Java8流式编程常见用法及案例

groupby统计数量

import java.util.Objects;

public class Student {

    /**
     * 学号
     */
    private long id;

    private String name;

    private int age;

    /**
     * 年级
     */
    private int grade;

    /**
     * 专业
     */
    private String major;

    /**
     * 学校
     */
    private String school;

    public Student() {
    }

    public Student(long id, String name, int age, int grade, String major, String school) {
        this.id = id;
        this.name = name;
        this.age = age;
        this.grade = grade;
        this.major = major;
        this.school = school;
    }

    public long getId() {
        return id;
    }

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

    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;
    }

    public int getGrade() {
        return grade;
    }

    public void setGrade(int grade) {
        this.grade = grade;
    }

    public String getMajor() {
        return major;
    }

    public void setMajor(String major) {
        this.major = major;
    }

    public String getSchool() {
        return school;
    }

    public void setSchool(String school) {
        this.school = school;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Student student = (Student) o;
        return Objects.equals(school, student.school);
    }

    @Override
    public int hashCode() {
        return Objects.hash(school);
    }

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

    @Test
    public void FileTest1() throws IOException {
        List<Student> students = new ArrayList<Student>() {
            {
                add(new Student(20160001, "孔明", 20, 1, "土木工程", "武汉大学"));
                add(new Student(20160003, "玄德", 22, 3, "经济管理", "武汉大学"));
                add(new Student(20160004, "云长", 21, 2, "信息安全", "武汉大学"));
                add(new Student(20161001, "翼德", 21, 2, "机械与自动化", "华中科技大学"));
                add(new Student(20161002, "元直", 23, 4, "土木工程", "华中科技大学"));
                add(new Student(20161003, "奉孝", 23, 4, "计算机科学", "华中科技大学"));
                add(new Student(20162001, "仲谋", 22, 3, "土木工程", "浙江大学"));
                add(new Student(20162002, "鲁肃", 23, 4, "计算机科学", "浙江大学"));
                add(new Student(20163001, "丁奉", 24, 5, "土木工程", "南京大学"));
            }
        };
        Map<String, Long> group2 = students.stream().collect(
                Collectors.groupingBy(Student::getSchool,
                        Collectors.counting()));
        System.out.println(group2);
    }

{南京大学=1, 武汉大学=3, 浙江大学=2, 华中科技大学=3}

Process finished with exit code 0