntry(HashMap.java:894)
        at java.util.HashMap$EntryIterator.next(HashMap.java:934)
        at java.util.HashMap$EntryIterator.next(HashMap.java:932)
        at cn.java.test.map.MapTest.main(MapTest.java:22)

#### 三、解决方法 ####

**1、使用新创建map存值**

Map<String,String> map = new HashMap<String,String>();
    Map<String,String> newMap = new HashMap<String,String>();
    map.put("name","ccshen");
    map.put("age","30");
    map.forEach((key,value) ->{
        if("age".equeals(key)){
            newMap.put(key,"29");
        }else{
            newMap.put(key,value);
        }
    });

**2、使用线程安全的ConcurrentHashMap**

Map<String,String> map = new ConcurrentHashMap<String,String>();
    map.put("name","ccshen");
    map.put("age","30");
    for(Map.Entry<String,String> m : map.entrySet()){
    	if(m.getKey().equals("age")){
    		map.put(m.getKey(),"29");
    	}
    }