lambda表达式处理集合List方法总结

Dear 丶 2023-10-10 16:25 30阅读 0赞

别在遇到循环就是for了，以下这些玩法，让你的代码像诗一样优雅。不在看着像小白写的了,

**当你需要list求和时，是否还在这样写？**

List<Integer> list = new ArrayList<>();
    Integer sum = 0;
    for (Integer integer : list) {
        
        sum+=integer;
    }

**当你需要判断集合中是否有张三这个人时，是否还在这样写?**

List<Student> students = new ArrayList<>();
      boolean isExist = false;
      for (Student student : students) {
        
          if(student.getName().equals("张三")){
        
                isExist = true;
                break;
          }
    }

*今天这些都不会再出现，耐心往下看，看完以后，你以后也别用上面这种写法了行吗？*

*  去重
 *  过滤
 *  抽取
 *  分组
 *  计数
 *  最大值、最小值
 *  平均值
 *  排序
 *  匹配
 *  求和

### 模拟数据 ###

//模拟数据
    @Data
        static class Student{
        
            //学号
            private Integer no;
            //姓名
            private String name;
            //年龄
            private Integer age;
            //成绩
            private Integer socre;
    
            public Student(Integer no,String name,Integer age,Integer socre){
        
                this.no = no;
                this.name = name;
                this.age =  age;
                this.socre =socre;
            }
        }
        public static List<Student> mockdata(){
        
            List<Student> students = new ArrayList<>();
            Student student1 = new Student(1001,"张三",20,100);
            Student student2 = new Student(1002,"张三",30,80);
            Student student3 = new Student(1003,"李四",25,70);
            Student student4 = new Student(1004,"王五",25,65);
            Student student5 = new Student(1005,"赵六",25,40);
            students.add(student1);
            students.add(student2);
            students.add(student3);
            students.add(student4);
            students.add(student5);
            return students;
        }

### 根据名称去重数据（去重） ###

//根据名字去重
    List<Student> result = data.stream().collect(Collectors.collectingAndThen(
    Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(Student::getName))), ArrayList::new
    ));
    System.out.println(result.toString());
    //输出如下:
    [Test.Student(name=张三, age=20, socre=100), Test.Student(name=李四, age=25, socre=70), Test.Student(name=王五, age=25, socre=65), Test.Student(name=赵六, age=25, socre=40)]

### 根据一定条件筛选数据（过滤） ###

//20<筛选年龄<30的学生
    List<Student> filterResult = data.stream().filter(item -> item.getAge() > 20 && item.getAge() < 30).collect(Collectors.toList());
    System.out.println(filterResult);
    //输出如下
    [Test.Student(name=李四, age=25, socre=70), Test.Student(name=王五, age=25, socre=65), Test.Student(name=赵六, age=25, socre=40)]

### （数据抽取） ###

//转为Map，key为学号，value为Student对象
    Map<Integer, Student> studentMap = data.stream().collect(Collectors.toMap(s -> s.getNo(), s -> s));
    //转为Map,key为学号，value为分数
    Map<Integer, Integer> scoreMap = data.stream().collect(Collectors.toMap(s -> s.getNo(), s -> s.getSocre()));
    //转为Set，获取所有的学号
    Set<Integer> nameset = data.stream().map(s -> s.getNo()).collect(Collectors.toSet());
    //转为List，获取所有的学号
    List<Integer> namelist = data.stream().map(s->s.getNo()).collect(Collectors.toList());
    System.out.println(studentMap);
    System.out.println(scoreMap);
    System.out.println(nameset);
    System.out.println(namelist);
    //输出如下:
    {
        1001=Test.Student(no=1001, name=张三, age=20, socre=100), 1002=Test.Student(no=1002, name=张三, age=30, socre=80), 1003=Test.Student(no=1003, name=李四, age=25, socre=70), 1004=Test.Student(no=1004, name=王五, age=25, socre=65), 1005=Test.Student(no=1005, name=赵六, age=25, socre=40)}
    {
        1001=100, 1002=80, 1003=70, 1004=65, 1005=40}
    [1001, 1002, 1003, 1004, 1005]
    [1001, 1002, 1003, 1004, 1005]

