Shulou(Shulou.com)06/02 Report--

Today, I will talk to you about how to operate the collection of Stream pairs in Java8. Many people may not know much about it. In order to make you understand better, the editor has summarized the following content for you. I hope you can get something according to this article.

First of all, in order to illustrate the operation of Stream on the object collection, create a new Student class (student class) and override the equals () and hashCode () methods

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 (screening) public static void main (String [] args) {Student S1 = new Student (1L, "Xiao Zhan", 15, "Zhejiang"); Student S2 = new Student (2L, "Wang Yibo", 15, "Hubei"); Student S3 = new Student (3L, "Yang Zi", 17, "Beijing") Student S4 = new Student (4L, "Li Xian", 17, "Zhejiang"); 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) } / * screening of collections * @ param students * @ return * / private static List testFilter (List students) {/ / screening students over 15 years old / / return students.stream () .filter (s-> s.getAge () > 15) .screening (Collectors.toList ()) / / screen students living in Zhejiang Province return students.stream (). Filter (s-> "Zhejiang" .equals (s.getAddress () .students (Collectors.toList ());}

Running result:

Here we created four students, after filter screening, screened out the address is a collection of students from Zhejiang.

Map (conversion) public static void main (String [] args) {Student S1 = new Student (1L, "Xiao Zhan", 15, "Zhejiang"); Student S2 = new Student (2L, "Wang Yibo", 15, "Hubei"); Student S3 = new Student (3L, "Yang Zi", 17, "Beijing"); Student S4 = new Student (4L, "Li Xian", 17, "Zhejiang") List students = new ArrayList (); students.add (S1); students.add (S2); students.add (S3); students.add (S4); testMap (students) } / * set conversion * @ param students * @ return * / private static void testMap (List students) {/ / add some information in front of the address to get only the address output List addresses = students.stream () .map (s-> "residential address:" + s.getAddress ()) .address (Collectors.toList ()); addresses.forEach (a-> System.out.println (a)) }

Running result

Map is to transform the corresponding elements according to the given method.

Distinct (deduplication) public static void main (String [] args) {testDistinct1 ();} / * set deduplication (basic type) * / private static void testDistinct1 () {/ / simple string deduplication List list = Arrays.asList ("111,222,333,111,222,222") List.stream (). Distinct (). ForEach (System.out::println);}

Running result:

Public static void main (String [] args) {testDistinct2 ();} / * collection to deduplicate (reference object) * / private static void testDistinct2 () {/ / reference object to implement hashCode and equal methods, otherwise deduplicated invalid Student S1 = new Student (1L, "Xiao Zhan", 15, "Zhejiang") Student S2 = new Student (2L, "Wang Yibo", 15, "Hubei"); Student S3 = new Student (3L, "Yang Zi", 17, "Beijing"); Student S4 = new Student (4L, "Li Xian", 17, "Zhejiang"); Student S5 = new Student (1L, "Xiao Zhan", 15, "Zhejiang"); 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);}

Running result:

It can be seen that two repeated "Xiao Zhan" students have removed the weight, not only because of the use of the distinct () method, but also because the Student object overrides the equals and hashCode () methods, otherwise deduplication is invalid.

Sorted (sort) public static void main (String [] args) {testSort1 ();} / * set sort (default sort) * / private static void testSort1 () {List list = Arrays.asList ("333,222,111"); list.stream (). Sorted (). ForEach (System.out::println);}

Running result:

Public static void main (String [] args) {testSort2 ();} / * set sort (specify collation) * / private static void testSort2 () {Student S1 = new Student (1L, "Xiao Zhan", 15, "Zhejiang"); Student S2 = new Student (2L, "Wang Yibo", 15, "Hubei") Student S3 = new Student (3L, "Yang Zi", 17, "Beijing"); Student S4 = new Student (4L, "Li Xian", 17, "Zhejiang"); List students = new ArrayList (); students.add (S1); students.add (S2); students.add (S2); 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);}

Running result:

The sorting rules specified above are sorted in descending order according to the students'id, and then by age.

Limit (limit the number of returns) public static void main (String [] args) {testLimit ();} / * set limit, return the first few elements * / private static void testLimit () {List list = Arrays.asList ("333,222,111"); list.stream () .limit (2) .forEach (System.out::println);}

Running result:

Skip (delete elements) public static void main (String [] args) {testSkip ();} / * set skip, delete the first n elements * / private static void testSkip () {List list = Arrays.asList ("333,222,111"); list.stream () .skip (2) .forEach (System.out::println);}

Running result:

Reduce (aggregation) public static void main (String [] args) {testReduce ();} / * set reduce, aggregating each element in the collection into a piece of data * / private static void testReduce () {List list = Arrays.asList ("Huan", "Ying", "you"); String appendStr = list.stream (). Reduce ("Beijing", (Azob)-> aqimb) System.out.println (appendStr);}

Running result:

Min (minimum) public static void main (String [] args) {testMin ();} / * find the minimum value of the elements in the set * / private static void testMin () {Student S1 = new Student (1L, "Xiao Zhan", 14, "Zhejiang"); Student S2 = new Student (2L, "Wang Yibo", 15, "Hubei") Student S3 = new Student (3L, "Yang Zi", 17, "Beijing"); Student S4 = new Student (4L, "Li Xian", 17, "Zhejiang"); List students = new ArrayList (); students.add (S1); students.add (S2); students.add (S2); students.add (S4) Student minS = students.stream () .min ((stu1,stu2)-> Integer.compare (stu1.getAge (), stu2.getAge ()) .get (); System.out.println (minS.toString ());}

Running result:

The above is to find the youngest of all the students, max in the same way, to find the maximum.

AnyMatch/allMatch/noneMatch (matching) public static void main (String [] args) {testMatch ();} private static void testMatch () {Student S1 = new Student (1L, "Xiao Zhan", 15, "Zhejiang"); Student S2 = new Student (2L, "Wang Yibo", 15, "Hubei"); Student S3 = new Student (3L, "Yang Zi", 17, "Beijing") Student S4 = new Student (4L, "Li Xian", 17, "Zhejiang"); List students = new ArrayList (); students.add (S1); students.add (S2); students.add (S3); students.add (S4); Boolean anyMatch = students.stream (). AnyMatch (s-> Hubei .equals (s.getAddress () If (anyMatch) {System.out.println ("Hubei people");} Boolean allMatch = students.stream (). AllMatch (s-> s.getAge () > = 15); if (allMatch) {System.out.println ("all students have reached the age of 15") } Boolean noneMatch = students.stream (). NoneMatch (s-> "Yang Yang" .equals (s.getName ()); if (noneMatch) {System.out.println ("no classmate named Yang Yang");}} after reading the above, do you have any further understanding of how to operate Stream sets in Java8? If you want to know more knowledge or related content, please follow the industry information channel, thank you for your support.

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