Java8 Stream 操作总结
1 map与方法引用
map()要么改变了element的值，要么改变了element中某属性的值
只要参数返回一致就可方法引用，被引用方法根据其方法类型有几种不用的引用姿势
public class Main {
public void mapString() {
Stream<String> arrayStream = Arrays.asList("555", "333", "444", "111", "222", "666").stream();
arrayStream.map(this::mapString);
arrayStream.map((s) -> mapString(s));
arrayStream.forEach(System.out::println);
}
private String mapString(String str) {
return str + ",";
}
}
2 flatmap展开多个Collection
flatMap用于把Stream中的层级结构扁平化并返回Stream，入参出参都是Stream
public static void main(String[] args) {
List<Student> classOneStudents = new ArrayList<>();
List<Student> classTwoStudents = new ArrayList<>();
List<Student> classThreeStudents = new ArrayList<>();
List<List<Student>> students = new ArrayList<>();
classOneStudents.add(Student.builder().age(18).name("Weison").score(90).build());
classOneStudents.add(Student.builder().age(19).name("Evan").score(80).build());
classOneStudents.add(Student.builder().age(20).name("Jack").score(70).build());
classOneStudents.add(Student.builder().age(21).name("Luis").score(60).build());
classTwoStudents.add(Student.builder().age(19).name("Elen").score(80).build());
classTwoStudents.add(Student.builder().age(20).name("Obam").score(70).build());
classTwoStudents.add(Student.builder().age(21).name("Bush").score(60).build());
classThreeStudents.add(Student.builder().age(20).name("Jackson").score(70).build());
classThreeStudents.add(Student.builder().age(21).name("Linn").score(60).build());
students.add(classOneStudents);
students.add(classTwoStudents);
students.add(classThreeStudents);
//5 flatMap 把Stream中的层级结构扁平化并返回Stream
List<Student> studentList = students
.stream()
.flatMap(List::stream)
.collect(Collectors.toList());
//展开多个List合并到一个新list
studentList
.stream()
.forEach(System.out::println);
}
3 reduce字符串、数值操作
reduce字符串、数值操作
String str3 = Stream.of("A", "B", "C", "D")
.reduce("", (s1, s2) -> s1 + "1" + s2 + "2", (s1, s2) -> s1 + "*" + s2);
System.out.println("拼接字符串str3：" + str3); //拼接字符串str3：1A21B21C21D2
int sumValue1 = Stream.of(1, 2, 3, 4)
.reduce(10, Integer::sum);
int sumValue2 = Stream.of(1, 2, 3, 4)
.reduce(10, (v1, v2) -> v1 + v2);
System.out.println("求和有起始值：" + sumValue1);//求和有起始值：20
System.out.println("求和有起始值：" + sumValue2);//求和有起始值：20
4 stream中的排序
sorted()对stream进行自然顺序排序，或传入Comparator实现自定义的排序
public static void main(String[] args) {
List<Student> classOneStudents = new ArrayList<>();
List<String> enrolOrders = Arrays.asList("Luis", "Weison", "Jack", "Evan");
classOneStudents.add(Student.builder().age(19).name("Evan").score(80).build());
classOneStudents.add(Student.builder().age(18).name("Weison").score(90).build());
classOneStudents.add(Student.builder().age(21).name("Luis").score(60).build());
classOneStudents.add(Student.builder().age(20).name("Jack").score(70).build());
// 按照入学顺序enrolOrders进行展示
//1 生成一个Map<studentName,Student>
Map<String, Student> nameStudentMap= classOneStudents.stream()
.collect(Collectors.toMap(student -> student.getName(), Function.identity(),
(student1, student2) -> student2));
//2 按照enrolOrders中的name顺序去Map<studentName,Student>中拿Student,并生成新的List
enrolOrders.stream()
.map(enrolOrder->nameStudentMap.get(enrolOrder))
.forEach(System.out::println);
}
5 collect 分组操作
toMap()时，最好加上mergeFucntion，以免java.lang.IllegalStateException: Duplicate key
public static void main(String[] args) {
List<String> strings = Arrays.asList("555", "333", "444", "111", "222", "666", "555", null);
List<Integer> integers = Arrays.asList(555, 333, 444, 111, 222, 666);
List<Student> classOneStudents = new ArrayList<>();
List<Student> classTwoStudents = new ArrayList<>();
List<Student> classThreeStudents = new ArrayList<>();
List<List<Student>> students = new ArrayList<>();
classOneStudents.add(Student.builder().age(5).name("Weison").sex("男").score(90).citationCount(1).build());
classOneStudents.add(Student.builder().age(6).name("Evan").sex("女").score(80).citationCount(2).build());
classOneStudents.add(Student.builder().age(7).name("Jack").sex("男").score(70).citationCount(2).build());
classOneStudents.add(Student.builder().age(8).name("Luis").sex("女").score(60).citationCount(4).build());
classTwoStudents.add(Student.builder().age(9).name("Elen").sex("女").score(80).citationCount(2).build());
classTwoStudents.add(Student.builder().age(10).name("Obam").sex("男").score(70).citationCount(5).build());
classTwoStudents.add(Student.builder().age(11).name("Bush").sex("女").score(60).citationCount(3).build());
classThreeStudents.add(Student.builder().age(12).name("Jackson").sex("男").score(70).citationCount(1).build());
classThreeStudents.add(Student.builder().age(13).name("Linn").sex("女").score(60).citationCount(1).build());
students.add(classOneStudents);
students.add(classTwoStudents);
students.add(classThreeStudents);
// toMap
// 不添加mergeFucntion 会报 java.lang.IllegalStateException: Duplicate key 5551
Map<String, String> stringsMap =
strings.stream()
.filter(element -> null != element)
.collect(Collectors.toMap(stringsValue -> stringsValue,
stringsValue -> stringsValue + "1",
(stringsValue1, stringsValue2) -> stringsValue2));
System.out.println("stringsMap-->" + stringsMap);
Map<String, Student> studentMap =
classOneStudents.stream()
.collect(Collectors.toMap(student -> student.getName(),
Function.identity(),
(student1, student2) -> student2));
System.out.println("studentMap-->" + studentMap);
// groupingBy 分组并求每组的数量
Map<String, Long> stringLongMap = students.stream()
.flatMap(student -> student.stream())
.collect(Collectors.groupingBy(Student::getSex, Collectors.counting()));
System.out.println("stringLongMap--->" + stringLongMap); // {女=5, 男=4}
}
