Stream API in Java
In Java, Stream was introduced in Java 8, the Stream API is used to process collections of objects in a functional and declarative style. A stream is a sequence of objects that supports various methods. A Stream is not a data structure; it just takes input from Collections, Arrays or I/O channels.
Streams do not modify the original data ; they only produce results using their methods. Mainly used for filtering, mapping, reducing, and collecting data, often replacing traditional loop-based approaches.
Syntax:
Stream<T> stream;
Here, T is either a class, object or data type depending upon the declaration.
Key Concepts of the Stream API:
Intermediate Operations: These operations transform a stream into another stream. They are typically lazy, they don't process data until a terminal operation is invoked.
Terminal Operations: These operations produce a result. Once a terminal operation is called, the stream cannot be reused. Examples include forEach(), collect(), reduce(), count(), min(), max(), findFirst().
Intermediate Operations in Java:
1. map() is an intermediate operation. These operations are always lazy.
An object that maps keys to values. A map cannot contain duplicate keys; each key can map to at most one value.
Syntax:
<R> Stream<R> map(Function<? super T, ? extends R> mapper)
2. filter(): The filter method is used to select elements as per the Predicate passed as an argument.
Syntax:
Stream<T> filter(Predicate<? super T> predicate)
3. sorted(): The sorted method is used to sort the stream elements.
Syntax:
Stream<T> sorted()
Stream<T> sorted(Comparator<? super T> comparator)
4. flatMap(): The flatMap operation in Java Streams is used to flatten a stream of collections into a single stream of elements.
Syntax:
<R> Stream<R> flatMap(Function<? super T, ? extends Stream<? extends R>> mapper)
5. distinct(): Removes duplicate elements. It returns a stream consisting of the distinct elements.
Syntax:
Stream<T> distinct()
6. peek(): Performs an action on each element without modifying the stream. It returns a stream consisting of the elements of this stream.
Syntax:
Stream<T> peek(Consumer<? super T> action)
Terminal Operations in Java:
Terminal Operations are the type of Operations that returns only result. These Operations are not processed further just return a final result value on stream.
1. collect(): The collect method is used to return the result of the intermediate operations performed on the stream.
Syntax:
<R, A> R collect(Collector<? super T, A, R> collector)
2. forEach(): The forEach method is used to iterate every element of the stream.
Syntax:
void forEach(Consumer<? super T> action)
3. reduce(): The reduce method is used to reduce the elements of a stream to a single value.
Syntax:
T reduce(T identity, BinaryOperator<T> accumulator)
Optional<T> reduce(BinaryOperator<T> accumulator)
4. count(): Returns the count of elements in the stream.
Syntax:
long count()
5. findFirst(): Returns the first element of the stream.
Syntax:
Optional<T> findFirst()
6. allMatch(): Checks if all elements of the stream match a given predicate.
Syntax:
boolean allMatch(Predicate<? super T> predicate)
7. anyMatch(): Checks if any element of the stream matches a given predicate.
Syntax:
boolean anyMatch(Predicate<? super T> predicate)
Example Java Program:
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
public class StreamApiExample {
public static void main(String[] args) {
List<String> names = Arrays.asList("Alice", "Bob", "Charlie", "David", "Eve");
// Filter names starting with 'A' and convert to uppercase
List<String> filteredAndMappedNames = names.stream()
.filter(name -> name.startsWith("A")) // Intermediate operation: filter
.map(String::toUpperCase) // Intermediate operation: map
.collect(Collectors.toList()); // Terminal operation: collect
System.out.println(filteredAndMappedNames); // Output: [ALICE]
}
}
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Stream API in Java?
