Lambda Expressions

Lambda Expressions

Lambdas are anonymous functions — a concise way to pass behaviour as data.

Before lambdas (anonymous class)

list.sort(new Comparator<String>() {
    @Override
    public int compare(String a, String b) {
        return a.compareTo(b);
    }
});

With lambda

list.sort((a, b) -> a.compareTo(b));
// Or even shorter with method reference:
list.sort(String::compareTo);

Functional Interfaces

A lambda can only be used where a functional interface (interface with exactly one abstract method) is expected.

Predicate<String> isLong = s -> s.length() > 5;
Function<String, Integer> length = String::length;
Consumer<String> printer = System.out::println;
Supplier<List<String>> newList = ArrayList::new;

Closure — capturing variables

Lambdas can capture local variables, but only if they are effectively final (never modified after assignment):

String prefix = "Hello, ";
Consumer<String> greet = name -> System.out.println(prefix + name); // ✅
prefix = "Hi, "; // ❌ compile error: prefix must be effectively final

Interview Tip

Know the four core functional interfaces (Supplier, Consumer, Function, Predicate) and when to use each. Interviewers also test whether you understand that lambdas don't create a new scope — this inside a lambda refers to the enclosing class, not the lambda.