Functional Programming Overview

A programming paradigm where functions are first-class citizens - treated like values.
In Java, functional programming was introduced with Java 8, enabling:

• more concise, expressive code,
• declarative data processing (Streams),
• use of lambda expressions, functional interfaces and method references.

Functional vs Imperative Style

Imperative (Old School)	Functional (Modern Java)
“How to do it” - step-by-step	”What to do” - describe the operation
Uses loops, conditionals	Uses functions like map(), filter()
Verbose	Concise, expressive

// Imperative
List<String> result = new ArrayList<>();
for (String name : names) {
    if (name.startsWith("A")) result.add(name);
}
 
// Functional
List<String> result = names.stream()
    .filter(n -> n.startsWith("A"))
    .collect(Collectors.toList());

Key Functional Concepts in Java

Concept	Purpose
Lambda Expressions	Inline functions (no boilerplate)
Functional Interfaces	Interfaces with a single abstract method
Streams API	Functional-style data processing
Method References	Cleaner alternative to lambdas
Optional	Avoiding nulls in a functional way

Characteristics of Functional Code

• No side effects.
• Pure functions.
• Immutability.
• Higher-order functions (accept or return other functions).

Tip

Be ready to:

• explain what FP is and how Java supports it since Java 8,
• show for-loop → stream().filter() conversion,
• mention lambda expressions and Streams API as key tools.

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Parent: _Multithreading