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Stream In Java - GeeksforGeeks

Stream was introduced in Java 8, the Stream API is used to process collections of objects. A stream in Java is a sequence of objects that supports various methods that can be pipelined to produce the desired result. 

The uses of Stream in Java are mentioned below:

• Stream API is a way to express and process collections of objects.
• Enable us to perform operations like filtering, mapping, reducing, and sorting.

Java Stream Creation is one of the most basic steps before considering the functionalities of the Java Stream. Below is the syntax given for declaring a Java Stream.

Stream<T> stream;

Here, T is either a class, object, or data type depending upon the declaration.

The features of Java streams are mentioned below:

• A stream is not a data structure; instead, it takes input from the Collections, Arrays, or I/O channels.
• Streams don’t change the original data structure, they only provide the result as per the pipelined methods.
• Each intermediate operation is lazily executed and returns a stream as a result, hence, various intermediate operations can be pipelined. Terminal operations mark the end of the stream and return the result.

There are two types of Operations in Streams:

1. Intermediate Operations
2. Terminal Operations

Intermediate Operations are the types of operations in which multiple methods are chained in a row.

• Methods are chained together.
• Intermediate operations transform a stream into another stream.
• It enables the concept of filtering where one method filters data and passes it to another method after processing.

There are some benefits because of which we use Stream in Java as mentioned below:

• No Storage
• Pipeline of Functions
• Laziness
• Can be infinite
• Can be parallelized
• Can be created from collections, arrays, Files Lines, Methods in Stream, IntStream etc.

There are a few Intermediate Operations mentioned below:

1. map(): The map method is used to return a stream consisting of the results of applying the given function to the elements of this stream.

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.

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 (according to Object.equals(Object)).

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, additionally performing the provided action on each element as elements are consumed from the resulting stream.

Syntax:

Stream<T> peek(Consumer<? super T> action)

Java program that demonstrates the use of all the intermediate operations:

import java.util.Arrays;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
import java.util.stream.Collectors;

public class StreamIntermediateOperationsExample {
    public static void main(String[] args) {
        // List of lists of names
        List<List<String>> listOfLists = Arrays.asList(
            Arrays.asList("Reflection", "Collection", "Stream"),
            Arrays.asList("Structure", "State", "Flow"),
            Arrays.asList("Sorting", "Mapping", "Reduction", "Stream")
        );

        // Create a set to hold intermediate results
        Set<String> intermediateResults = new HashSet<>();

        // Stream pipeline demonstrating various intermediate operations
        List<String> result = listOfLists.stream()
            .flatMap(List::stream)               // Flatten the list of lists into a single stream
            .filter(s -> s.startsWith("S"))      // Filter elements starting with "S"
            .map(String::toUpperCase)            // Transform each element to uppercase
            .distinct()                          // Remove duplicate elements
            .sorted()                            // Sort elements
            .peek(s -> intermediateResults.add(s)) // Perform an action (add to set) on each element
            .collect(Collectors.toList());       // Collect the final result into a list

        // Print the intermediate results
        System.out.println("Intermediate Results:");
        intermediateResults.forEach(System.out::println);

        // Print the final result
        System.out.println("Final Result:");
        result.forEach(System.out::println);
    }
}

Intermediate Results:
STRUCTURE
STREAM
STATE
SORTING
Final Result:
SORTING
STATE
STREAM
STRUCTURE

List of Lists Creation:

• The listOfLists is created as a list containing other lists of strings.

Stream Operations:

• flatMap(List::stream): Flattens the nested lists into a single stream of strings.
• filter(s -> s.startsWith("S")): Filters the strings to only include those that start with "S".
• map(String::toUpperCase): Converts each string in the stream to uppercase.
• distinct(): Removes any duplicate strings.
• sorted(): Sorts the resulting strings alphabetically.
• peek(...): Adds each processed element to the intermediateResults set for intermediate inspection.
• collect(Collectors.toList()): Collects the final processed strings into a list called result.

The program prints the intermediate results stored in the intermediateResults set. Finally, it prints the result list, which contains the fully processed strings after all stream operations.

This example showcases how Java Streams can be used to process and manipulate collections of data in a functional and declarative manner, applying transformations and filters in a sequence of operations.

Terminal Operations are the type of Operations that return the result. These Operations are not processed further just return a final result value.

There are a few Terminal Operations mentioned below:

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 through 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. The reduce method takes a BinaryOperator as a parameter.

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, if present.

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)

Here ans variable is assigned 0 as the initial value and i is added to it.

Note: Intermediate Operations are running based on the concept of Lazy Evaluation, which ensures that every method returns a fixed value(Terminal operation) before moving to the next method.

Java Program Using all Terminal Operations:

import java.util.*;
import java.util.stream.Collectors;

public class StreamTerminalOperationsExample {
    public static void main(String[] args) {
        // Sample data
        List<String> names = Arrays.asList(
            "Reflection", "Collection", "Stream",
            "Structure", "Sorting", "State"
        );

        // forEach: Print each name
        System.out.println("forEach:");
        names.stream().forEach(System.out::println);

        // collect: Collect names starting with 'S' into a list
        List<String> sNames = names.stream()
                                   .filter(name -> name.startsWith("S"))
                                   .collect(Collectors.toList());
        System.out.println("\ncollect (names starting with 'S'):");
        sNames.forEach(System.out::println);

        // reduce: Concatenate all names into a single string
        String concatenatedNames = names.stream().reduce(
            "",
            (partialString, element) -> partialString + " " + element
        );
        System.out.println("\nreduce (concatenated names):");
        System.out.println(concatenatedNames.trim());

        // count: Count the number of names
        long count = names.stream().count();
        System.out.println("\ncount:");
        System.out.println(count);

        // findFirst: Find the first name
        Optional<String> firstName = names.stream().findFirst();
        System.out.println("\nfindFirst:");
        firstName.ifPresent(System.out::println);

        // allMatch: Check if all names start with 'S'
        boolean allStartWithS = names.stream().allMatch(
            name -> name.startsWith("S")
        );
        System.out.println("\nallMatch (all start with 'S'):");
        System.out.println(allStartWithS);

        // anyMatch: Check if any name starts with 'S'
        boolean anyStartWithS = names.stream().anyMatch(
            name -> name.startsWith("S")
        );
        System.out.println("\nanyMatch (any start with 'S'):");
        System.out.println(anyStartWithS);
    }
}

Output:

List Creation:

• The names list is created with sample strings.

Stream Operations:

• forEach: Prints each name in the list.
• collect: Filters names starting with 'S' and collects them into a new list.
• reduce: Concatenates all names into a single string.
• count: Counts the total number of names.
• findFirst: Finds and prints the first name in the list.
• allMatch: Checks if all names start with 'S'.
• anyMatch: Checks if any name starts with 'S'.

The program prints each name, names starting with 'S', concatenated names, the count of names, the first name, whether all names start with 'S', and whether any name starts with 'S'.

Streams are widely used in modern Java applications for:

• Data Processing
• For processing JSON/XML responses
• For database Operations
• Concurrent Processing

Important Points:

• A stream consists of a source followed by zero or more intermediate methods combined together (pipelined) and a terminal method to process the objects obtained from the source as per the methods described.
• Stream is used to compute elements as per the pipelined methods without altering the original value of the object.

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