Lambda 表达式
Student
import java.io.Serializable;
import java.util.Objects;
public class Student implements Serializable {
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);
}
//省略setter/getter。。。
}
使用场景
过滤
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<Student> students = new ArrayList<>();
students.add(s1);
students.add(s2);
students.add(s3);
students.add(s4);
//筛选年龄大于15岁的学生
List <Student> stuList = students.stream()
.filter(s -> s.getAge()>15)
.collect(Collectors.toList());
//筛选住在浙江省的学生
List<Student> stuList2 = students.stream()
.filter(s -> "浙江".equals(s.getAddress()))
.collect(Collectors.toList());
转换
.map
map: 对于Stream中包含的元素使用给定的转换函数进行转换操作,新生成的Stream只包含转换生成的元素。这个方法有三个对于原始类型的变种方法,分别是:mapToInt,mapToLong和mapToDouble。这三个方法也比较好理解,比如mapToInt就是把原始Stream转换成一个新的Stream,这个新生成的Stream中的元素都是int类型。之所以会有这样三个变种方法,可以免除自动装箱/拆箱的额外消耗;
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<Student> students = new ArrayList<>();
students.add(s1);
students.add(s2);
students.add(s3);
students.add(s4);
//获取地址集合
List<String> addresses1 = students.stream()
.map(Student::getAddress)
.collect(Collectors.toList());
//在地址前面加上部分信息,只获取地址输出
List<String> addresses = students.stream()
.map(s -> "住址:" + s.getAddress())
.collect(Collectors.toList());
//获取的地址以逗号隔开拼成字符串
String addresses2 = students.stream()
.map(Student::getAddress)
.collect(Collectors.joining(","));
//集合以逗号隔开拼成字符串
String addresses3 = StringUtils.join(addresses1.toArray(), ",");
.mapToLong
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<Student> students = new ArrayList<>();
students.add(s1);
students.add(s2);
students.add(s3);
students.add(s4);
//年龄求和
int sum = students.stream().mapToLong(Student::getAge).sum();
.collect
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, "袁天罡", 17, "浙江");
Student s5 = new Student(4L, "李淳风", 17, "浙江");
List<Student> students = new ArrayList<>();
students.add(s1);
students.add(s2);
students.add(s3);
students.add(s4);
students.add(s5);
//键相同,新的值不会覆盖旧的值
Map<Long, String> map1 = students.stream()
.collect(Collectors.toMap(Student::getId, Student::getName,
(oldVal, newVal) -> oldVal));
//键相同,新的值会覆盖旧的值
Map<Long, String> map2 = students.stream()
.collect(Collectors.toMap(Student::getId, Student::getName,
(oldVal, newVal) -> newVal));
Map<String, String> map = new HashMap<>();
map.put("a", "路飞1");
map.put("b", "索隆23");
map.put("c", "三治456");
map.put("d", "布鲁克7890");
//获取新的map,值为原来值的长度
Map<String, Integer> map2 = map.entrySet().stream()
.collect(Collectors.toMap(Map.Entry::getKey, e -> e.getValue().length()));
//获取新的map,值为原来值的长度
Map<String, Integer> map3 = map.entrySet().stream()
.collect(HashMap::new, (m, v)
-> m.put(v.getKey(), v.getValue().length()),
HashMap::putAll);
去重
基本类型
//简单字符串的去重
List<String> list = Arrays.asList("111", "222", "333", "111", "222");
list.stream().distinct().forEach(System.out::println);
集合
//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<Student> students = new ArrayList<>();
students.add(s1);
students.add(s2);
students.add(s3);
students.add(s4);
students.add(s5);
List <Student> students2 = students.stream().distinct().collect(Collectors.toList());
排序
默认排序
List<String> list = Arrays.asList("333", "222", "111");
list.stream().sorted().forEach(System.out::println);
指定排序规则
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", 14, "浙江");
List<Student> students = new ArrayList<>();
students.add(s1);
students.add(s2);
students.add(s3);
students.add(s4);
// 按id和年龄倒序排列
students.stream()
.sorted((stu1, stu2) -> Long.compare(stu2.getId(), stu1.getId()))
.sorted((stu1, stu2) -> Integer.compare(stu2.getAge(), stu1.getAge()))
.collect(Collectors.toList());
students.stream()
// 按年龄升序排序,相同年龄按id降序排序
.sorted((stu1, stu2) -> Long.compare(stu2.getId(), stu1.getId()))
.sorted(Comparator.comparingInt(Student::getAge))
.forEach(System.out::println);
students.stream()
// 按年龄升序排序,相同年龄按id降序排序
.sorted(Comparator.comparingLong(Student::getId).reversed())
.sorted(Comparator.comparingInt(Student::getAge))
.forEach(System.out::println);
students.stream()
// 按年龄升序排序,相同年龄按id升序排序
.sorted(Comparator.comparingInt(Student::getAge).thenComparingLong(Student::getId))
.forEach(System.out::println);
students.stream()
// 按年龄升序排序,相同年龄按id升序排序,反向取值,即按年龄排序,年龄相同按id升序
.sorted(Comparator.comparingInt(Student::getAge)
.reversed()
.thenComparingLong(Student::getId)
.reversed())
.forEach(System.out::println);
截取
截取前N个
//集合limit,返回前几个元素
List<String> list = Arrays.asList("333", "222", "111");
list.stream().limit(2).forEach(System.out::println);
跳过前N个
//集合skip,删除前n个元素
List<String> list = Arrays.asList("333", "222", "111");
list.stream().skip(2).forEach(System.out::println);
聚合
//集合reduce,将集合中每个元素聚合成一条数据
List<String> list = Arrays.asList("欢", "迎", "你");
String appendStr = list.stream().reduce("北京", (a, b) -> a + b);
极值
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<Student> students = new ArrayList<>();
students.add(s1);
students.add(s2);
students.add(s3);
students.add(s4);
// 求年龄最小的学生
Student minS = students.stream()
.min(Comparator.comparingInt(Student::getAge))
.orElse(new Student(0L, null, 0, null));
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
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<Student> 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");
}
分组
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(5L, "肖战", 17, "浙江");
Student s6 = new Student(6L, "肖战", 15, "浙江");
List<Student> students = new ArrayList<>();
students.add(s1);
students.add(s2);
students.add(s3);
students.add(s4);
students.add(s5);
students.add(s6);
// 按照姓名和年龄分组
Map<String, Map<Integer, List<Student>>> groupMap = students.stream()
.collect(Collectors.groupingBy(Student::getName, Collectors.groupingBy(Student::getAge)));
结果如下:
{
"肖战": {
"15": [
{
"address": "浙江",
"age": 15,
"id": 1,
"name": "肖战"
},
{
"address": "浙江",
"age": 15,
"id": 6,
"name": "肖战"
}
],
"17": [
{
"address": "浙江",
"age": 17,
"id": 5,
"name": "肖战"
}
]
}
...
}