Master Java Stream API: From Basics to Advanced Operations

1. Overview of Stream API

Java 8 introduced two major features: Lambda expressions and the Stream API, which brings functional programming concepts into Java, greatly improving developer productivity by enabling concise, clean data processing.

1.1 Why Use Stream API

Streams allow operations on data sources such as collections, arrays, and databases (including NoSQL) without storing elements, providing a pipeline for filtering, mapping, and aggregating data.

1.2 What Is a Stream

A Stream is a sequence of elements generated from a data source, focusing on computation rather than storage.

Streams do not store elements.

Streams do not modify the source; they produce a new Stream.

Stream operations are lazy and execute only when a terminal operation is invoked.

2. Creating Streams

Since Stream is an interface, you cannot instantiate it directly. Four creation methods are provided:

2.1 From Collections

default Stream<T> stream()

: returns a sequential stream.

default Stream<T> parallelStream()

: returns a parallel stream.

<code>package blogs.blog13;
import day33.java.Employee;
import day33.java.EmployeeData;
import java.util.List;
import java.util.stream.Stream;
public class StreamAPITest {
    public static void main(String[] args) {
        List&lt;Employee&gt; employeeList = EmployeeData.getEmployees();
        Stream&lt;Employee&gt; stream = employeeList.stream();
        System.out.println(stream);
        Stream&lt;Employee&gt; employeeStream = employeeList.parallelStream();
        System.out.println(employeeStream);
    }
}
</code>

2.2 From Arrays

static Stream<T> stream(T[] array)

: creates a stream from an array.

Overloads for primitive arrays:

IntStream stream(int[])

,

LongStream stream(long[])

,

DoubleStream stream(double[])

.

<code>import java.util.Arrays;
import java.util.stream.IntStream;
public class StreamAPITest {
    public static void main(String[] args) {
        int[] arr = {1,2,3,4,5,6};
        IntStream stream = Arrays.stream(arr);
        System.out.println(stream);
    }
}
</code>

2.3 Using Stream.of()

<code>import java.util.stream.Stream;
public class StreamAPITest {
    public static void main(String[] args) {
        Stream&lt;Integer&gt; stream = Stream.of(1,2,3,4,5,6);
        System.out.println(stream);
    }
}
</code>

2.4 Creating Infinite Streams

Stream.iterate(seed, UnaryOperator)

: generates an iterative infinite stream.

Stream.generate(Supplier)

: generates an infinite stream of supplied values.

<code>Stream.iterate(0, t -> t + 2).limit(10).forEach(System.out::println);
Stream.generate(Math::random).limit(10).forEach(System.out::println);
</code>

3. Intermediate Operations

3.1 Filtering and Slicing

<code>stream.filter(e -> e.getSalary() > 7000).forEach(System.out::println);
stream.distinct();
stream.limit(3);
stream.skip(3);
</code>

3.2 Mapping

<code>Stream&lt;String&gt; upper = list.stream().map(String::toUpperCase);
Stream&lt;Character&gt; chars = stream.flatMap(s -> fromStringToStream(s));
</code>

3.3 Sorting

<code>stream.sorted(); // natural order (requires Comparable
stream.sorted((e1,e2) -> Integer.compare(e1.getAge(), e2.getAge()));
</code>

4. Terminal Operations

4.1 Matching and Finding

allMatch(Predicate)

anyMatch(Predicate)

noneMatch(Predicate)

findFirst()

findAny()

4.2 Reduction

<code>Integer sum = list.stream().reduce(0, Integer::sum);
Optional&lt;Double&gt; total = salaries.reduce((a,b) -> a + b);
</code>

4.3 Collecting

<code>List&lt;Employee&gt; highEarners = list.stream()
    .filter(e -> e.getSalary() > 6000)
    .collect(Collectors.toList());
</code>

5. Optional Class

Optional is a container that may hold a non‑null value or be empty, helping to avoid

NullPointerException

. Creation methods include

Optional.of(value)

,

Optional.empty()

, and

Optional.ofNullable(value)

. Common methods:

isPresent()

,

ifPresent()

,

get()

,

orElse()

,

orElseGet()

, and

orElseThrow()

.

<code>Optional&lt;Girl&gt; opt = Optional.ofNullable(girl);
String name = opt.orElse(new Girl(new Boy("Default"))).getBoy().getName();
</code>

6. Summary

Streams provide a functional way to process collections and arrays.

Operations follow three steps: create, intermediate, terminal.

Four ways to create streams: from collections, arrays,

Stream.of()

, and infinite streams.

Streams are single‑use; after a terminal operation a new stream must be created.

Intermediate operations are lazy; terminal operations trigger processing.

Optional helps handle potentially null values safely.