Java8 array, list operation sink "5")-Java8 LAMBDA list statistics (sum, Maximum, minimum, average)

 Public classApple {PrivateInteger ID; PrivateString name; PrivateBigDecimal money; PrivateInteger num;  PublicApple (integer ID, String name, BigDecimal money, integer num) { This. ID =ID;  This. Name =name;  This. Money =Money ;  This. num =num; }}

 list<apple> applelist = new  arraylist<> (); //  Store Apple Object collection  apple apple1  = new  Apple (1, "Apple 1", new  BigDecimal ("3.25"), 10 = new  Apple (1, "Apple 2", new  BigDecimal ("1.35"), 20 = new  Apple (2, "banana", new  BigDecimal ("2.89"), 30 = new  Apple (3, "Lychee", new  BigDecimal ("9.99"), 40 

List.stream (). maptodouble (User::getheight). SUM ()//And List.stream (). maptodouble (User::getheight). Max ()// Maximum List.stream (). maptodouble (user::getheight). Min ()//min. list.stream (). maptodouble (User::getheight). Average ()// Average

1. List to MapID to Key,apple object as value, you can do this:/**  * List---MAP * Note that: * Tomap if the collection object has duplicate key, will error duplicate Key .... *  the ID of the APPLE1,APPLE12 is 1. *  */Map<integer, apple> applemap = Applelist.stream (). Collect (Collectors.tomap ( Apple::getid, A-A, (K1,K2)-K1); print applemap:{1=apple{id=1, name= ' Apple 1 ', money=3.25, num=10}, 2=apple{ id=2, name= ' banana ', money=2.89, num=30}, 3=apple{id=3, Name= ' lychee ', money=9.99, num=40}

2. Group the object elements in the List, grouped by an attribute, for example, by grouping IDs together with the same ID://list is grouped by ID map<integer,list<apple> >Map<integer, list<apple>> groupBy = applelist.stream (). Collect (Collectors.groupingby ( Apple::getid)); System.err.println ("groupBy:" +groupBy); {1=[apple{id=1, name= ' Apple 1 ', money=3.25, num=10}, apple{id=1, Name= ' Apple 2 ', money=1.35, num=20}], 2=[apple{id=2, name = ' banana ', money=2.89, num=30}], 3=[apple{id=3, name= ' lychee ', money=9.99, num=40}]}

3Filter: Filter out the set of eligible elements:// filter out eligible data list<apple> filterlist = Applelist.stream (). Filter (A-a.getname (). Equals ("banana")). Collect (Collectors.tolist ()); System.err.println ("filterlist:" +filterlist); [Apple{id=2, name= ' banana ', money=2.89, num=30}]

4. Sum: Sums the data in the collection according to a property: BigDecimal://Calculate Total AmountBigDecimal Totalmoney =Applelist.stream (). Map (Apple::getmoney). Reduce (Bigdecimal.zero, bigdecimal::add); System.err.println ("Totalmoney:" +totalmoney);//totalmoney:17.48Integer://Calculate Quantityintsum =Applelist.stream (). Maptoint (Apple::getnum). sum (); System.err.println ("Sum:" +sum);//sum:100List<Integer> cc =NewArraylist<>(); Cc.add (1); Cc.add (2); Cc.add (3);intsum = Cc.stream (). Maptoint (Integer::intvalue). sum ();//6

★5Collectors.groupingby to group, sort, and so on:ImportjavaX.Model.Student;Importjava.util.Arrays;ImportJava.util.HashMap;Importjava.util.List;ImportJava.util.Map;Importjava.util.function.Function;Importjava.util.stream.Collectors; Public classFunctionx { Public Static voidMain (string[] args) {//1. Grouping CountList<student> list1= Arrays.aslist (NewStudent (1, "One", "Zhao"),NewStudent (2, "one", "Qian"),NewStudent (3, "I", "Sun")); //1.1 grouping counts based on an attributeMap<string,long> result1=List1.stream (). Collect (Collectors.groupingby (student::getgroupid,collectors.counting ()));        System.out.println (RESULT1); //1.2 counts against the entire entity object, when it is used as a stringMap<student,long> result2=List1.stream (). Collect (Collectors.groupingby (function.identity (), collectors.counting ()));        System.out.println (RESULT2); //1.3 Sorts the result according to the key value of the grouping, puts it in another map, and outputsMap<string,long> xmap=NewHashmap<>(); Result1.entryset (). Stream (). Sorted (map.entry.<string,long>comparingbykey (). Reversed ())//reversed does not take effect. foreachordered (x->Xmap.put (X.getkey (), X.getvalue ()));        System.out.println (XMAP); //2. Group and count one of the attributes worth sum or avg:id sumMap<string,integer> result3=List1.stream (). Collect (Collectors.groupingby (Student::getgroupid,collectors.summingint (Student::getid)        )        );    System.out.println (RESULT3); }}

Https://www.cnblogs.com/yangweiqiang/p/6934671.html

Java8 array, list operation sink "5")-Java8 LAMBDA list statistics (sum, Maximum, minimum, average)