简介

Stream 作为 java8 的新特性，基于 lambda 表达式，是对集合对象功能的增强，它专注于对集合对象进行各种高效、便利的聚合操作或者大批量的数据操作，提高了编程效率和代码可读性。

Stream 的原理：将要处理的元素看做一种流，流在管道中传输，并且可以在管道的节点上处理，包括过滤筛选、去重、排序、聚合等。元素流在管道中经过中间操作的处理，最后由最终操作得到前面处理的结果。
集合有两种方式生成流：
在这里插入图片描述
上图中是 Stream 类的类结构图，里面包含了大部分的中间和终止操作。

举例说明

首先为了说明 Stream 对对象集合的操作，新建一个 Student 类（学生类）, 覆写了 equals() 和 hashCode() 方法

public class Student1{
    
    

    private Long id;
    private String name;
    private int age;
    private String address;

    
   public Student1(){
    
    }

  public Student1(Long id,String name,int age,String address)
 {
    
    
        this.id =id;
        this.name =name;
        this.age =age;
        this.address =address;
    }

    
	@Override
   public String toString() {
    
    
        return "Student1{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", age=" + age +
                ", address='" + address + '\'' +
                '}';
    }

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

    @Override
    public int hashCode() {
    
    
        return Objects.hash(id, name, age, address);
    }

    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 String getAddress() {
    
    
        return address;
    }

    public void setAddress(String address) {
    
    
        this.address = address;
    }

}

filter（筛选）

public class StreamDemo {
    
    
    public static void main(String[] args) {
    
    
        Student1 s1 = new Student1(1L, "张三", 15, "浙江");
        Student1 s2 = new Student1(2L, "李四", 15, "湖北");
        Student1 s3 = new Student1(3L, "王五", 17, "北京");
        Student1 s4 = new Student1(4L, "赵六", 17, "浙江");
        List<Student1> student1s = new ArrayList<>();
        student1s.add(s1);
        student1s.add(s2);
        student1s.add(s3);
        student1s.add(s4);

        List<Student1> streamStudents = testFilter(student1s);
        streamStudents.forEach(System.out::println);

    }
    private static List<Student1>testFilter(List<Student1>student1s){
    
    
        //筛选年龄大于15岁的学生
//        return student1s.stream().filter(s->s.getAge()>15).collect(Collectors.toList());
//        //筛选住在浙江的学生
        return student1s.stream().filter(s->"浙江".equals(s.getAddress())).collect(Collectors.toList());
    }

运行结果：
在这里插入图片描述
这里我们创建了四个学生，经过 filter 的筛选，筛选出地址是浙江的学生集合。

map(转换)

  public static void main(String[] args) {
    
    
        Student1 s1 = new Student1(1L, "张三", 15, "浙江");
        Student1 s2 = new Student1(2L, "李四", 15, "湖北");
        Student1 s3 = new Student1(3L, "王五", 17, "北京");
        Student1 s4 = new Student1(4L, "赵六", 17, "浙江");
        List<Student1> student1s = new ArrayList<>();
        student1s.add(s1);
        student1s.add(s2);
        student1s.add(s3);
        student1s.add(s4);
        
        testMap(student1s);
    }
    private static void testMap(List<Student1>student1s){
    
    
        //地址前面加上部分.信息，只获取地址输出
        List<String> addresses = student1s.stream().map(s -> "地址:" + s.getAddress()).collect(Collectors.toList());
        addresses.forEach(a-> System.out.println(a));
    }