java8 List 转 Set、Map（高级）

实体类

public class Student {

    private int id;
    private String name;
    private String score;
    private int classNo;

    public Student(int id, String name, String score, int classNo) {
        this.id = id;
        this.name = name;
        this.score = score;
        this.classNo = classNo;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getScore() {
        return score;
    }

    public void setScore(String score) {
        this.score = score;
    }

    public int getClassNo() {
        return classNo;
    }

    public void setClassNo(int classNo) {
        this.classNo = classNo;
    }

    @Override
    public String toString() {
        return "Student{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", score='" + score + '\'' +
                ", classNo=" + classNo +
                '}';
    }
}

Main

import com.demo.entity.Student;

import java.util.*;
import java.util.stream.Collectors;


public class Main {

    private static List<String> simpleList = new ArrayList<>();
    private static List<Student> normalList = new ArrayList<>();
    static {
        simpleList.add("apple");
        simpleList.add("apple");
        simpleList.add("banana");
        simpleList.add("orange");

        normalList.add(new Student(1, "Emma", "A", 701));
        normalList.add(new Student(2, "Larissa", "S", 701));
        normalList.add(new Student(3, "Sophia", "B", 701));
        normalList.add(new Student(4, "Ashley", "B", 702));
        normalList.add(new Student(5, "May", "C", 702));
        normalList.add(new Student(6, "Hailey", "D", 702));
        normalList.add(new Student(7, "Kelly", "S", 703));
        normalList.add(new Student(8, "Amy", "A", 703));
        normalList.add(new Student(9, "Wesley", "C", 703));
    }
    public static void main(String[] args){
        // TODO
    }

}

转换

System.out.println("----------------简单List---------------");
simpleList.forEach(System.out::println);
System.out.println("----------------简单List转Set---------------");
Set<String> simpleSet = new HashSet<>(simpleList);
simpleSet.forEach(System.out::println);

输出：

----------------简单List---------------
apple
apple
banana
orange
----------------简单List转Set---------------
banana
orange
apple

---

System.out.println("----------------普通List---------------");
normalList.forEach(System.out::println);
System.out.println("----------------普通List转Set---------------");
Set<Integer> normalSet = normalList.stream().map(Student::getClassNo).collect(Collectors.toSet());
normalSet.forEach(System.out::println);

输出：

----------------普通List---------------
Student{id=1, name='Emma', score='A', classNo=701}
Student{id=2, name='Larissa', score='S', classNo=701}
Student{id=3, name='Sophia', score='B', classNo=701}
Student{id=4, name='Ashley', score='B', classNo=702}
Student{id=5, name='May', score='C', classNo=702}
Student{id=6, name='Hailey', score='D', classNo=702}
Student{id=7, name='Kelly', score='S', classNo=703}
Student{id=8, name='Amy', score='A', classNo=703}
Student{id=9, name='Wesley', score='C', classNo=703}
----------------普通List转Set---------------
701
702
703

---

System.out.println("----------------普通List转List---------------");
List<String> list = normalList.stream().map(Student::getName).collect(Collectors.toList());
list.forEach(System.out::println);

输出：

----------------普通List转List---------------
Emma
Larissa
Sophia
Ashley
May
Hailey
Kelly
Amy
Wesley

---

System.out.println("----------------普通List转Map---------------");
Map<Integer,Student> normalMap = normalList.stream().collect(Collectors.toMap(Student::getId,(b)->b));
normalMap.forEach((id, student) -> {
    System.out.println(id + "::" + student);
});

输出：

----------------普通List转Map---------------
1::Student{id=1, name='Emma', score='A', classNo=701}
2::Student{id=2, name='Larissa', score='S', classNo=701}
3::Student{id=3, name='Sophia', score='B', classNo=701}
4::Student{id=4, name='Ashley', score='B', classNo=702}
5::Student{id=5, name='May', score='C', classNo=702}
6::Student{id=6, name='Hailey', score='D', classNo=702}
7::Student{id=7, name='Kelly', score='S', classNo=703}
8::Student{id=8, name='Amy', score='A', classNo=703}
9::Student{id=9, name='Wesley', score='C', classNo=703}

复杂一点的转换：

List<Student> students = new ArrayList<>(normalList);
System.out.println("----------------原数据---------------");
students.forEach(System.out::println);
System.out.println("----------------List<Student>转Map<String, Student>重复key只保留前者---------------");
// 重复key处理 (s1, s2) -> s1)
Map<Integer, Student> classStudentMap = students.stream().collect(Collectors.toMap(Student::getClassNo, s -> s, (s1, s2) -> s1));
classStudentMap.forEach((classNo, student) -> System.out.println(classNo + "::" + student));

