Java8中怎么操作Stream 对集合

今天就跟大家聊聊有关 Java8中怎么操作Stream 对集合，可能很多人都不太了解，为了让大家更加了解，小编给大家总结了以下内容，希望大家根据这篇文章可以有所收获。

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 Student {    private Long id;    private String name;    private int age;    private String address;    public Student() {}    public Student(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 "Student{" +                "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 static void main(String [] args) {        Student s1 = new Student(1L, "肖战", 15, "浙江");        Student s2 = new Student(2L, "王一博", 15, "湖北");        Student s3 = new Student(3L, "杨紫", 17, "北京");        Student s4 = new Student(4L, "李现", 17, "浙江");        List students = new ArrayList<>();        students.add(s1);        students.add(s2);        students.add(s3);        students.add(s4);        List streamStudents = testFilter(students);        streamStudents.forEach(System.out::println);    }    /**     * 集合的筛选     * @param students     * @return     */    private static List testFilter(List students) {        //筛选年龄大于15岁的学生//        return students.stream().filter(s -> s.getAge()>15).collect(Collectors.toList());        //筛选住在浙江省的学生        return students.stream().filter(s ->"浙江".equals(s.getAddress())).collect(Collectors.toList());    }

运行结果：

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) {        Student s1 = new Student(1L, "肖战", 15, "浙江");        Student s2 = new Student(2L, "王一博", 15, "湖北");        Student s3 = new Student(3L, "杨紫", 17, "北京");        Student s4 = new Student(4L, "李现", 17, "浙江");        List students = new ArrayList<>();        students.add(s1);        students.add(s2);        students.add(s3);        students.add(s4);        testMap(students);    }    /**     * 集合转换     * @param students     * @return     */    private static void testMap(List students) {        //在地址前面加上部分信息，只获取地址输出        List addresses = students.stream().map(s ->"住址:"+s.getAddress()).collect(Collectors.toList());        addresses.forEach(a ->System.out.println(a));    }

运行结果

map就是将对应的元素按照给定的方法进行转换。

distinct(去重)

    public static void main(String [] args) {      testDistinct1();    }    /**     * 集合去重（基本类型）     */    private static void testDistinct1() {        //简单字符串的去重        List list = Arrays.asList("111","222","333","111","222");        list.stream().distinct().forEach(System.out::println);    }

运行结果：

public static void main(String [] args) {      testDistinct2();    }    /**     * 集合去重（引用对象）     */    private static void testDistinct2() {        //引用对象的去重，引用对象要实现hashCode和equal方法，否则去重无效        Student s1 = new Student(1L, "肖战", 15, "浙江");        Student s2 = new Student(2L, "王一博", 15, "湖北");        Student s3 = new Student(3L, "杨紫", 17, "北京");        Student s4 = new Student(4L, "李现", 17, "浙江");        Student s5 = new Student(1L, "肖战", 15, "浙江");        List students = new ArrayList<>();        students.add(s1);        students.add(s2);        students.add(s3);        students.add(s4);        students.add(s5);        students.stream().distinct().forEach(System.out::println);    }

运行结果：

可以看出，两个重复的"肖战"同学进行了去重，这不仅因为使用了distinct()方法，而且因为Student对象重写了equals和hashCode()方法，否则去重是无效的。

sorted(排序)

    public static void main(String [] args) {        testSort1();    }    /**     * 集合排序（默认排序）     */    private static void testSort1() {        List list = Arrays.asList("333","222","111");        list.stream().sorted().forEach(System.out::println);    }

运行结果：

    public static void main(String [] args) {        testSort2();    }    /**     * 集合排序（指定排序规则）     */    private static void testSort2() {        Student s1 = new Student(1L, "肖战", 15, "浙江");        Student s2 = new Student(2L, "王一博", 15, "湖北");        Student s3 = new Student(3L, "杨紫", 17, "北京");        Student s4 = new Student(4L, "李现", 17, "浙江");        List students = new ArrayList<>();        students.add(s1);        students.add(s2);        students.add(s3);        students.add(s4);        students.stream()                .sorted((stu1,stu2) ->Long.compare(stu2.getId(), stu1.getId()))                .sorted((stu1,stu2) -> Integer.compare(stu2.getAge(),stu1.getAge()))                .forEach(System.out::println);    }

运行结果：

上面指定排序规则，先按照学生的id进行降序排序，再按照年龄进行降序排序

limit（限制返回个数）

    public static void main(String [] args) {        testLimit();    }    /**     * 集合limit，返回前几个元素     */    private static void testLimit() {        List list = Arrays.asList("333","222","111");        list.stream().limit(2).forEach(System.out::println);    }

运行结果：

skip(删除元素)

    public static void main(String [] args) {        testSkip();    }    /**     * 集合skip，删除前n个元素     */    private static void testSkip() {        List list = Arrays.asList("333","222","111");        list.stream().skip(2).forEach(System.out::println);    }

运行结果：

reduce(聚合)

    public static void main(String [] args) {        testReduce();    }    /**     * 集合reduce,将集合中每个元素聚合成一条数据     */    private static void testReduce() {        List list = Arrays.asList("欢","迎","你");        String appendStr = list.stream().reduce("北京",(a,b) -> a+b);        System.out.println(appendStr);    }

运行结果：

min(求最小值)

    public static void main(String [] args) {        testMin();    }    /**     * 求集合中元素的最小值     */    private static void testMin() {        Student s1 = new Student(1L, "肖战", 14, "浙江");        Student s2 = new Student(2L, "王一博", 15, "湖北");        Student s3 = new Student(3L, "杨紫", 17, "北京");        Student s4 = new Student(4L, "李现", 17, "浙江");        List students = new ArrayList<>();        students.add(s1);        students.add(s2);        students.add(s3);        students.add(s4);        Student minS = students.stream().min((stu1,stu2) ->Integer.compare(stu1.getAge(),stu2.getAge())).get();        System.out.println(minS.toString());    }

运行结果：

上面是求所有学生中年龄最小的一个，max同理，求最大值。

anyMatch/allMatch/noneMatch（匹配）

    public static void main(String [] args) {        testMatch();    }    private static void testMatch() {        Student s1 = new Student(1L, "肖战", 15, "浙江");        Student s2 = new Student(2L, "王一博", 15, "湖北");        Student s3 = new Student(3L, "杨紫", 17, "北京");        Student s4 = new Student(4L, "李现", 17, "浙江");        List students = new ArrayList<>();        students.add(s1);        students.add(s2);        students.add(s3);        students.add(s4);        Boolean anyMatch = students.stream().anyMatch(s ->"湖北".equals(s.getAddress()));        if (anyMatch) {            System.out.println("有湖北人");        }        Boolean allMatch = students.stream().allMatch(s -> s.getAge()>=15);        if (allMatch) {            System.out.println("所有学生都满15周岁");        }        Boolean noneMatch = students.stream().noneMatch(s -> "杨洋".equals(s.getName()));        if (noneMatch) {            System.out.println("没有叫杨洋的同学");        }    }

看完上述内容，你们对 Java8中怎么操作Stream 对集合有进一步的了解吗？如果还想了解更多知识或者相关内容，请关注行业资讯频道，感谢大家的支持。