jdk8 Stream流中将集合转成map，重复key处理，统计最大值，获取某个属性集合等10种最常用方法
jdk8 Stream流中将集合转成map，重复key处理，统计最大值，获取某个属性集合等10种最常用方法🔊stream10种常用方法🧣最后的话
🔊stream10种常用方法
//1、list转map，指定key-value，key,value是对象中的某个属性值.
Map<String,String>userMap1=userList.stream().collect(Collectors.toMap(User::getId,User::getName));
System.out.println("1->"+userMap1);
//2、list转map 指定key-value，key为属性值，value是对象本身
Map<String,User>userMap2=userList.stream().collect(Collectors.toMap(User::getId,User->User));
System.out.println("2->"+userMap2);
//3、list转map 指定key-value，value是对象本身，Function.identity()是简洁写法，返回对象本身
Map<String,User>userMap3=userList.stream().collect(Collectors.toMap(User::getId, Function.identity()));
System.out.println("3->"+userMap3);
//4、指定key-value，value是对象本身，Function.identity()是简洁写法，返回对象本身，遇到key 冲突的解决办法，选择第二个key的值value2覆盖第一个key的值value1。
Map<String,User>userMap4=userList.stream().collect(Collectors.toMap(User::getName, Function.identity(),(value1,value2)->value2));
System.out.println("4->"+userMap4);
//5、指定key-value，按照name进行分组 value是对象集合
Map<String, List<User>>userMap5=userList.stream().collect(Collectors.groupingBy(User::getName));
System.out.println("5->"+userMap5);
//6.统计所有age
IntegertotalAge=userList.stream().map(User::getAge).reduce(0, Integer::sum);
System.out.println("6总年龄->"+totalAge);
//7.只对名称为zhangsan的age 进行统计汇总
IntegertotalAgeFilter=userList.stream().filter(t->t.getName().equals("zhangsan")).map(User::getAge).reduce(0, Integer::sum);
System.out.println("7总年龄filter->"+totalAgeFilter);
//8.获取最大age
IntegermaxAge=userList.stream().map(User::getAge).max((x, y) ->x-y).get();
System.out.println("8最大年龄->"+maxAge);
//9.循环打印
userList.stream().forEach(user->System.out.println("9->"+user));
//10.获取名称集合
List<String>nameList=userList.stream().map(User::getName).collect(Collectors.toList());
System.out.println("10名称集合->"+JSON.toJSONString(nameList));
/**
* @author gzh -》javaNice
* 用户类
*/
publicclassUser {
/**
* id
*/
privateStringid;
/**
* 名称
*/
privateStringname;
/**
* 年龄
*/
privateIntegerage;
publicStringgetId() {
returnid;
}
publicvoidsetId(Stringid) {
this.id=id;
}
publicStringgetName() {
returnname;
}
publicvoidsetName(Stringname) {
this.name=name;
}
publicIntegergetAge() {
returnage;
}
publicvoidsetAge(Integerage) {
this.age=age;
}
@Override
publicStringtoString() {
return"User{"+
"id='"+id+'\''+
", name='"+name+'\''+
", age="+age+
'}';
}
publicUser() {
}
publicUser(Stringid, Stringname, Integerage) {
this.id=id;
this.name=name;
this.age=age;
}
}
importcom.alibaba.fastjson.JSON;
importorg.assertj.core.util.Lists;
importjava.util.List;
importjava.util.Map;
importjava.util.function.Function;
importjava.util.stream.Collectors;
/**
* @author gzh -》javaNice
* *
*/
publicclassStreamDemo {
publicstaticvoidmain(String[] args) {
Useruser1=newUser("1","zhangsan",1);
Useruser2=newUser("2","zhangsan",2);
Useruser3=newUser("3","lisi",2);
List<User>userList=Lists.newArrayList();
userList.add(user1);
userList.add(user2);
userList.add(user3);
//1、list转map，指定key-value，key,value是对象中的某个属性值.
Map<String,String>userMap1=userList.stream().collect(Collectors.toMap(User::getId,User::getName));
System.out.println("1->"+userMap1);
//2、list转map 指定key-value，key为属性值，value是对象本身
Map<String,User>userMap2=userList.stream().collect(Collectors.toMap(User::getId,User->User));
System.out.println("2->"+userMap2);
//3、list转map 指定key-value，value是对象本身，Function.identity()是简洁写法，返回对象本身
Map<String,User>userMap3=userList.stream().collect(Collectors.toMap(User::getId, Function.identity()));
System.out.println("3->"+userMap3);
//4、指定key-value，value是对象本身，Function.identity()是简洁写法，返回对象本身，遇到key 冲突的解决办法，选择第二个key的值value2覆盖第一个key的值value1。
Map<String,User>userMap4=userList.stream().collect(Collectors.toMap(User::getName, Function.identity(),(value1,value2)->value2));
System.out.println("4->"+userMap4);
//5、指定key-value，按照name进行分组 value是对象集合
Map<String, List<User>>userMap5=userList.stream().collect(Collectors.groupingBy(User::getName));
System.out.println("5->"+userMap5);
//6.统计所有age
IntegertotalAge=userList.stream().map(User::getAge).reduce(0, Integer::sum);
System.out.println("6总年龄->"+totalAge);
//7.只对名称为zhangsan的age 进行统计汇总
IntegertotalAgeFilter=userList.stream().filter(t->t.getName().equals("zhangsan")).map(User::getAge).reduce(0, Integer::sum);
System.out.println("7总年龄filter->"+totalAgeFilter);
//8.获取最大age
IntegermaxAge=userList.stream().map(User::getAge).max((x, y) ->x-y).get();
System.out.println("8最大年龄->"+maxAge);
//9.循环打印
userList.stream().forEach(user->System.out.println("9->"+user));
//10.获取名称集合
List<String>nameList=userList.stream().map(User::getName).collect(Collectors.toList());
System.out.println("10名称集合->"+JSON.toJSONString(nameList));
}
}
🧣最后的话
🖲要熟练掌握技巧，一定多多练习：纸上得来终觉浅，绝知此事要躬行。
