Difference Between map() And flatMap() In Java Stream

<R> Stream<R> map(Function<? super T, ? extends R> mapper)

<R> Stream<R> flatMap(Function<? super T, ? extends Stream<? extends R>> mapper)

map()	flatMap()
The function passed to map() operation returns a single value for a single input.	The function you pass to flatmap() operation returns an arbitrary number of values as the output.
One-to-one mapping occurs in map().	One-to-many mapping occurs in flatMap().
Only perform the mapping.	Perform mapping as well as flattening.
Produce a stream of value.	Produce a stream of stream value.
map() is used only for transformation.	flatMap() is used both for transformation and mapping.

                             
                                    // Java program using map() function
                                   
                                    import
                                   
                                    java.io.*;
                                   
                                    import
                                   
                                    java.util.*;
                                   
                                    import
                                   
                                    java.util.ArrayList;
                                   
                                    import
                                   
                                    java.util.List;
                                   
                                    import
                                   
                                    java.util.stream.Collectors;
                                   
                                    class
                                   
                                    GFG {
                                   
                                    public
                                   
                                    static
                                   
                                    void
                                   
                                    main(String[] args)
                                   
                                    {
                                   
                                    // making the array list object
                                   
                                    ArrayList<String> fruit =
                                   
                                    new
                                   
                                    ArrayList<>();
                                   
                                    fruit.add(
                                   
                                    "Apple"
                                   
                                    );
                                   
                                    fruit.add(
                                   
                                    "mango"
                                   
                                    );
                                   
                                    fruit.add(
                                   
                                    "pineapple"
                                   
                                    );
                                   
                                    fruit.add(
                                   
                                    "kiwi"
                                   
                                    );
                                   
                                    System.out.println(
                                   
                                    "List of fruit-"
                                   
                                    + fruit);
                                   
                                    // lets use map() to convert list of fruit
                                   
                                    List list = fruit.stream()
                                   
                                    .map(s -> s.length())
                                   
                                    .collect(Collectors.toList());
                                   
                                    System.out.println(
                                   
                                    "List generated by map-"
                                   
                                    + list);
                                   
                                    }
                                   
                                    }

List of fruit-[Apple, mango, pineapple, kiwi]
List generated by map-[5, 5, 9, 4]

                                 
                                        // Java program using flatMap() function
                                       
                                        import
                                       
                                        java.io.*;
                                       
                                        import
                                       
                                        java.util.*;
                                       
                                        import
                                       
                                        java.util.ArrayList;
                                       
                                        import
                                       
                                        java.util.List;
                                       
                                        import
                                       
                                        java.util.stream.Collectors;
                                       
                                        class
                                       
                                        GFG {
                                       
                                        public
                                       
                                        static
                                       
                                        void
                                       
                                        main(String[] args)
                                       
                                        {
                                       
                                        // making the arraylist object of List of Integer
                                       
                                        List<List<Integer> > number =
                                       
                                        new
                                       
                                        ArrayList<>();
                                       
                                        // adding the elements to number arraylist
                                       
                                        number.add(Arrays.asList(
                                       
                                        1
                                       
                                        ,
                                       
                                        2
                                       
                                        ));
                                       
                                        number.add(Arrays.asList(
                                       
                                        3
                                       
                                        ,
                                       
                                        4
                                       
                                        ));
                                       
                                        number.add(Arrays.asList(
                                       
                                        5
                                       
                                        ,
                                       
                                        6
                                       
                                        ));
                                       
                                        number.add(Arrays.asList(
                                       
                                        7
                                       
                                        ,
                                       
                                        8
                                       
                                        ));
                                       
                                        System.out.println(
                                       
                                        "List of list-"
                                       
                                        + number);
                                       
                                        // using flatmap() to flatten this list
                                       
                                        List<Integer> flatList
                                       
                                        = number.stream()
                                       
                                        .flatMap(list -> list.stream())
                                       
                                        .collect(Collectors.toList());
                                       
                                        // printing the list
                                       
                                        System.out.println(
                                       
                                        "List generate by flatMap-"
                                       
                                        + flatList);
                                       
                                        }
                                       
                                        }