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In this guide, you will learn about the Stream collect() method in Java programming and how to use it with an example.
1. Stream collect() Method Overview
Definition:
The Stream.collect() method is a terminal operation that transforms the elements of a stream into a different kind of result, e.g., a collection or an arbitrary value. It performs a mutable reduction operation on the elements of the stream.
Syntax:
<R, A> R collect(Collector<? super T, A, R> collector)Parameters:
- collector: A Collector which specifies how the stream elements should be accumulated, possibly transforming the accumulated result, and returning it.
Key Points:
- The collect() method is a versatile method and can transform a stream's elements into various types of outcomes including collections like lists, sets, or maps.
- Since collect() is a terminal operation, it closes the stream after its execution.
- Common collectors are provided by the Collectors class, which includes utility methods like toList(), toSet(), joining(), and more.
- Custom collectors can be implemented using the Collector interface for more advanced use cases.
- If the stream is parallel, and the collector is concurrent, then the collection process will also be done in parallel.
2. Stream collect() Method - Collect to List Example
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;
public class StreamCollectExample {
public static void main(String[] args) {
Stream<String> fruitsStream = Stream.of("Apple", "Banana", "Cherry", "Date", "Elderberry");
// Using collect to gather stream elements into a list
List<String> fruitsList = fruitsStream.collect(Collectors.toList());
System.out.println(fruitsList);
}
}
Output:
[Apple, Banana, Cherry, Date, Elderberry]
Explanation:
In the example, we have a stream of fruit names. We use the collect() method in conjunction with the Collectors.toList() collector to gather all the names into a List. The resulting list is then printed, demonstrating the use of collect() to produce a collection from a stream.
3. Stream collect() Method - Collect to Set Example
import java.util.Set;
import java.util.stream.Collectors;
import java.util.stream.Stream;
public class SetExample {
public static void main(String[] args) {
Stream<String> stream = Stream.of("Apple", "Banana", "Cherry", "Apple");
Set<String> set = stream.collect(Collectors.toSet());
System.out.println(set); // Output: [Apple, Cherry, Banana]
}
}
Output:
[Apple, Cherry, Banana]
4. Stream collect() Method - Collect to Map Example
import java.util.Map;
import java.util.stream.Collectors;
import java.util.stream.Stream;
class Employee {
private final String name;
private final int id;
public Employee(String name, int id) {
this.name = name;
this.id = id;
}
public String getName() {
return name;
}
public int getId() {
return id;
}
}
public class MapExample {
public static void main(String[] args) {
Stream<Employee> stream = Stream.of(new Employee("John", 1), new Employee("Jane", 2));
Map<Integer, String> map = stream.collect(Collectors.toMap(Employee::getId, Employee::getName));
System.out.println(map); // Output: {1=John, 2=Jane}
}
}
Output:
{1=John, 2=Jane}
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