System.out.println("----------------List<Student>转Map<String, List<Student>>---------------");
// 重复key处理成一个集合
Map<Integer, List<Student>> listMap = students.stream().collect(Collectors.toMap(Student::getClassNo, s -> {
    List<Student> l = new ArrayList<>();
    l.add(s);
    return l;
}, (List<Student> s1, List<Student> s2) -> {
    s1.addAll(s2);
    return s1;
}));
listMap.forEach((learn, student) -> System.out.println(learn + "::" + student));

输出：

----------------原数据---------------
Student{id=1, name='Emma', score='A', classNo=701}
Student{id=2, name='Larissa', score='S', classNo=701}
Student{id=3, name='Sophia', score='B', classNo=701}
Student{id=4, name='Ashley', score='B', classNo=702}
Student{id=5, name='May', score='C', classNo=702}
Student{id=6, name='Hailey', score='D', classNo=702}
Student{id=7, name='Kelly', score='S', classNo=703}
Student{id=8, name='Amy', score='A', classNo=703}
Student{id=9, name='Wesley', score='C', classNo=703}
----------------List<Student>转Map<String, Student>重复key只保留前者---------------
701::Student{id=1, name='Emma', score='A', classNo=701}
702::Student{id=4, name='Ashley', score='B', classNo=702}
703::Student{id=7, name='Kelly', score='S', classNo=703}
----------------List<Student>转Map<String, List<Student>>---------------
701::[Student{id=1, name='Emma', score='A', classNo=701}, Student{id=2, name='Larissa', score='S', classNo=701}, Student{id=3, name='Sophia', score='B', classNo=701}]
702::[Student{id=4, name='Ashley', score='B', classNo=702}, Student{id=5, name='May', score='C', classNo=702}, Student{id=6, name='Hailey', score='D', classNo=702}]
703::[Student{id=7, name='Kelly', score='S', classNo=703}, Student{id=8, name='Amy', score='A', classNo=703}, Student{id=9, name='Wesley', score='C', classNo=703}]

---

List<Student> students = new ArrayList<>(normalList);
// 先分组，准备好数据
Map<String, List<Student>> grouping = students.stream().collect(Collectors.groupingBy(Student::getScore));
System.out.println("----------------Map<String, List<Student>>转List<Student>---------------");
// 把map的values全部拆出来
List<Student> collect = grouping.entrySet().stream()
        .flatMap(map -> map.getValue().stream())
        .collect(Collectors.toList());
collect.forEach(System.out::println);

System.out.println("----------------Map<String, List<Student>>转List<List<Student>>---------------");
// 只要map的value，但是不改变格式
grouping.values().forEach(System.out::println);

输出：

----------------Map<String, List<Student>>转List<Student>---------------
Student{id=1, name='Emma', score='A', classNo=701}
Student{id=8, name='Amy', score='A', classNo=703}
Student{id=3, name='Sophia', score='B', classNo=701}
Student{id=4, name='Ashley', score='B', classNo=702}
Student{id=5, name='May', score='C', classNo=702}
Student{id=9, name='Wesley', score='C', classNo=703}
Student{id=2, name='Larissa', score='S', classNo=701}
Student{id=7, name='Kelly', score='S', classNo=703}
Student{id=6, name='Hailey', score='D', classNo=702}
----------------Map<String, List<Student>>转List<List<Student>>---------------
[Student{id=1, name='Emma', score='A', classNo=701}, Student{id=8, name='Amy', score='A', classNo=703}]
[Student{id=3, name='Sophia', score='B', classNo=701}, Student{id=4, name='Ashley', score='B', classNo=702}]
[Student{id=5, name='May', score='C', classNo=702}, Student{id=9, name='Wesley', score='C', classNo=703}]
[Student{id=2, name='Larissa', score='S', classNo=701}, Student{id=7, name='Kelly', score='S', classNo=703}]
[Student{id=6, name='Hailey', score='D', classNo=702}]

---

分组

List<Student> students = new ArrayList<>(normalList);
System.out.println("----------------分组---------------");
// 根据key分组
Map<String, List<Student>> grouping = students.stream().collect(Collectors.groupingBy(Student::getScore));
grouping.forEach((score, student) -> System.out.println(score + "::" + student));
System.out.println("----------------按照多个属性分组---------------");
// 根据多个key的组合分组
grouping = students.stream().collect(Collectors.groupingBy( e -> e.getClassNo() + "_" + e.getScore()));
grouping.forEach((learn, student) -> System.out.println(learn + "::" + student));

