工作中常用lambda表达式-增加代码的逼格

• List 转 String 字符串用逗号分割

普通方式
List<Long> useridList = authoritativeVO.getUserList();
int count = 0;
StringBuilder targetStr = new StringBuilder();
for(int i=0;i<useridList.size();i++){
	targetStr.append(useridList.get(i));
	count++;
	if(count<useridList.size()){
		targetStr.append(",");
	}
}
System.out.println(targetStr.toString());  

lambda方式
String deptIds = authoritativeVO.getUseridList().stream().map(i -> String.valueOf(i)).collect(Collectors.joining(","));

• 查询18岁学生的id列表

##1.查询18岁的学生列表
List<Student> studentList = studentService.list(Wrappers.<Student>lambdaQuery().eq(Student::getAge,18)；

##2.普通方式获取18岁学生列表的id
List<Long> idList = new ArrayList<>()；
for(Student stu : studentList){
  idList.add(stu.getId());
}

##2.lambda获取18岁学生列表的id
List<Long> ids = studentList.stream().map(Student::getId).collect(Collectors.toList());

• List 转Map

//list 转map
普通方式
Map<String,Person> map = new HashMap<>();
for (Person person:list) {
    map.put(person.getName(),person);
}

lambda方式
Map<String,Person> map1 = list.stream().collect(Collectors.toMap(Person::getName, t->t));
Map<String,Address> map2 = list.stream().collect(Collectors.toMap(Person::getName, t->t.getAddress()));
Map<String,Integer> map4 = list.stream().collect(Collectors.toMap(Person::getName, t->t.getAge()));
Map<String,Integer> map3 = list.stream().collect(Collectors.toMap(Person::getName, Person::getAge));

• List 分组

普通方式
Map<Integer,List<Person>> mapt = new HashMap<>();
for (Person person:list){
    if(mapt.containsKey(person.getAge())){
        List<Person> people = mapt.get(person.getAge());
        people.add(person);
    }else{
        List<Person> people = new ArrayList<>();
        people.add(person);
        mapt.put(person.getAge(),people);
    }
}

lambda方式
Map<Integer,List<Person>> listMap = list.stream().collect(Collectors.groupingBy(Person::getAge));

• Map 转 List

List collect = listMap.values().stream().collect(Collectors.toList());

• 过滤

List<String> stringList = list.stream().map(s -> s.getName()).filter(s -> !s.isEmpty()).collect(Collectors.toList());
List<Person> personList = list.stream().filter(s -> !s.getName().isEmpty()).collect(Collectors.toList());

• 所使用的JAVA测试类如下

@Data
@Accessors(chain = true)
@AllArgsConstructor
public class Address {
    private String province; //省
    private String city; //市
    private String county; //县
}

@Data
@Accessors(chain = true)
@AllArgsConstructor
public class Person {
    private String name;
    private int age;
    private Address address;
}