### 分组 ###

//分组，根据学生姓名分组
    Map<String, List<Student>> groupMap = data.stream().collect(Collectors.groupingBy(Student::getName));
    //分组后保持有序
    LinkedHashMap<String, ArrayList<Student>> groupMapSort = data.stream().collect(Collectors.groupingBy(Student::getName, LinkedHashMap::new, Collectors.toCollection(ArrayList::new)));
    //按学生姓名分组,List存放学号
    Map<String, List<Integer>> map1 = data.stream().collect(Collectors.groupingBy(Student::getName, Collectors.mapping(Student::getNo, Collectors.toList())));
    //按学生姓名分组,Set存放学号
    Map<String, Set<Integer>> map2 = data.stream().collect(Collectors.groupingBy(Student::getName, Collectors.mapping(Student::getNo, Collectors.toSet())));
    System.out.println(groupMap);
    System.out.println(groupMap);
    System.out.println(map1);
    System.out.println(map2);
    //输出如下：
    {
        李四=[Test.Student(no=1003, name=李四, age=25, socre=70)], 张三=[Test.Student(no=1001, name=张三, age=20, socre=100), Test.Student(no=1002, name=张三, age=30, socre=80)], 王五=[Test.Student(no=1004, name=王五, age=25, socre=65)], 赵六=[Test.Student(no=1005, name=赵六, age=25, socre=40)]}
    {
        李四=[Test.Student(no=1003, name=李四, age=25, socre=70)], 张三=[Test.Student(no=1001, name=张三, age=20, socre=100), Test.Student(no=1002, name=张三, age=30, socre=80)], 王五=[Test.Student(no=1004, name=王五, age=25, socre=65)], 赵六=[Test.Student(no=1005, name=赵六, age=25, socre=40)]}
    {
        李四=[1003], 张三=[1001, 1002], 王五=[1004], 赵六=[1005]}
    {
        李四=[1003], 张三=[1001, 1002], 王五=[1004], 赵六=[1005]}

### 计数 ###

//计数，根据name分组，并获取每个分组的数量
    Map<String, Long> count = data.stream().collect(Collectors.groupingBy(Student::getName, Collectors.counting()));
    System.out.println(count);
    //输出如下:
    {
        李四=1, 张三=2, 王五=1, 赵六=1}

### 最大值、最小值 ###

//最小年龄
    Integer min = data.stream().mapToInt(Student::getAge).min().getAsInt();
    //最大年龄
    Integer max = data.stream().mapToInt(Student::getAge).max().getAsInt();
    // 最大对象
    Student maxObject = data.stream().max(Comparator.comparing(Student::getAge)).get();
    // 最小对象
    Student minObject = data.stream().min(Comparator.comparing(Student::getAge)).get();
    System.out.println(min);
    System.out.println(max);
    System.out.println(maxObject);
    System.out.println(minObject);
    //输出如下：
    20
    30
    Test.Student(no=1002, name=张三, age=30, socre=80)
    Test.Student(no=1001, name=张三, age=20, socre=100)

### 平均值 ###

// 年龄平均数
    double asDouble = data.stream().mapToLong(Student::getAge).average().getAsDouble();
    System.out.println(asDouble);
    //输出如下:
    25.0

### 排序 ###

// 对象根据年龄属性升序排序
    List<Student> ageAsc = data.stream().sorted(Comparator.comparing(Student::getAge)).collect(Collectors.toList());
    // 对象根据年龄属性降序排序
    List<Student> ageDesc = data.stream().sorted(Comparator.comparing(Student::getAge).reversed()).collect(Collectors.toList());
    // 双字段排序，年龄升序，分数降序
    List<Student> ageScoreSort = data.stream().sorted(Comparator.comparing(Student::getAge).thenComparing(Student::getSocre).reversed()).collect(Collectors.toList());
    System.out.println(ageAsc);
    System.out.println(ageDesc);
    System.out.println(ageScoreSort);
    //输出如下：
    [Test.Student(no=1001, name=张三, age=20, socre=100), Test.Student(no=1003, name=李四, age=25, socre=70), Test.Student(no=1004, name=王五, age=25, socre=65), Test.Student(no=1005, name=赵六, age=25, socre=40), Test.Student(no=1002, name=张三, age=30, socre=80)]
    [Test.Student(no=1002, name=张三, age=30, socre=80), Test.Student(no=1003, name=李四, age=25, socre=70), Test.Student(no=1004, name=王五, age=25, socre=65), Test.Student(no=1005, name=赵六, age=25, socre=40), Test.Student(no=1001, name=张三, age=20, socre=100)]
    [Test.Student(no=1002, name=张三, age=30, socre=80), Test.Student(no=1003, name=李四, age=25, socre=70), Test.Student(no=1004, name=王五, age=25, socre=65), Test.Student(no=1005, name=赵六, age=25, socre=40), Test.Student(no=1001, name=张三, age=20, socre=100)]

### 匹配 ###

//查找list中是否都是张三
    boolean result1 = data.stream().allMatch((s) -> s.getName().equals("张三"));
    //查找list中是否有一个是张三
    boolean result2 = data.stream().anyMatch((s) -> s.getName().equals("张三"));
    //判断list中没有张三
    boolean result3 = data.stream().noneMatch((s) -> s.getName().equals("张三"));
    System.out.println(result1);
    System.out.println(result2);
    System.out.println(result3);
    //输出如下:
    false
    true
    false

### 求和 ###

//成绩总和
    Integer scoreSum = data.stream().mapToInt(Student::getAge).sum();
    System.out.println(scoreSum);
    //输出如下：
    125