Advanced⏱️ 9 min📘 Topic 18 of 22

λ Java Lambda Expressions and Functional Interfaces

Master Java lambda expressions and functional interfaces — syntax, method references, the java.util.function package (Function, Predicate, Consumer, Supplier) and closures. With examples.

Lambdas (Java 8) let you pass behavior as data — the foundation of functional-style Java and the Streams API.

📜 Syntax

(params) -> expression
(params) -> { statements; }

Runnable r = () -> System.out.println("run");
Comparator<String> byLen = (a, b) -> a.length() - b.length();

A lambda is an instance of a functional interface — an interface with exactly one abstract method.

🧩 The java.util.function toolbox

Function<T,R> — takes T, returns R (apply)
Predicate<T> — takes T, returns boolean (test)
Consumer<T> — takes T, returns nothing (accept)
Supplier<T> — takes nothing, returns T (get)
BiFunction<T,U,R>, UnaryOperator<T>, etc.

🔗 Method references

Shorthand for lambdas that just call a method:

String::toUpperCase (instance method of type)
System.out::println (instance method of object)
Integer::parseInt (static method)
ArrayList::new (constructor)

🎒 Closures

Lambdas capture effectively final local variables — referenced but not reassigned. They can read outer variables but not mutate them.

💻 Code Examples

Predicate filter

Predicate<Integer> isEven = n -> n % 2 == 0;
List<Integer> evens = new ArrayList<>();
for (int n : List.of(1,2,3,4)) if (isEven.test(n)) evens.add(n);
System.out.println(evens);

Output:

[2, 4]

Sort with a lambda comparator

List<String> names = new ArrayList<>(List.of("bob","al","cara"));
names.sort((a, b) -> a.length() - b.length());
System.out.println(names);

Output:

[al, bob, cara]

Method reference

List.of("a","b","c").forEach(System.out::println);

Output:

a
b
c

⚠️ Common Mistakes

Trying to mutate a captured local variable inside a lambda — it must be effectively final.
Writing a verbose lambda when a method reference is clearer (s -> s.toUpperCase() vs String::toUpperCase).
Implementing a lambda for an interface with more than one abstract method — only functional interfaces qualify.
Forgetting the lambda's type is inferred from context — it needs a target functional interface.

🎯 Interview Questions

Real questions asked at top product and service-based companies.

Q1.What is a lambda expression?Beginner

A concise way to provide the implementation of a functional interface's single abstract method, written as (params) -> body. It lets you treat behavior as a value you can pass around.

Q2.What is a functional interface?Beginner

An interface with exactly one abstract method (e.g., Runnable, Comparator, Function). It can be the target type of a lambda or method reference. @FunctionalInterface enforces the single-method rule.

Q3.Name the core functional interfaces in java.util.function.Intermediate

Function<T,R> (apply), Predicate<T> (test), Consumer<T> (accept), Supplier<T> (get), plus BiFunction, UnaryOperator, BinaryOperator and primitive specializations like IntFunction.

Q4.What is a method reference?Intermediate

Shorthand for a lambda that only calls an existing method. Forms: Type::staticMethod, object::instanceMethod, Type::instanceMethod, and Type::new (constructor reference).

Q5.What does 'effectively final' mean for lambda capture?Advanced

A local variable a lambda captures must not be reassigned after initialization (even without the final keyword). This avoids data races and matches the value-capture semantics of closures in Java.

🧠 Quick Summary

Lambdas implement a functional interface's single abstract method.
java.util.function: Function, Predicate, Consumer, Supplier.
Method references (Class::method) are concise lambda shorthand.
Lambdas capture effectively-final locals (read, not reassign).
Lambdas power the Streams API and functional-style Java.