Java 8 example – Stream Collectors groupingBy

Group By, Count and Sort

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<string> items =
                Arrays.asList("apple", "apple", "banana",
                        "apple", "orange", "banana", "papaya");

        Map<string, long> result =
                items.stream().collect(
                        Collectors.groupingBy(
                                Function.identity(), Collectors.counting()
                        )
                );

        System.out.println(result);


    }
}

Output:

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

sorting

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 items =
                Arrays.asList("apple", "apple", "banana",
                        "apple", "orange", "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

Group by Price – Collectors.groupingBy and Collectors.mapping example.

import java.math.BigDecimal; import java.util.Arrays; import java.util.List; import java.util.Map; import java.util.Set; import java.util.stream.Collectors;  public class Java8Examples4 {      public static void main(String[] args) {          //3 apple, 2 banana, others 1         List items = Arrays.asList(                 new Item("apple", 10, new BigDecimal("9.99")),                 new Item("banana", 20, new BigDecimal("19.99")),                 new Item("orang", 10, new BigDecimal("29.99")),                 new Item("watermelon", 10, new BigDecimal("29.99")),                 new Item("papaya", 20, new BigDecimal("9.99")),                 new Item("apple", 10, new BigDecimal("9.99")),                 new Item("banana", 10, new BigDecimal("19.99")),                 new Item("apple", 20, new BigDecimal("9.99"))                 );  		//group by price         Map<bigdecimal,  list<Item>> groupByPriceMap = 			items.stream().collect(Collectors.groupingBy(Item::getPrice));          System.out.println(groupByPriceMap);  		// group by price, uses 'mapping' to convert List to Set        Map<bigdecimal, set<string>> result =                 items.stream().collect(                         Collectors.groupingBy(Item::getPrice,                                 Collectors.mapping(Item::getName, Collectors.toSet())                         )                 );          System.out.println(result);      } }