1.概述
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：
- stream() − 为集合创建串行流
- parallelStream() - 为集合创建并行流
- 中间操作主要有以下方法（此类型方法返回的都是 Stream）：map (mapToInt, flatMap 等)、 filter、
distinct、 sorted、 peek、 limit、 skip、 parallel、 sequential、 unordered - 终止操作主要有以下方法：forEach、 forEachOrdered、 toArray、 reduce、 collect、 min、
max、 count、 anyMatch、 allMatch、 noneMatch、 findFirst、 findAny、
iterator
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; }
}1.1.filter（筛选）
/** * 利用Stream筛选处符合条件的集合 * @param student * @return */ private static List<Student> testFilter() { 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,"样子",17,"北京"); Student s4=new Student(4L,"李贤",17,"浙江");
List<Student> student=new ArrayList<Student>(); student.add(s1); student.add(s2); student.add(s3); student.add(s4); //筛选年龄大于15的学生 return student.stream().filter(s->s.getAge()>15).collect(Collectors.toList());//筛选居住在在浙江的学生// return student.stream().filter(s->"浙江".equals(s.getAddress())).collect(Collectors.toList()); }1.2.map(转换)
/** * 集合转换 * @param student */ private static void testMap(List<Student> student) { //在地址前面加上部分信息,只获取地址输出 List<String> address=student.stream().map(s->"地址是："+s.getAddress()).collect(Collectors.toList()); address.forEach(a->System.out.println(a)); }1.3.distinct(去重)
/** * 去重(基本类型) * @param student */ private static void testDistinct1() { List<String> list=Arrays.asList("111","222","111","333","444","222"); list.stream().distinct().forEach(s->System.out.println(s));;// list.forEach(s->System.out.println(s)); }/** * 去重(引用对象) * 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 */ private static void testDistinct2() { Student s1=new Student(1L,"肖战",15,"浙江"); Student s7=new Student(1L,"肖战",15,"浙江"); Student s2=new Student(2L,"王一博",15,"湖北"); Student s3=new Student(3L,"样子",17,"北京"); Student s4=new Student(4L,"李贤",18,"浙江"); Student s8=new Student(4L,"李贤",18,"浙江"); Student s5=new Student(2L,"李一鸣",16,"广州"); Student s6=new Student(1L,"张锋",13,"湛江");
List<Student> student=new ArrayList<Student>(); student.add(s1); student.add(s7); student.add(s8); student.add(s2); student.add(s3); student.add(s4); student.add(s5); student.add(s6);
student.stream().distinct().forEach(s->System.out.println(s)); }1.4.sorted(去重)
/** * 排序(默认排序) */ private static void testSorted() { List<String> list=Arrays.asList("111","333","222"); list.stream().sorted().forEach(s->System.out.println(s)); }
/** * 集合排序（指定规则） */ private static void testSorted2() { Student s1=new Student(1L,"肖战",15,"浙江"); Student s7=new Student(1L,"肖战",15,"浙江"); Student s2=new Student(2L,"王一博",15,"湖北"); Student s3=new Student(3L,"样子",17,"北京"); Student s4=new Student(4L,"李贤",18,"浙江"); Student s8=new Student(4L,"李贤",18,"浙江"); Student s5=new Student(2L,"李一鸣",16,"广州"); Student s6=new Student(1L,"张锋",13,"湛江");
List<Student> student=new ArrayList<Student>(); student.add(s1); student.add(s7); student.add(s8); student.add(s2); student.add(s3); student.add(s4); student.add(s5); student.add(s6);
//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 student.stream() .sorted((stu1,stu2)->Long.compare(stu2.getId(),stu1.getId())) //按照id进行倒叙排序 .sorted((stu1,stu2)->Integer.compare(stu2.getAge(),stu1.getAge())) //按照年龄进行倒叙排序 .forEach(s->System.out.println(s)); }1.5.limit(限制返回的个数)
/** * 限制返回的个数 * 集合limit，返回前几个元素 */ private static void testlimit() { List<String> list=Arrays.asList("111","222","333"); List<String> list2=list.stream().limit(2).collect(Collectors.toList()); list2.forEach(s->System.out.println(s)); }1.6.skip(删除前几个元素)
/** * 集合skip，删除前n个元素 */ private static void testSkip() { List<String> list=Arrays.asList("333","222","111"); List<String> collect = list.stream().skip(2).collect(Collectors.toList()); collect.forEach(s->System.out.println(s)); }1.7.reduce(将集合中的每一个元素聚合成一条数据)
/** * 集合Reduce，将集合中的每一个元素聚合成一条数据 */ private static void testReduce() { List<String> list=Arrays.asList("欢","迎","你"); String reduce = list.stream().reduce("北京",(a,b)->a+b); System.out.println(reduce); }1.8.min(求集合中的最小值)
/** * 求min集合中元素的最小值 * 求所有学生中年龄最小的一个，max 同理，求最大值。 */ private static void testMin() { Student s1=new Student(1L,"肖战",15,"浙江"); Student s7=new Student(1L,"肖战",15,"浙江"); Student s2=new Student(2L,"王一博",15,"湖北"); Student s3=new Student(3L,"样子",17,"北京"); Student s4=new Student(4L,"李贤",18,"浙江"); Student s8=new Student(4L,"李贤",18,"浙江"); Student s5=new Student(2L,"李一鸣",16,"广州"); Student s6=new Student(1L,"张锋",13,"湛江");
List<Student> student=new ArrayList<Student>(); student.add(s1); student.add(s7); student.add(s8); student.add(s2); student.add(s3); student.add(s4); student.add(s5); student.add(s6); //求年龄最小的人 Student student2 = student.stream().min((stu1,stu2)->Integer.compare(stu1.getAge(),stu2.getAge())).get(); System.out.println(student2); }1.8.anyMatch/allMatch/noneMatch(匹配)
/** * 测试匹配 *anyMatch/allMatch/noneMatch（匹配） *anyMatch：Stream 中任意一个元素符合传入的 predicate，返回 true allMatch：Stream 中全部元素符合传入的 predicate，返回 true noneMatch：Stream 中没有一个元素符合传入的 predicate，返回 true */ private static void testMatch() { Student s1=new Student(1L,"肖战",15,"浙江"); Student s7=new Student(1L,"肖战",15,"浙江"); Student s2=new Student(2L,"王一博",15,"湖北"); Student s3=new Student(3L,"样子",17,"北京"); Student s4=new Student(4L,"李贤",18,"浙江"); Student s8=new Student(4L,"李贤",18,"浙江"); Student s5=new Student(2L,"李一鸣",16,"广州"); Student s6=new Student(1L,"张锋",13,"湛江");
List<Student> student=new ArrayList<Student>(); student.add(s1); student.add(s7); student.add(s8); student.add(s2); student.add(s3); student.add(s4); student.add(s5); student.add(s6);
boolean anyMatch = student.stream().anyMatch(s->"湖北".equals(s.getAddress())); if (anyMatch) { System.out.println("有湖北人"); }
boolean allMatch = student.stream().allMatch(s->s.getAge()>=15); if (allMatch) { System.out.println("所有学生都满15周岁"); }
boolean noneMatch = student.stream().noneMatch(s->"杨洋".equals(s.getName())); if (noneMatch) { System.out.println("没有叫杨洋的人"); } }更多资源和教程请关注公众号：非科班的科班。
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原文地址：https://www.cnblogs.com/liduchang/p/11779009.html
时间： 2024-10-08 11:36:59