Java8流式程式設計常見用法及案例
阿新 • • 發佈:2022-02-10
groupby統計數量
import java.util.Objects; public class Student { /** * 學號 */ private long id; private String name; private int age; /** * 年級 */ private int grade; /** * 專業 */ private String major; /** * 學校 */ 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", "武漢大學")); add(new Student(20160003, "玄德", 22, 3, "經濟管理", "武漢大學")); add(new Student(20160004, "雲長", 21, 2, "資訊保安", "武漢大學")); add(new Student(20161001, "翼德", 21, 2, "機械與自動化", "華中科技大學")); add(new Student(20161002, "元直", 23, 4, "土木工程", "華中科技大學")); add(new Student(20161003, "奉孝", 23, 4, "電腦科學", "華中科技大學")); add(new Student(20162001, "仲謀", 22, 3, "土木工程", "浙江大學")); add(new Student(20162002, "魯肅", 23, 4, "電腦科學", "浙江大學")); add(new Student(20163001, "丁奉", 24, 5, "土木工程", "南京大學")); } }; Map<String, Long> group2 = students.stream().collect( Collectors.groupingBy(Student::getSchool, Collectors.counting())); System.out.println(group2); }
{南京大學=1, 武漢大學=3, 浙江大學=2, 華中科技大學=3}
Process finished with exit code 0