Java8 实现stream将对象集合list中抽取属性集合转化为map或list
作者：文耀文耀
这篇文章主要介绍了Java8 实现stream将对象集合list中抽取属性集合转化为map或list的操作，具有很好的参考价值，希望对大家有所帮助。一起跟随小编过来看看吧
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;
}
}
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脚本之家。如有错误或未考虑完全的地方，望不吝赐教。