Java8 将一个List转为Map的操作

2023-07-05 04:31:03 260

将List转为Map

publicclassAnswerApp{
publicstaticvoidmain(String[]args)throwsException{
Listnames=Lists.newArrayList("Answer","AnswerAIL","AI");
Mapmap=names.stream().collect(Collectors.toMap(v->v,v->1));
System.out.println(map);
}
}

程序运行输出

{Answer=1,AnswerAIL=1,AI=1}

将List转为Map

publicstaticvoidmain(String[]args)throwsException{
Listusers=newArrayList<>();
for(inti=0;i<3;i++){
users.add(newUser("answer"+i,newRandom().nextInt(100)));
}
System.out.println(JSON.toJSONString(users));
System.out.println();
Mapmap=users.stream().collect(Collectors.toMap(User::getName,User::getAge));
System.out.println(map);
}

程序运行输出

[{"age":78,"name":"answer0"},{"age":89,"name":"answer1"},{"age":72,"name":"answer2"}]
{answer2=72,answer1=89,answer0=78}

将List转为Map

实现方式1

publicclassAnswerApp{
publicstaticvoidmain(String[]args)throwsException{
Listusers=newArrayList<>();
for(inti=0;i<3;i++){
//改为此代码,转map时会报错DuplicatekeyUser
//users.add(newUser("answer",newRandom().nextInt(100)));
users.add(newUser("answer"+i,newRandom().nextInt(100)));
}
System.out.println(JSON.toJSONString(users));
System.out.println();
Mapmap=users.stream().collect(Collectors.toMap(User::getName,Function.identity()));
System.out.println(JSON.toJSONString(map));
}
}

该方式如果map的key(如上述例子的User::getName的值)重复，会抛错java.lang.IllegalStateException:DuplicatekeyUser

程序运行输出

[{"age":22,"name":"answer0"},{"age":79,"name":"answer1"},{"age":81,"name":"answer2"}]
{"answer2":{"age":81,"name":"answer2"},"answer1":{"age":79,"name":"answer1"},"answer0":{"age":22,"name":"answer0"}}

实现方式2

publicclassAnswerApp{
publicstaticvoidmain(String[]args)throwsException{
Listusers=newArrayList<>();
for(inti=0;i<3;i++){
users.add(newUser("answer",newRandom().nextInt(100)));
}
System.out.println(JSON.toJSONString(users));
System.out.println();

//如果key重复,则根据冲突方法·(key1,key2)->key2·判断.解释:key1key2冲突时取key2
Mapmap=users.stream().collect(Collectors.toMap(User::getName,Function.identity(),(key1,key2)->key2));
System.out.println(JSON.toJSONString(map));
}
}

程序运行输出

[{"age":24,"name":"answer"},{"age":89,"name":"answer"},{"age":68,"name":"answer"}]
{"answer":{"age":68,"name":"answer"}}

如果改为(key1,key2)->key1则输出{"answer":{"age":24,"name":"answer"}}

User实体

@Data
@NoArgsConstructor
@AllArgsConstructor
publicclassUser{
privateLongid;
privateStringname;
privateIntegerage;
publicUser(Stringname){
this.name=name;
}
publicUser(Stringname,Integerage){
this.name=name;
this.age=age;
}
}

补充：java8中使用Lambda表达式将list中实体类的两个字段转Map

代码：

Listlist=newArrayList<>();
Mapmap=list.stream().collect(Collectors.toMap(Entity::getId,Entity::getType));

常用的lambda表达式：

**
*List->Map
*需要注意的是：
*toMap如果集合对象有重复的key，会报错Duplicatekey....
*apple1,apple12的id都为1。
*可以用(k1,k2)->k1来设置，如果有重复的key,则保留key1,舍弃key2
*/
MapappleMap=appleList.stream().collect(Collectors.toMap(Apple::getId,a->a,(k1,k2)->k1));

安照某一字段去重
list=list.stream().filter(distinctByKey(p->((ModCreditColumn)p).getFieldCode())).collect(Collectors.toList());

ListunitNetValue=listIncreaseDto.stream().map(IncreaseDto::getUnitNetValue).collect(Collectors.toList());

//求和对象List
BigDecimalallFullMarketPrice=entityList.stream().filter(value->value.getFullMarketPrice()!=null).map(SceneAnalysisRespVo::getFullMarketPrice).reduce(BigDecimal.ZERO,BigDecimal::add);

ListnaturalDayList;
BigDecimaltotal=naturalDayList.stream().reduce(BigDecimal.ZERO,BigDecimal::add);

分组函数
Map>groupMap=total.getGroupList().stream().collect(Collectors.groupingBy(SceneAnalysisRespVo::getVmName));

//DV01之和
BigDecimalallDV01=values.stream().filter(sceneAnalysisRespVo->sceneAnalysisRespVo.getDv()!=null).map(SceneAnalysisRespVo::getDv).reduce(BigDecimal.ZERO,BigDecimal::add);

以上为个人经验，希望能给大家一个参考，也希望大家多多支持毛票票。如有错误或未考虑完全的地方，望不吝赐教。

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Java8 将一个List转为Map的操作

将List转为Map

将List转为Map

将List转为Map

常用的lambda表达式：

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