输出：

----------------分组---------------
A::[Student{id=1, name='Emma', score='A', classNo=701}, Student{id=8, name='Amy', score='A', classNo=703}]
B::[Student{id=3, name='Sophia', score='B', classNo=701}, Student{id=4, name='Ashley', score='B', classNo=702}]
C::[Student{id=5, name='May', score='C', classNo=702}, Student{id=9, name='Wesley', score='C', classNo=703}]
S::[Student{id=2, name='Larissa', score='S', classNo=701}, Student{id=7, name='Kelly', score='S', classNo=703}]
D::[Student{id=6, name='Hailey', score='D', classNo=702}]
----------------按照多个属性分组---------------
702_C::[Student{id=5, name='May', score='C', classNo=702}]
703_A::[Student{id=8, name='Amy', score='A', classNo=703}]
702_B::[Student{id=4, name='Ashley', score='B', classNo=702}]
701_S::[Student{id=2, name='Larissa', score='S', classNo=701}]
703_C::[Student{id=9, name='Wesley', score='C', classNo=703}]
703_S::[Student{id=7, name='Kelly', score='S', classNo=703}]
702_D::[Student{id=6, name='Hailey', score='D', classNo=702}]
701_B::[Student{id=3, name='Sophia', score='B', classNo=701}]
701_A::[Student{id=1, name='Emma', score='A', classNo=701}]

---

排序

根据名字排序

System.out.println("----------------List排序---------------");
// 虽然这里是浅拷贝，但是只影响修改而不影响排序
List<Student> students = new ArrayList<>(normalList);
Collections.sort(students, Comparator.comparing(Student::getName));
// 比上面更简洁
// students.sort(Comparator.comparing(Student::getName));
students.forEach(System.out::println);

输出：

----------------List排序---------------
Student{id=8, name='Amy', score='A', classNo=703}
Student{id=4, name='Ashley', score='B', classNo=702}
Student{id=1, name='Emma', score='A', classNo=701}
Student{id=6, name='Hailey', score='D', classNo=702}
Student{id=7, name='Kelly', score='S', classNo=703}
Student{id=2, name='Larissa', score='S', classNo=701}
Student{id=5, name='May', score='C', classNo=702}
Student{id=3, name='Sophia', score='B', classNo=701}
Student{id=9, name='Wesley', score='C', classNo=703}

---

System.out.println("----------------简单List排序---------------");
List<String> list = new ArrayList<>(simpleList);
System.out.println("----------------正序---------------");
list.sort((a,b) -> a.compareTo(b));
// 更简洁的方式
// list.sort(Comparator.naturalOrder());
list.forEach(System.out::println);
System.out.println("----------------倒序---------------");
list.sort(Comparator.reverseOrder());
list.forEach(System.out::println);

输出：

----------------简单List排序---------------
----------------正序---------------
apple
apple
banana
orange
----------------倒序---------------
orange
banana
apple
apple

Map排序

List<Student> students = new ArrayList<>(normalList);
// 先按照成绩分组，准备好数据
Map<String, List<Student>> grouping = students.stream().collect(Collectors.groupingBy(Student::getScore));
System.out.println("----------------Map<String, List<Student>>排序---------------");
Map<String, List<Student>> result = new LinkedHashMap<>();
// Map的key有特殊处理
grouping.entrySet().stream()
        .sorted((o1,o2) -> {
            Integer k1 = getWeight(o1.getKey());
            Integer k2 = getWeight(o2.getKey());
            return k1.compareTo(k2);
        })
        .forEachOrdered(x -> result.put(x.getKey(), x.getValue()));
result.forEach((learn, student) -> System.out.println(learn + "::" + student));
System.out.println("----------------");
Map<String, List<Student>> result2 = new LinkedHashMap<>();
// 仅仅按照key排序
grouping.entrySet().stream()
        .sorted(Map.Entry.comparingByKey())
        .forEachOrdered(x -> result2.put(x.getKey(), x.getValue()));
result2.forEach((learn, student) -> System.out.println(learn + "::" + student));
System.out.println("----------------");
Map<String, List<Student>> result3 = new LinkedHashMap<>();
// 等价第一个，只是省去了getKey方法
grouping.entrySet().stream()
        .sorted(Map.Entry.comparingByKey((o1,o2) -> {
            Integer k1 = getWeight(o1);
            Integer k2 = getWeight(o2);
            return k1.compareTo(k2);
        }))
        .forEachOrdered(x -> result3.put(x.getKey(), x.getValue()));
result3.forEach((learn, student) -> System.out.println(learn + "::" + student));

