What's the difference between map() and flatMap() methods in Java 8?

Both map and flatMap can be applied to a Stream<T> and they both return a Stream<R>. The difference is that the map operation produces one output value for each input value, whereas the flatMap operation produces an arbitrary number (zero or more) values for each input value.

This is reflected in the arguments to each operation.

The map operation takes a Function, which is called for each value in the input stream and produces one result value, which is sent to the output stream.

The flatMap operation takes a function that conceptually wants to consume one value and produce an arbitrary number of values. However, in Java, it's cumbersome for a method to return an arbitrary number of values, since methods can return only zero or one value. One could imagine an API where the mapper function for flatMap takes a value and returns an array or a List of values, which are then sent to the output. Given that this is the streams library, a particularly apt way to represent an arbitrary number of return values is for the mapper function itself to return a stream! The values from the stream returned by the mapper are drained from the stream and are passed to the output stream. The "clumps" of values returned by each call to the mapper function are not distinguished at all in the output stream, thus the output is said to have been "flattened."

Typical use is for the mapper function of flatMap to return Stream.empty() if it wants to send zero values, or something like Stream.of(a, b, c) if it wants to return several values. But of course any stream can be returned.

Stream.flatMap, as it can be guessed by its name, is the combination of a map and a flat operation. That means that you first apply a function to your elements, and then flatten it. Stream.map only applies a function to the stream without flattening the stream.

To understand what flattening a stream consists in, consider a structure like [ [1,2,3],[4,5,6],[7,8,9] ] which has "two levels". Flattening this means transforming it in a "one level" structure : [ 1,2,3,4,5,6,7,8,9 ].

I would like to give 2 examples to get a more practical point of view:
First example making usage of map:

@Test
public void convertStringToUpperCaseStreams() {
    List<String> collected = Stream.of("a", "b", "hello") // Stream of String 
            .map(String::toUpperCase) // Returns a stream consisting of the results of applying the given function to the elements of this stream.
            .collect(Collectors.toList());
    assertEquals(asList("A", "B", "HELLO"), collected);
}

Nothing special in the first example, a Function is applied to return the String in uppercase.

Second example making usage of flatMap:

@Test
public void testflatMap() throws Exception {
    List<Integer> together = Stream.of(asList(1, 2), asList(3, 4)) // Stream of List<Integer>
            .flatMap(List::stream)
            .map(integer -> integer + 1)
            .collect(Collectors.toList());
    assertEquals(asList(2, 3, 4, 5), together);
}

In the second example, a Stream of List is passed. It is NOT a Stream of Integer!
If a transformation Function has to be used (through map), then first the Stream has to be flattened to something else (a Stream of Integer).
If flatMap is removed then the following error is returned: The operator + is undefined for the argument type(s) List, int.
It is NOT possible to apply + 1 on a List of Integers!