Java Functional Programming Guide | Generated by AI

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Java has embraced functional programming principles since Java 8, introducing lambdas, streams, and other features that make code more concise, readable, and declarative. This guide covers the fundamentals to advanced concepts, assuming familiarity with core Java. We’ll use Java 8+ syntax, as that’s when these features debuted. Examples are in Java code blocks for easy copying.

1. Introduction to Functional Programming in Java

Functional programming (FP) emphasizes:

Java isn’t purely functional like Haskell, but it blends FP with its object-oriented roots. Key enablers:

Benefits: Reduced boilerplate, easier parallelism, better composability.

2. Lambda Expressions

Lambdas are anonymous functions used for short, one-off implementations. They’re the gateway to FP in Java.

Basic Syntax

A lambda is: (parameters) -> { body }

// Traditional anonymous inner class
Runnable r = new Runnable() {
    @Override
    public void run() {
        System.out.println("Hello, World!");
    }
};

// Lambda equivalent
Runnable lambda = () -> System.out.println("Hello, World!");
lambda.run();

With Parameters

// Binary operator example
BinaryOperator<Integer> add = (a, b) -> a + b;
System.out.println(add.apply(2, 3)); // 5

// Multi-line body
Comparator<String> comparator = (s1, s2) -> {
    int lenDiff = s1.length() - s2.length();
    return lenDiff != 0 ? lenDiff : s1.compareTo(s2);
};

Capturing Variables (Effectively Final)

Lambdas can access outer variables, but they must be effectively final (not reassigned).

int threshold = 10;
Predicate<Integer> isHigh = x -> x > threshold; // OK
// threshold = 20; // Error: not effectively final

3. Functional Interfaces

A functional interface has exactly one abstract method (SAM - Single Abstract Method). Java provides built-ins in java.util.function.

Built-in Examples

Custom ones:

@FunctionalInterface  // Optional, but good practice
interface Transformer {
    String transform(String input);
}

Transformer upper = s -> s.toUpperCase();
System.out.println(upper.transform("java")); // JAVA

Use @FunctionalInterface to enforce SAM.

Default and Static Methods

Functional interfaces can have defaults (Java 8+), like Optional.orElse().

default int compare(String a, String b) { ... } // Allowed
static void utility() { ... } // Allowed

4. Method References

Shorthand for lambdas invoking existing methods. Syntax: Class::method or instance::method.

Types:

Examples:

// Lambda: x -> System.out.println(x)
Consumer<String> printer = System.out::println;

// Static method
Function<String, Integer> length = String::length;

// Instance method
List<String> list = Arrays.asList("a", "b");
list.forEach(System.out::println); // Prints each

// Constructor
Supplier<List<String>> listSupplier = ArrayList::new;

5. Streams API

Streams process collections declaratively: create → transform → collect. Lazy evaluation (intermediate ops don’t run until terminal op).

Creating Streams

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

List<String> names = Arrays.asList("Alice", "Bob", "Charlie");

// From collection
Stream<String> stream = names.stream();

// From array
String[] arr = {"x", "y"};
Stream<String> arrStream = Arrays.stream(arr);

// Infinite
Stream<Integer> infinite = Stream.iterate(0, n -> n + 1);

Intermediate Operations (Lazy)

Chain them; no computation until terminal.

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);
List<Integer> evensSquared = numbers.stream()
    .filter(n -> n % 2 == 0)
    .map(n -> n * n)
    .collect(Collectors.toList()); // [4, 16]

Terminal Operations (Eager)

Trigger computation and return a result.

// Reduce: sum
int sum = numbers.stream().reduce(0, Integer::sum); // 15

// Collect to map
Map<String, Integer> nameLengths = Stream.of("Alice", "Bob")
    .collect(Collectors.toMap(s -> s, String::length)); // {Alice=5, Bob=3}

// Grouping
Map<Integer, List<String>> byLength = names.stream()
    .collect(Collectors.groupingBy(String::length)); // {5=[Alice, Charlie], 3=[Bob]}

Parallel Streams

For parallelism: parallelStream() or .parallel(). Use cautiously (debugging harder).

long count = names.parallelStream().count(); // 3

6. Collectors

From java.util.stream.Collectors. Build complex reductions.

Common:

// Custom collector for max by length
String longest = names.stream()
    .collect(Collectors.maxBy(Comparator.comparingInt(String::length)))
    .orElse(""); // "Charlie"

// Partition evens/odds
Map<Boolean, List<Integer>> partitions = numbers.stream()
    .collect(Collectors.partitioningBy(n -> n % 2 == 0));

7. Optional

Avoids NullPointerException by wrapping potentially null values. Encourages explicit null handling.

Creation:

Operations:

Optional<String> opt = Optional.ofNullable(getName()); // Assume may return null

String name = opt.orElse("Unknown");
opt.ifPresent(System.out::println);

String upper = opt.map(String::toUpperCase).orElse("DEFAULT");

Streams often return Optional (e.g., findFirst()).

8. Advanced Topics

Composable Functions

Function.andThen(), Function.compose() for chaining.

Function<String, Integer> len = String::length;
Function<Integer, String> toStr = i -> "Len: " + i;

Function<String, String> chain = len.andThen(toStr);
System.out.println(chain.apply("Java")); // Len: 4

Recursion and Tail Calls

Java lacks optimization, but use Stream.iterate() for iterative recursion.

Immutability Helpers

Use Collections.unmodifiableList() or libraries like Guava/Immutable Collections (though built-in since Java 10+ with List.of()).

List.of("a", "b") creates immutable lists (Java 9+).

Pattern Matching (Java 21+ Preview/Stable)

Enhances FP with destructuring in switches.

// Preview feature; enable with --enable-preview
String desc = switch (obj) {
    case Integer i -> "int: " + i;
    case String s -> "str: " + s.length();
    default -> "unknown";
};

Virtual Threads (Java 21+)

FP shines with lightweight threads for concurrent streams.

9. Best Practices

Common pitfalls:

10. Example: Functional Pipeline

Process a list of employees functionally.

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

class Employee {
    String name;
    int salary;
    Department dept;

    Employee(String name, int salary, Department dept) {
        this.name = name;
        this.salary = salary;
        this.dept = dept;
    }
}

enum Department { ENG, HR }

public class FunctionalExample {
    public static void main(String[] args) {
        List<Employee> employees = Arrays.asList(
            new Employee("Alice", 50000, Department.ENG),
            new Employee("Bob", 60000, Department.HR),
            new Employee("Charlie", 55000, Department.ENG)
        );

        // High-paid engineers' names, sorted
        List<String> result = employees.stream()
            .filter(e -> e.dept == Department.ENG)
            .filter(e -> e.salary > 52000)
            .map(Employee::getName)  // Assume getter
            .sorted()
            .collect(Collectors.toList());

        System.out.println(result); // [Charlie]
    }
}

Conclusion

Lambdas and FP transform Java from imperative to expressive. Start with simple replacements (e.g., forEach over loops), then build pipelines. Practice on real projects for mastery. For deeper dives, experiment in an IDE.

References

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