Stream Collectors Groupingby Example

In this article, we'll show you how to group, count, sum, and sort lists by using Java 8 Stream collectors. 1. Group by, Count and Sort

1.1 Group by A-list and display the total count of it. (Grouped by list and display totals) Java8example1.java

Package com.mkyong.java8;

Import Java.util.Arrays;
Import java.util.List;
Import Java.util.Map;
Import java.util.function.Function;
Import java.util.stream.Collectors;

public class Java8example1 {public

    static void Main (string[] args) {

        //3 Apple, 2 banana, others 1
        list<st ring> items =
                arrays.aslist ("Apple", "apple", "banana",
                        "apple", "orange", "banana", "papaya");

        map<string, long> result =
                Items.stream (). Collect (Collectors.groupingby (
                                ), collectors.counting ()
                        )
                ;

        SYSTEM.OUT.PRINTLN (result);


    }

Output

{
	papaya=1, orange=1, banana=2, Apple=3
}

1.2 Add sorting. (add sort implementation) Java8example2.java

Package com.mkyong.java8;
Import Java.util.Arrays;
Import Java.util.LinkedHashMap;
Import java.util.List;
Import Java.util.Map;
Import java.util.function.Function;

Import java.util.stream.Collectors;
        public class Java8example2 {public static void main (string[] args) {//3 Apple, 2 banana, others 1 list<string> items = arrays.aslist ("Apple", "apple", "banana", "apple", "Ora

        Nge "," banana "," papaya ");
                                map<string, long> result = Items.stream (). Collect (Collectors.groupingby (

        Function.identity (), collectors.counting ());

        map<string, long> finalmap = new linkedhashmap<> (); Sort a map and add to Finalmap Result.entryset (). Stream (). Sorted (map.entry.<string, long> Comparingbyvalue (). Reversed ()). foreachordered (E-> Finalmap.put (E.getkey (), E.getvalue ());


    System.out.println (FINALMAP); }
}

Output

{
	apple=3, banana=2, Papaya=1, Orange=1
}

2. List Objects

Examples to ' GROUP by ' a list of user defined Objects. (an example of grouping with the user-defined Objects list.) ）

2.1 A Pojo. Item.java

Package com.mkyong.java8;

Import Java.math.BigDecimal;

public class Item {

    private String name;
    private int qty;
    Private BigDecimal Price;

    constructors, Getter/setters
}

2.2 Group BY the name + count or sum the Qty. (name + Count group or to Qty sum) Java8examples3.java

package Com.mkyong.java8;
Import Java.math.BigDecimal;
Import Java.util.Arrays;
Import java.util.List;
Import Java.util.Map;

Import java.util.stream.Collectors;
        public class Java8examples3 {public static void main (string[] args) {//3 Apple, 2 banana, others 1 list<item> items = arrays.aslist (new Item ("Apple", New BigDecimal ("9.99")), NE W Item ("Banana", new BigDecimal