Some useful stream operations

Separate Odd and Even numbers (List<Integer> to Map<String, List<Integer>>)

Map<String, List<Integer>> oddEvenMap = numbers.stream()  
        .collect(Collectors.groupingBy(i -> (i % 2 != 0 ? "Odd" : "Even")));

Find distinct values from Map<String, List<Integer>

Set<Integer> set = map.values().stream()
					.flatMap(Collection::stream)
					.collect(Collectors.toSet());

Biggest Odd and Even from List (List<Integer> to Map<String, Integer>)

Map<String, Integer> bigOddEven = numbers.stream().collect(Collectors.toMap(i -> (i % 2 == 0 ? "Even" : "Odd"),  Function.identity(), (i1 , i2) -> i1 > i2 ? i1 : i2));

Use value as key and create new map from existing

Map<String, Integer> map = new HashMap<>();  
map.put("a", 1);  
map.put("b", 1);  
map.put("c", 2);  
  
Map<Integer, List<String>> ans = map.keySet().stream().collect(Collectors.groupingBy(map::get));  
System.out.println(ans);

Output

{1=[a, b], 2=[c]}

Find frequency of each number in list

List<Integer> numbers = new ArrayList<>();  
numbers.add(1);  
numbers.add(2);  
numbers.add(3);  
numbers.add(4);  
numbers.add(4);  
  
Map<Integer, Long> ans = numbers.stream()  
        .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));  
System.out.println(ans);

Output

{1=1, 2=1, 3=1, 4=2}

Remove from map based on condition on values

Map<String, Integer> map = new HashMap<>();  
map.put("a", 1);  
map.put("b", 1);  
map.put("c", 2);  
  
map.values().removeIf(i -> i == 1);  
System.out.println(map);

Output

{c=2}

Distinct and Sum of numbers

Stream<Integer> distinct = numbers.stream().distinct();
int s = numbers.stream().mapToInt(Integer::intValue).sum();

IntSummaryStatistics for Average, Sum, Min/Max

IntSummaryStatistics intSummaryStatistics = numbers.stream().mapToInt(Integer::intValue).summaryStatistics();
double average = intSummaryStatistics.getAverage();
long sum = intSummaryStatistics.getSum();
int max = intSummaryStatistics.getMax();
int min = intSummaryStatistics.getMin();
long count = intSummaryStatistics.getCount();