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 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 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]
