Java中list转map的常用方法

我们开发中经常要用到list转map，下面我整理了几种方法供参考：

先创建对象：

package com.yabao.testvo;

public class Student extends Man {

    private String name;

    private int age;

    public Student() {
    }

    public Student(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

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

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

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

1.传统的方法经过遍历list然后put进map中。这个方法就不演示了。

下面三种就直接在代码里面展示了。

@Test
    public void test4() {
        Student stu1 = new Student("张三", 23);
        Student stu2 = new Student("李四", 24);
        Student stu3 = new Student("王五", 25);
        Student stu4 = new Student("赵六", 23);
        Student stu5 = new Student("前七", 25);

        List<Student> strList = new ArrayList<>();
        Collections.addAll(strList, stu1, stu2, stu3, stu4,stu5);
        // 1.根据age作为key,name作为value转map（age相同时前面覆盖后面的数据）
        Map<Integer, String> collect = strList.stream().collect(Collectors.toMap(Student::getAge, Student::getName, (key1,key2) -> key1 ));
        for (Map.Entry<Integer, String> integerStudentEntry : collect.entrySet()) {
            System.out.println(integerStudentEntry.getKey() + ":" + String.valueOf(integerStudentEntry.getValue()));
        }

        // 2.根据age作为key，student对象作为value转map（age相同时前面覆盖后面的数据）
        Map<Integer, Student> collectStu = strList.stream().collect(Collectors.toMap(Student::getAge, Function.identity(), (key1, key2) -> key2));
        for (Map.Entry<Integer, Student> integerStudentEntry : collectStu.entrySet()) {
            System.out.println(integerStudentEntry.getKey() + "::" + String.valueOf(integerStudentEntry.getValue()));
        }

        // 3.根据age作为key，student对象作为value分组转map（age相同时前面覆盖后面的数据）
        Map<Integer, List<Student>> listMap = strList.stream().collect(Collectors.groupingBy(Student::getAge));
        for (Map.Entry<Integer, List<Student>> integerStudentEntry : listMap.entrySet()) {
            System.out.println(integerStudentEntry.getKey() + "::" + String.valueOf(integerStudentEntry.getValue()));
        }
    }

总结：平时需要用到的list转map的简单方法基本都列举出来了。