Java8 新特性之Stream

1、Student

package com.elk.log.myTest;


public class Student implements Comparable<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;

    // 省略getter和setter


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

    @Override
    public int compareTo(Student o) {
        return (int) (this.id-o.id);
    }
}

View Code

2、操作

       // 初始化
        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(20160002, "伯约", 21, 2, "信息安全", "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"));
                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"));
            }
        };

        //过滤--->列表钟只含有名字为"孔明"
        List<Student> whuStudents = students.stream().filter(student -> "孔明".equals(student.getName())).collect(Collectors.toList());

        //排序
        List<Student> sortStudents = students.stream().sorted(new Comparator<Student>() {
            @Override
            public int compare(Student o1, Student o2) {
                return o1.getGrade()-o2.getGrade();
            }
        }).collect(Collectors.toList());

        //将实体对象(Student)映射为字符串(String)
        List<String> names =  students.stream().filter(student -> "土木工程".equals(student.getMajor())).map(Student::getName).collect(Collectors.toList());

        //统计分数和
        int gradeSum =  students.stream().mapToInt(Student::getGrade).sum();

        String[] strs = {"java8", "is", "easy", "to", "use"};

        //扁平化操作？？？？
        List<String[]> distinctStrs = Arrays.stream(strs)
                .map(str -> str.split(""))  // 映射成为Stream<String[]>
                .distinct()
                .collect(Collectors.toList());

        List<String> distinctStrs1 = Arrays.stream(strs)
                .map(str -> str.split(""))  // 映射成为Stream<String[]>
                .flatMap(Arrays::stream)  // 扁平化为Stream<String>
                .distinct()
                .collect(Collectors.toList());

        //查询列表对象是否存在某个对象
        boolean ret1 = students.stream().allMatch(student -> "鲁肃".equals(student.getName()));

        //年龄的平均值
        Double age  = students.stream().collect(Collectors.averagingInt(Student::getAge));

        //获取最大的分数对象
        Student s = students.stream().max(new Comparator<Student>() {
            @Override
            public int compare(Student o1, Student o2) {
                return o1.getGrade() - o2.getGrade();
            }
        }).get();

        //计算机科学年龄总和
        int totalAge2 = students.stream()
                .filter(student -> "计算机科学".equals(student.getMajor()))
                .map(Student::getAge)
                .reduce(0, Integer::sum);
        
        //list大小
        Long l = students.stream().count();

具体可查看：https://blog.csdn.net/anthony_ju/article/details/82078450