A detailed analysis of flatMap method in Java 8 Stream API

Preface

The flatMap method is an important intermediate operation in the Stream API, which not only converts each element in the stream into a new stream, but also combines multiple streams into a single stream.

The flatMap method is often used to handle nested collections or to flatten elements in a collection.

1. Definition of flatMap method

The flatMap method is defined as follows:

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

This method takes a function type parameter, i.e. a function that converts each element in the stream into another stream, and returns a new flattened stream containing the converted elements.

2. Use scenarios of flatMap method

The flatMap method is widely used in the following scenarios:

• Nested Collection Flattening: Convert nested collections into a single stream.
• One-to-many mapping: Convert each element in the stream into a stream of multiple elements.

3. Example of flatMap method

Here are some examples of flatMap methods that show their application in handling nested collections and one-to-many mappings.

3.1 Nested collection flattening

Suppose there is a list of multiple strings, which we want to flatten into a single list of all strings.

List<List<String>> nestedList = (
    ("apple", "banana"),
    ("cherry", "date"),
    ("elderberry", "fig", "grape")
);

List<String> flatList = ()
    .flatMap(List::stream)
    .collect(());

("Flat List: " + flatList);

Output result:

Flat List: [apple, banana, cherry, date, elderberry, fig, grape]

With the flatMap method, we can flatten the nested list into a single list containing all elements.

3.2 One-to-many mapping

Suppose there is a list of strings, we want to split each string into a single character and collect all characters into a list.

List<String> words = ("apple", "banana", "cherry");

List<String> characters = ()
    .flatMap(word -> (("")))
    .collect(());

("Characters: " + characters);

Output result:

Characters: [a, p, p, l, e, b, a, n, a, n, a, c, h, e, r, r, y]

With the flatMap method, we can split each string into a single character and collect all characters into a new list.

3.3 Handling complex objects

Suppose there is a list containing user objects, each user has a list containing multiple addresses, we want to extract all addresses of all users and collect the results into a new list.

class User {
    String name;
    List<String> addresses;

    User(String name, List<String> addresses) {
         = name;
         = addresses;
    }

    public List<String> getAddresses() {
        return addresses;
    }
}

List<User> users = (
    new User("Alice", ("123 Main St", "456 Oak St")),
    new User("Bob", ("789 Pine St")),
    new User("Charlie", ("101 Maple St", "202 Birch St", "303 Cedar St"))
);

List<String> allAddresses = ()
    .flatMap(user -> ().stream())
    .collect(());

("All Addresses: " + allAddresses);

Output result:

All Addresses: [123 Main St, 456 Oak St, 789 Pine St, 101 Maple St, 202 Birch St, 303 Cedar St]

With the flatMap method, we can extract all addresses of all users and collect them into a new list.

4. Things to note about flatMap method

When using the flatMap method, you need to pay attention to the following points:

• Function implementation: The flatMap method depends on the implementation of the Function interface, so the implementation logic needs to be accurate to ensure the correct conversion result.
• Flattening of streams: The flatMap method not only transforms elements in the stream, but also merges multiple streams into a single stream, eliminating nested structures.
• Performance impact: For large data sets, frequent use of complex flatMap operations may affect performance, and the transformation logic should be optimized as much as possible.

Through the understanding and application of the flatMap method, we can efficiently process and transform nested collections, thus writing simpler and easier to maintain code.

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