Flatten a Stream of Lists in Java using forEach loop
Last Updated : 11 Dec, 2018
Given a Stream of Lists in Java, the task is to Flatten the Stream using forEach() method.
Examples: Input: lists = [ [1, 2], [3, 4, 5, 6], [8, 9] ] Output: [1, 2, 3, 4, 5, 6, 7, 8, 9] Input: lists = [ ['G', 'e', 'e', 'k', 's'], ['F', 'o', 'r'] ] Output: [G, e, e, k, s, F, o, r]
Approach: - Get the Lists in the form of 2D list.
- Create an empty list to collect the flattened elements.
- With the help of forEach loop, convert each elements of the list into stream and add it to the list
- Now convert this list into stream using stream() method.
- Now flatten the stream by converting it into list using collect() method.
Below is the implementation of the above approach:
Example 1: Using lists of integer.
Java // Java program to flatten a stream of lists // using forEach() method import java.util.ArrayList; import java.util.Arrays; import java.util.List; import java.util.stream.*; class GFG { // Function to flatten a Stream of Lists public static <T> Stream<T> flattenStream(List<List<T> > lists) { // Create an empty list to collect the stream List<T> finalList = new ArrayList<>(); // Using forEach loop // convert the list into stream // and add the stream into list for (List<T> list : lists) { list.stream() .forEach(finalList::add); } // Convert the list into Stream and return it return finalList.stream(); } public static void main(String[] args) { // Get the lists to be flattened. List<Integer> a = Arrays.asList(1, 2); List<Integer> b = Arrays.asList(3, 4, 5, 6); List<Integer> c = Arrays.asList(7, 8, 9); List<List<Integer> > arr = new ArrayList<List<Integer> >(); arr.add(a); arr.add(b); arr.add(c); // Flatten the Stream List<Integer> flatList = new ArrayList<Integer>(); flatList = flattenStream(arr) .collect(Collectors.toList()); // Print the flattened list System.out.println(flatList); } } Output: [1, 2, 3, 4, 5, 6, 7, 8, 9]
Example 2: Using lists of Characters.
Java // Java program to flatten a stream of lists // using forEach() method import java.util.ArrayList; import java.util.Arrays; import java.util.List; import java.util.stream.*; class GFG { // Function to flatten a Stream of Lists public static <T> Stream<T> flattenStream(List<List<T> > lists) { // Create an empty list to collect the stream List<T> finalList = new ArrayList<>(); // Using forEach loop // convert the list into stream // and add the stream into list for (List<T> list : lists) { list.stream() .forEach(finalList::add); } // Convert the list into Stream and return it return finalList.stream(); } public static void main(String[] args) { // Get the lists to be flattened. List<Character> a = Arrays.asList('G', 'e', 'e', 'k', 's'); List<Character> b = Arrays.asList('F', 'o', 'r'); List<Character> c = Arrays.asList('G', 'e', 'e', 'k', 's'); List<List<Character> > arr = new ArrayList<List<Character> >(); arr.add(a); arr.add(b); arr.add(c); // Flatten the Stream List<Character> flatList = new ArrayList<Character>(); flatList = flattenStream(arr) .collect(Collectors.toList()); // Print the flattened list System.out.println(flatList); } } Output: [G, e, e, k, s, F, o, r, G, e, e, k, s]
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