Mastering Java Streams and Collectors: Modern Collection Processing
Mastering Java Streams and Collectors: Modern Collection Processing
Java 8 introduced Streams and Collectors, revolutionizing how we process collections. This post delves into these powerful features, showcasing how they streamline code, enhance readability, and improve performance compared to traditional approaches.
1. Enhanced Collection Sorting
Sorting collections has become significantly more concise and readable since Java 8. Let's illustrate this with an example of sorting and modifying users based on their points.
Modern Code (Java 8+)
The following code efficiently sorts users by points (descending order), selects the top 10, and promotes them to moderators. Notice its compactness and clarity:
usuarios.sort(Comparator.comparing(Usuario::getPontos).reversed());
usuarios
.subList(0,10)
.forEach(Usuario::tornaModerador);
File: Seleciona10.java
Legacy Code (Pre-Java 8)
The pre-Java 8 approach is verbose and less intuitive. It requires explicit Comparator implementation, list reversal, and manual iteration:
Collections.sort(usuarios, new Comparator<Usuario>() {
@Override
public int compare(Usuario u1, Usuario u2) {
return u1.getPontos() - u2.getPontos();
}
});
Collections.reverse(usuarios);
List<Usuario> top10 = usuarios.subList(0, 10);
for (Usuario usuario : top10) {
usuario.tornaModerador();
}
File: Seleciona10Antigo.java
2. Streams: Efficiently Filtering and Transforming Collections
Before Java 8, filtering collections often involved explicit loops and conditional statements. Let's look at how to make moderators of users with over 100 points.
Traditional Filtering in Java (Pre-Java 8)
Imperative and verbose, this approach requires multiple code blocks for every added condition:
for (Usuario usuario : usuarios) {
if (usuario.getPontos() > 100) {
usuario.tornaModerador();
}
}
File: Moderador100Ponto.java
The Absence of filter in the Collections API
Unlike sort (in List) and forEach (in Iterable), a dedicated filter method wasn't added directly to Collection or List. This decision was influenced by the already large Collection API and the emerging need for fluent, chained operations in the new Streams API.
Introducing Java 8 Streams
Streams provide a functional and fluent way to process collections. Crucially, they don't modify the original collection, preventing unexpected side effects.
Creating a Stream from a List
The stream() method, added to the Collection interface, creates a stream:
Stream<Usuario> stream = usuarios.stream();
The filter() method, accepting a Predicate, allows for element filtering:
Stream<Usuario> stream = usuarios.stream()
.filter(u -> u.getPontos() > 100);
This can be further simplified:
usuarios.stream()
.filter(u -> u.getPontos() > 100);
File: Moderador100PontoStreams.java
Streams Don't Modify the Original Collection
filter() and other stream operations leave the original list unchanged:
usuarios.stream()
.filter(u -> u.getPontos() > 100);
usuarios.forEach(System.out::println); // Prints all users
To apply the transformation, the resulting stream must be processed:
usuarios.stream()
.filter(u -> u.getPontos() > 100)
.forEach(System.out::println);
File: MantemOriginal.java
Streams Aren't Collections
Unlike List, a Stream doesn't store data. It operates more like an Iterator, processing elements on demand. This allows for fluent chaining of operations.
Fluent Stream Usage
The best practice is to chain operations directly:
usuarios.stream()
.filter(u -> u.getPontos() > 100)
.forEach(System.out::println);
forEach() returns void, ending the stream. To perform further operations, stream() must be called again.
Complete Example: Filtering and Modifying Users
Optimized code using Streams:
usuarios.stream()
.filter(u -> u.getPontos() > 100)
.forEach(Usuario::tornaModerador);
File: FiltraModerador.java
Using Method References in filter
When filter calls a single method, a method reference can be used:
usuarios.stream()
.filter(Usuario::isModerador);
File: FiltraModerador.java
Conclusion
Java Streams and Collectors provide a powerful, efficient, and readable way to process collections. By understanding their capabilities and best practices, developers can write cleaner, more maintainable code. This shift towards functional programming paradigms offers significant advantages in terms of conciseness and expressiveness.
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