Java 8 Lambda Expressions and Stream API

Introduction

Java 8 introduced Lambda Expressions and the Stream API, revolutionizing how developers write code. These features brought functional programming concepts into Java, enabling concise, expressive, and efficient data processing.

Lambda Expressions

Syntax and Basics

List names = Arrays.asList("Alice", "Bob", "Charlie");
names.forEach(name -> System.out.println(name));
  

Functional Interfaces

Lambdas work with functional interfaces like Predicate, Function, Consumer, and Supplier.

Examples

// Using Predicate
Predicate isEven = n -> n % 2 == 0;
System.out.println(isEven.test(4)); // true

// Using Comparator
List list = Arrays.asList("banana", "apple", "cherry");
list.sort((a, b) -> a.compareTo(b));
System.out.println(list);
  

Advanced Lambda Usage

• Closures and variable capture
• Method references
• Constructor references

Stream API

Introduction

Streams provide a declarative way to process collections. They support operations like filtering, mapping, reducing, and can run in parallel.

Stream Creation

Stream stream = Stream.of("a", "b", "c");
List list = Arrays.asList("x", "y", "z");
Stream listStream = list.stream();
  

Intermediate Operations

• filter()
• map()
• flatMap()
• distinct()
• sorted()
• peek()

Terminal Operations

• forEach()
• collect()
• reduce()
• count()
• findFirst()
• findAny()

Examples

List numbers = Arrays.asList(1,2,3,4,5);
int sum = numbers.stream()
                 .filter(n -> n % 2 == 0)
                 .mapToInt(n -> n)
                 .sum();
System.out.println(sum); // 6
  

Collectors

Collectors provide powerful ways to group, partition, and summarize data from streams.

Map> grouped = 
    names.stream().collect(Collectors.groupingBy(String::length));
System.out.println(grouped);
  

Parallel Streams

Streams can run in parallel to leverage multi-core processors. However, they must be used carefully to avoid performance pitfalls.

Performance Analysis

Lambdas and streams improve readability but may introduce overhead. Benchmarking is essential to optimize performance.

Interview-Ready Notes

• Q: Difference between streams and collections?
  A: Collections store data; streams process data.
• Q: Why Optional?
  A: To avoid null checks and NullPointerException.
• Q: Lambda vs anonymous class?
  A: Lambdas are concise and focus on behavior, anonymous classes are verbose.
• Q: What is CompletableFuture?
  A: A class for async programming with chaining and combining tasks.

Conclusion

Lambda Expressions and the Stream API transformed Java programming by introducing functional programming concepts. Mastering these features is essential for writing efficient, maintainable, and modern Java applications.