Java8 实现stream将对象集合list中抽取属性集合转化为map或list

首先新建一个实体类Person

@Data
public class Person {
 /** 编码 */
 private String code;
 /** 名字 */
 private String name;
 public Person(String code, String name) {
  this.code = code;
  this.name = name;
 }
}

实例化三个对象放入list集合中

public static void main(String[] args) {
 Person person1 = new Person("001", "张三");
 Person person2 = new Person("002", "李四");
 Person person3 = new Person("002", "王五");
 List<Person> personList = new ArrayList<>();
 personList.add(person1);
 personList.add(person2);
 personList.add(person3);
 personList.forEach(t -> System.out.println(t.toString()));
}

输出结果为：

Person(code=001, name=张三)

Person(code=002, name=李四)

Person(code=002, name=王五)

1.抽取对象的code作为key，name作为value转化为map集合

方法为

private static HashMap<String, String> listToMap(List<Person> personList) {
 return (HashMap<String, String>)personList.stream()
   .filter(t -> t.getName()!=null)
   .collect(Collectors.toMap(Person::getCode,Person::getName,(k1,k2)->k2));
}

filter() 方法作用是过滤掉名字为空的对象，当对象的名字为null时，会出现NPE空指针异常

(k1,k2)->k2 意思是遇到相同的key时取第二个值

(k1,k2)->k1 意思是遇到相同的key时取第一个值

调用这个方法

HashMap<String,String> personMap = listToMap(personList);
personMap.forEach((k,v)-> System.out.println(k.toString() + " - " + v.toString()));

输出结果为：

001 - 张三

002 - 王五

2.抽取对象的name得到name的list集合

方法为

private static List<String> getNameList(List<Person> personList) {
 return personList.stream().map(Person::getName).collect(Collectors.toList());
}

调用这个方法

List<String> nameList = getNameList(personList);
nameList.forEach(t -> System.out.println(t.toString()));

输出结果为：

张三

李四

王五

补充：java8 使用stream将List转成Map,或者从List对象中获取单个属性List,List中根据某个字段排序

1.学生类

import lombok.Data;
@Data
public class Student{
 private String stuId;
 private String name;
 private String age;
 private String sex;
}

2.测试类

public class Test {
 public static void main(String[] args) {

  // 创建学生List
  List<Student> list = createStudentList();

  // 1.获取value为Student对象，key为学生ID的Map
  getStudentObjectMap(list);

  // 2.获取value为学生姓名，key为学生ID的Map
  getStudentNameMap(list);

  // 3.获取学生姓名List
  getStudentNameList(list);

  //4.List中删除学生id = 1的对象

  list.removeIf(student -> student.getStuId().equals(1));

  //5.如果StudentId为Long类型如何转？

  Map<String, String> mapStr = list.stream().collect(Collectors.toMap(student -> student.getStuId().toString(), student -> JSON.toJSONString(student)));

  //6.根据List中Student的学生姓名排序
  Collections.sort(list, (o1, o2) -> {
   if (o1.getName().compareTo(o2.getName()) > 0) {
    return 1;
   } else if (o1.getName().compareTo(o2.getName()) < 0) {
    return -1;
   } else {
    return 0;
   }
  });
  //7.List遍历
  List<String> listStr = new ArrayList<>();
  List<Student> listStu = new ArrayList<>();
  listStr.forEach(studentStr -> {
   listStu.add(JSON.parseObject(studentStr, Student.class));

  });

  //List根据某个字段过滤、排序
  listStu.stream()
    .filter(student -> student.getSex().equals("女"))
    .sorted(Comparator.comparing(Student::getName))
    .collect(Collectors.toList());

  //List根据某个字段分组
  Map<String,List<Student>> sexGroupMap = listStu.stream()
              .collect(Collectors.groupingBy(Student::getSex));
  //如果Map中多个名称相同，则studentId用逗号间隔
  Map<String,String> studentNameIdMap = listStu.stream()
              .collect(toMap(Student::getName,Student::getStuId,(s,a)->s+","+a));
 }

 public static List<Student> createStudentList() {
  List<Student> list = new ArrayList<Student>();
  Student lily = new Student();
  lily.setStuId("1");
  lily.setName("lily");
  lily.setAge("14");
  lily.setSex("女");
  Student xiaoming = new Student();
  xiaoming.setStuId("2");
  xiaoming.setName("xiaoming");
  xiaoming.setAge("15");
  xiaoming.setSex("男");
  list.add(lily);
  list.add(xiaoming);
  return list;
 }

 public static Map<Object, Object> getStudentObjectMap(List<Student> list) {
  Map<Object, Object> map = list.stream().collect(Collectors.toMap(Student::getStuId, student -> student));
  map.forEach((key, value) -> {
   System.out.println("key:" + key + ",value:" + value);
  });
  return map;
 }

 public static Map<String, String> getStudentNameMap(List<Student> list) {
  Map<String, String> map = list.stream().collect(Collectors.toMap(Student::getStuId, Student::getName));
  map.forEach((key, value) -> {
   System.out.println("key:" + key + ",value:" + value);
  });
  return map;
 }

 public static List<String> getStudentNameList(List<Student> list) {
  List<String> result = list.stream().map(student -> student.getName()).collect(Collectors.toList());
  for (String name : result) {
   System.out.println("name:" + name);
  }
  return result;
 }
}

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