Java8特性之Stream流使用

前言

java8特性中新增了Stream流这个API，力求让更多的程序员写出更简洁，更高效率的代码。

获取流

在java8中有两种方法获取stream流，一种是stream()方法，另一种则是parallelStream()方法；这两者的区别是前者为串行流，后者为并行流。

流的常用方法使用

通用实体：

public class Student {
    private Integer id;
    private String name;
    private String classes;
    private Integer score;

    public Student(Integer id, String name, String classes, Integer score) {
        this.id = id;
        this.name = name;
        this.classes = classes;
        this.score = score;
    }

    public Integer getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

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

    public String getClasses() {
        return classes;
    }

    public void setClasses(String classes) {
        this.classes = classes;
    }

    public Integer getScore() {
        return score;
    }

    public void setScore(Integer score) {
        this.score = score;
    }

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

1、分组

public static void main(String[] args) {
        List<Student> studentList = new ArrayList<>();
        Student student1 = new Student(1,"张三","一班",100);
        Student student2 = new Student(2,"张四","一班",20);
        Student student3 = new Student(3,"张五","二班",150);
        Student student4 = new Student(4,"张六","二班",200);
        Student student5 = new Student(5,"张七","三班",233);
        studentList.add(student1);
        studentList.add(student2);
        studentList.add(student3);
        studentList.add(student4);
        studentList.add(student5);
        //根据班级分组,也可以多条件分组,多条件分组就在groupingBy中嵌套即可
        Map<String, List<Student>> collect = studentList.stream().collect(Collectors.groupingBy(i -> i.getClasses()));
        System.out.println(collect.toString());
    }

结果为：

{
	一班=[Student{id=1, name='张三', classes='一班', score=100}, Student{id=2, name='张四', classes='一班', score=20}],
	二班=[Student{id=3, name='张五', classes='二班', score=150}, Student{id=4, name='张六', classes='二班', score=200}],
	三班=[Student{id=5, name='张七', classes='三班', score=233}]
}

2、过滤

		//过滤出分数大于150分的学生
        List<Student> collect = studentList.stream().filter(student -> student.getScore() > 150).collect(Collectors.toList());
        System.out.println(collect.toString());

结果为：

[Student{id=4, name='张六', classes='二班', score=200}, Student{id=5, name='张七', classes='三班', score=233}]

3、转化为Map

		//用ID作为键，用对象作为值，转换为Map对象
        Map<Integer, Student> collect = studentList.stream().collect(Collectors.toMap(student -> student.getId(), student -> student));
        System.out.println(collect.toString());

结果为：

{1=Student{id=1, name='张三', classes='一班', score=100}, 2=Student{id=2, name='张四', classes='一班', score=20}, 3=Student{id=3, name='张五', classes='二班', score=150}, 4=Student{id=4, name='张六', classes='二班', score=200}, 5=Student{id=5, name='张七', classes='三班', score=233}}

4、Map转化为List

		//转换为List
		List<Student> collect = collect1.entrySet().stream().map(item -> {
            return item.getValue();
        }).collect(Collectors.toList());
        System.out.println(collect.toString());

结果为：

[Student{id=1, name='张三', classes='一班', score=100}, Student{id=2, name='张四', classes='一班', score=20}, Student{id=3, name='张五', classes='二班', score=150}, Student{id=4, name='张六', classes='二班', score=200}, Student{id=5, name='张七', classes='三班', score=233}]

5、求和

		//求分数和
        int sum = studentList.stream().mapToInt(Student::getScore).sum();
        System.out.println(sum);

结果为：

703

6、求最大最小值

		//求最大分数的学生
        Optional<Student> collect = studentList.stream().collect(Collectors.maxBy(Comparator.comparing(Student::getScore)));
        //求最小分数的学生
        Optional<Student> collect1 = studentList.stream().collect(Collectors.minBy(Comparator.comparing(Student::getScore)));
        if (collect.isPresent()){
            Student student = collect.get();
            System.out.println(student);
        }
        if (collect1.isPresent()){
            Student student = collect1.get();
            System.out.println(student);
        }

结果为：

Student{id=5, name='张七', classes='三班', score=233}
Student{id=2, name='张四', classes='一班', score=20}

7、去重

public static void main(String[] args) {
        List<Student> studentList = new ArrayList<>();
        Student student1 = new Student(1,"张三","一班",100);
        Student student2 = new Student(2,"张四","一班",20);
        Student student3 = new Student(3,"张五","二班",150);
        Student student4 = new Student(4,"张六","二班",200);
        Student student5 = new Student(5,"张七","三班",233);
        Student student6 = new Student(5,"张七","三班",233);
        studentList.add(student1);
        studentList.add(student2);
        studentList.add(student3);
        studentList.add(student4);
        studentList.add(student5);
        studentList.add(student6);
        List<Student> collect = studentList.stream().distinct().collect(Collectors.toList());
        System.out.println(collect);
    }

结果为：

[Student{id=1, name='张三', classes='一班', score=100}, Student{id=2, name='张四', classes='一班', score=20}, Student{id=3, name='张五', classes='二班', score=150}, Student{id=4, name='张六', classes='二班', score=200}, Student{id=5, name='张七', classes='三班', score=233}, Student{id=5, name='张七', classes='三班', score=233}]

8、排序

		//按照分数从高往低排列，如果需要多个，则在Comparator.comparing(Student::getScore).reversed()后新增thenComparing方法
		List<Student> collect = studentList.stream().sorted(Comparator.comparing(Student::getScore).reversed()).collect(Collectors.toList());
        System.out.println(collect);

结果为：

[Student{id=5, name='张七', classes='三班', score=233}, Student{id=4, name='张六', classes='二班', score=200}, Student{id=3, name='张五', classes='二班', score=150}, Student{id=1, name='张三', classes='一班', score=100}, Student{id=2, name='张四', classes='一班', score=20}]

9、平均值

		//计算学生平均分数
		OptionalDouble average = studentList.stream().mapToInt(Student::getScore).average();
        if (average.isPresent()){
            double asDouble = average.getAsDouble();
            System.out.println(asDouble);
        }

结果为：

140.6

注：Optional类为Java8中新增特性，它是一个可以为null的容器对象；如果值存在则isPresent()方法会返回true，调用get()方法会返回该对象。