Useful tools in java

Index

• 1. Grouping elements of a List -Java 8
• 2. Sorting a map by values - Java 8
• 3. Get first element in a map

1. Grouping elements of a List -Java 8

import java.io.*;
import java.util.*;
import java.util.Map;
import java.util.stream.Collectors;
import java.util.function.Function;
...

List<Integer> list = new ArrayList<Integer>();
list.add(1);
list.add(3);
list.add(1);
list.add(3);
list.add(2);
list.add(1);

Map<Integer,Long> group = list
                          .stream()
                          .collect
                           (
                             Collectors
                             .groupingBy
                              (
                                 Function.identity()
                                ,Collectors.counting()
                              )
                           )
  ;
System.out.println(group); // out: {1=3, 2=1, 3=2} 

2. Sorting a map by values - Java 8

 import java.io.*;
 import java.util.*;
...
 Map<Integer,Long> list = new HashMap<Integer,Long>();
list.put(3,0L);
list.put(2,5L);
list.put(1,3L);
list.put(4,2L);
 Map<Integer, Long> finalMap = new LinkedHashMap<>();
//This method does not mutate the original map
list
.entrySet()
.stream()
.sorted
(
  Map
  .Entry
  .<Integer,Long>comparingByValue()
  .reversed()
)
.forEachOrdered(e -> finalMap.put(e.getKey(),e.getValue()));

System.out.println(finalMap); //out: {2=5, 1=3, 4=2, 3=0}

3. Get first element in a map 

import java.io.*;
import java.util.*;
...
Map<Integer,Long> map = new HashMap<Integer,Long>();
map.put(3,0L);
map.put(2,5L);
map.put(1,3L);
map.put(4,2L);

System.out.println("Key:"+map.entrySet().stream().findFirst().get().getKey()); //out: Key:1
System.out.print("Value:"+map.entrySet().stream().findFirst().get().getValue());//out: Value:3