Java Stream Examples

Problem #1: Given an integer array sorted in increasing order, return an array of the squares of each number sorted in increasing order.

Solution:

    public static int[] sortedSquaresArray(int[] numbers) {
        return Arrays.stream(numbers).boxed()
                .map(val -> val * val)
                .sorted()
                .mapToInt(value -> value.intValue())
                .toArray();
    }

If you want to return the squares of each number sorted in descending order then you can use .sorted(Collections.reverseOrder())

Problem #2: Given ArrayList of Employee (name, age) object. Print the names of employees having age greater than 20 saperated with comma.

Solution:

    String str = employeeList.stream()
                        .filter(e -> e.age >20)
                        .map(e -> e.name)
                        .collect(Collectors.joining(","));

    System.out.println(str);

Problem#3: Create a reverse map in Java , which means change the key as value and value as key.

Solution:

     sourceMap.forEach((key,value) -> inverseMap.put(value,key))

This is equivalent to:

         for (Map.Entry entry : sourceMap.entrySet()){
            inverseMap.put(entry.getValue(), entry.getKey());
        }

Problem#4: Given a List of Integer. Find out sum of top 3 elements in the given list.

Solution:

     List intList = new ArrayList<>();

      intList.add(34);
      intList.add(45);
      intList.add(50);
      intList.add(300);

     int topElementSum = intList.stream().sorted(Comparator.reverseOrder())
              .limit(3)
              .mapToInt(Integer::intValue)
              .sum();
     System.out.println(topElementSum);

We create a stream of Integer objects via Collection.stream() and then perform Stream.sorted() to sort the stream in descending order followed by Stream.limit() to get first 3 element.
Then convert stream to IntStream using Stream.mapToInt() to get sum() of elements.

Problem#5:Use stream API to filter out all even numbers from a list of integers?

Solution:

     List numbers = Arrays.asList(1,2,3,4,5,6,7,8,9,10);

     List evenNumbers = intList.stream().filter(i -> i%2 == 0).collect(Collectors.toList());

Problem#6:What is difference between map and flatMap in Stream API?

Solution:

It is one of the most common question asked during interview.

Map:

map is an intermediate operation used to transforms each element of stream into another object, maintaining the one-to-one relationship.

Stream.of("apple", "banana", "carrot").map(e -> e.length());
-- output will be - [5, 6, 6].

FlatMap:

FlatMap is an intermediate operation used when one element can result in multiple elements. It flaten the structure into one stream.

Example:
The list before flattening:[ [8, 1, 6], [10, 11, 12], [13, 14, 15] ]
After Flattening: [ 8, 1, 6, 10, 11, 12, 12, 14, 15 ]

  List> listOfStringList = Arrays.asList(
            Arrays.asList("Hello", "how"),
            Arrays.asList("are", "you"),
            Arrays.asList("welcome", "to"),
            Arrays.asList("Java", "Stream programming")
        );

  listOfStringList.stream()
                   .flatMap(list -> list.stream())
                   .forEach(System.out::println);

Output:
Hello
how
are
you
welcome
to
Java
Stream programming

Problem#7:What is the use of reduce method in stream?

Solution:

reduce operation performs a reduction on the elements of the stream, using a binary operation to combine them.

Example:

Optional sum = Stream.of(1, 2, 3, 4).reduce(Integer::sum);

Here the stream of numbers is reduce to single element which is sum of all numbers in stream.