 

权重方法：

/**
 * 不同成绩有不同的排序权重
 * @param score
 * @return
 */
public static Integer getWeight(String score){
    switch (score){
        case "S": return 1;
        case "A": return 2;
        case "B": return 3;
        case "C": return 2;
        case "D": return 2;
        default:return 0;
    }
}

 

输出：

----------------Map<String, List<Student>>排序---------------
S::[Student{id=2, name='Larissa', score='S', classNo=701}, Student{id=7, name='Kelly', score='S', classNo=703}]
A::[Student{id=1, name='Emma', score='A', classNo=701}, Student{id=8, name='Amy', score='A', classNo=703}]
C::[Student{id=5, name='May', score='C', classNo=702}, Student{id=9, name='Wesley', score='C', classNo=703}]
D::[Student{id=6, name='Hailey', score='D', classNo=702}]
B::[Student{id=3, name='Sophia', score='B', classNo=701}, Student{id=4, name='Ashley', score='B', classNo=702}]
----------------
A::[Student{id=1, name='Emma', score='A', classNo=701}, Student{id=8, name='Amy', score='A', classNo=703}]
B::[Student{id=3, name='Sophia', score='B', classNo=701}, Student{id=4, name='Ashley', score='B', classNo=702}]
C::[Student{id=5, name='May', score='C', classNo=702}, Student{id=9, name='Wesley', score='C', classNo=703}]
D::[Student{id=6, name='Hailey', score='D', classNo=702}]
S::[Student{id=2, name='Larissa', score='S', classNo=701}, Student{id=7, name='Kelly', score='S', classNo=703}]
----------------
S::[Student{id=2, name='Larissa', score='S', classNo=701}, Student{id=7, name='Kelly', score='S', classNo=703}]
A::[Student{id=1, name='Emma', score='A', classNo=701}, Student{id=8, name='Amy', score='A', classNo=703}]
C::[Student{id=5, name='May', score='C', classNo=702}, Student{id=9, name='Wesley', score='C', classNo=703}]
D::[Student{id=6, name='Hailey', score='D', classNo=702}]
B::[Student{id=3, name='Sophia', score='B', classNo=701}, Student{id=4, name='Ashley', score='B', classNo=702}]

 

如果你要倒序的话

System.out.println("----------------倒序----------------");
Map<String, List<Student>> result4 = new LinkedHashMap<>();
// 仅仅按照key排序
grouping.entrySet().stream()
        .sorted(Map.Entry.<String, List<Student>>comparingByKey().reversed())
        .forEachOrdered(x -> result4.put(x.getKey(), x.getValue()));
result4.forEach((learn, student) -> System.out.println(learn + "::" + student));
System.out.println("----------------");
Map<String, List<Student>> result5 = new LinkedHashMap<>();
// 等价第一个，只是省去了getKey方法
grouping.entrySet().stream()
        .sorted(Map.Entry.<String, List<Student>>comparingByKey((o1,o2) -> {
            Integer k1 = getWeight(o1);
            Integer k2 = getWeight(o2);
            return k1.compareTo(k2);
        }).reversed())
        .forEachOrdered(x -> result5.put(x.getKey(), x.getValue()));
result5.forEach((learn, student) -> System.out.println(learn + "::" + student));

 

输出：

----------------倒序----------------
S::[Student{id=2, name='Larissa', score='S', classNo=701}, Student{id=7, name='Kelly', score='S', classNo=703}]
D::[Student{id=6, name='Hailey', score='D', classNo=702}]
C::[Student{id=5, name='May', score='C', classNo=702}, Student{id=9, name='Wesley', score='C', classNo=703}]
B::[Student{id=3, name='Sophia', score='B', classNo=701}, Student{id=4, name='Ashley', score='B', classNo=702}]
A::[Student{id=1, name='Emma', score='A', classNo=701}, Student{id=8, name='Amy', score='A', classNo=703}]
----------------
B::[Student{id=3, name='Sophia', score='B', classNo=701}, Student{id=4, name='Ashley', score='B', classNo=702}]
A::[Student{id=1, name='Emma', score='A', classNo=701}, Student{id=8, name='Amy', score='A', classNo=703}]
C::[Student{id=5, name='May', score='C', classNo=702}, Student{id=9, name='Wesley', score='C', classNo=703}]
D::[Student{id=6, name='Hailey', score='D', classNo=702}]
S::[Student{id=2, name='Larissa', score='S', classNo=701}, Student{id=7, name='Kelly', score='S', classNo=703}]

 

 

随时补充