Java 8 Features: Lambdas, Streams, and Functional Interfaces

Java 8 introduced major enhancements that brought functional programming concepts into the language. The three most important additions are:

Lambda Expressions

Streams API

Functional Interfaces

Together, these features allow Java developers to write cleaner, more concise, and more expressive code.

1. Lambda Expressions

What is a Lambda?

A lambda expression is a short block of code that:

Can be passed around as a method argument

Has no name

Implements a functional interface method

Basic Syntax

(parameters) -> expression

or

(parameters) -> { statements; }

Example: Before Java 8

Using an anonymous class:

Runnable r = new Runnable() {

    public void run() {

        System.out.println("Hello from Runnable!");

    }

};

Example: With Lambda

Runnable r = () -> System.out.println("Hello from Runnable!");

Common Lambda Examples

With multiple parameters

(int a, int b) -> a + b

With a block

(name) -> {

    System.out.println("Hello " + name);

}

2. Functional Interfaces

What is a Functional Interface?

A functional interface has exactly one abstract method.

It can have default and static methods as well.

Examples in Java 8:

Runnable — void run()

Callable<T> — T call()

Comparator<T> — int compare(T a, T b)

Predicate<T> — returns true/false

Function<T, R> — transforms a value

Consumer<T> — takes input, returns nothing

Supplier<T> — returns a value

Annotated Functional Interface

@FunctionalInterface

interface Greeting {

    void sayHello(String name);

}

Using with Lambda

Greeting g = (name) -> System.out.println("Hello " + name);

g.sayHello("Alice");

3. Streams API

The Streams API is one of the most powerful features of Java 8. It allows you to process collections of data in a functional and declarative way.

Key Concepts

Stream is not a data structure

It doesn’t store data; it processes data from collections, arrays, or I/O channels.

Operations

There are two kinds:

(1) Intermediate operations (return a stream):

map()

filter()

sorted()

distinct()

limit()

(2) Terminal operations (produce a result):

collect()

forEach()

reduce()

count()

findFirst(), findAny()

Stream Processing Pipeline

A stream pipeline typically has three parts:

Source (e.g., list, array)

Intermediate operations (transform)

Terminal operation (produce result)

Example: Without Streams

Filter even numbers and print them:

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6);

for (Integer n : numbers) {

    if (n % 2 == 0) {

        System.out.println(n);

    }

}

With Streams

numbers.stream()

       .filter(n -> n % 2 == 0)

       .forEach(System.out::println);

More Stream Examples

Mapping

List<String> upper = list.stream()

                         .map(String::toUpperCase)

                         .collect(Collectors.toList());

Sorting

list.stream()

    .sorted()

    .forEach(System.out::println);

Reducing

int sum = numbers.stream()

                 .reduce(0, (a, b) -> a + b);

Collecting

List<String> result = strings.stream()

                             .filter(s -> s.startsWith("A"))

                             .collect(Collectors.toList());

Benefits of Java 8 Functional Features

Cleaner and more readable code

Less boilerplate (especially with lambdas)

Easier to parallelize work using parallelStream()

More declarative style

Better use of modern multicore processors

Summary

Feature Purpose Example

Lambdas Concise functions as values x -> x * 2

Functional Interfaces Enable lambdas Predicate<T>

Streams API Functional data processing list.stream().filter(...).collect(...)

Java 8’s functional programming features significantly modernized the language and made it more expressive and powerful.

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Java Collections Framework: List, Set, Map

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November 28, 2025

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