java.util.Map中的putIfAbsent、computeIfAbsent、computeIfPresent、compute基本使用

本文主要是介绍java.util.Map中的putIfAbsent、computeIfAbsent、computeIfPresent、compute基本使用，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

1、put

插入或覆盖

map.put(K,V);

 

2、putIfAbsent

     以下情况插入新值

     1）key不存在

     2）key存在，但value==null

     插入新value

 

map.putIfAbsent(K,V)//个人理解,相当于
if(!map.containsKey(K)||map.get(K)==null){map.put(K,V)
}

 

3、computeIfAbsent

     以下情况插入新值

     1）key不存在

     2）key存在，但value==null

     插入新value

   与putIfAbsent不同的是，当插入的value==null时，computeIfAbsent是不做插入的，而putIfAbsent做插入。

map.computeIfAbsent(K,v->{return new Object();});//个人理解,相当于
if(!map.containsKey(K)||map.get(K)==null){if(V!=null){map.put(K,V);}else{}
}

 

4、computeIfPresent

    只有key存在才做操作

      1）新value != null，覆盖原值

      2）新value == null，移除对应的key

map.computeIfPresent(K,(k,v)->{return new Object();});//个人理解,相当于
if(map.containsKey(K)){if(V!=null){map.put(K,V);}else{map.remove(K);}
}

 

5、compute

    1）新value != null，有该key的值覆盖，无则插入

    2）新value == null，无该key什么都不做，有则移除

map.putIfAbsent(K,V);//个人理解,相当于if(V!=null){map.put(K,V);
}else{if(map.containsKey(K)){map.remove(K);}else{}
}

 

测试用例：

@Testpublic void test29(){Map<String,String> mm=new HashMap<>();mm.putIfAbsent("key1","val1");mm.putIfAbsent("key11",null);mm.putIfAbsent("key11","val11");mm.putIfAbsent("key11","val12");mm.putIfAbsent("key111",null);mm.putIfAbsent("key112",null);mm.computeIfAbsent("key2",v->{return "val2";});mm.computeIfAbsent("key22",v->{return null;});mm.computeIfAbsent("key22",v->{return "val22";});mm.computeIfAbsent("key22",v->{return "val23";});mm.computeIfAbsent("key222",v->{return null;});mm.computeIfAbsent("key112",v->{return "val112";});mm.computeIfAbsent("key3",v->{return "val3";});mm.computeIfAbsent("key4",v->{return "val4";});mm.computeIfAbsent("key5",v->{return "val5";});mm.computeIfAbsent("key5",v->{return "val555";});mm.computeIfAbsent("key6",v->{return null;});printMap(mm);System.out.println();mm.computeIfPresent("key1",(k,v)->{return "test1";});
//        mm.put("key1",null);mm.computeIfPresent("key100",(k,v)->{return "Test2";});mm.computeIfPresent("key3",(k,v)->{return null;});mm.compute("key4",(k,v)->{return "Test3";});mm.compute("key5",(k,v)->{return null;});mm.compute("key55",(k,v)->{return null;});mm.compute("key555",(k,v)->{return "val555";});printMap(mm);}private void printMap(Map<String,String> mm){Iterator<Map.Entry<String, String>> iterator = mm.entrySet().iterator();while(iterator.hasNext()){Map.Entry<String, String> next = iterator.next();System.out.println("key:"+next.getKey()+"  value:"+next.getValue());}}

输出：

key:key1  value:val1
key:key2  value:val2
key:key5  value:val5
key:key3  value:val3
key:key4  value:val4
key:key22  value:val22
key:key11  value:val11
key:key112  value:val112
key:key111  value:nullkey:key1  value:test1
key:key2  value:val2
key:key4  value:Test3
key:key555  value:val555
key:key22  value:val22
key:key11  value:val11
key:key112  value:val112
key:key111  value:null

 

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