A lambda expression in Java is a short, anonymous block of code that implements a single-method (functional) interface. It lets you pass behaviour as an argument: instead of writing a verbose anonymous inner class, you write (params) -> body. Lambdas were introduced in Java 8 (2014) and are still the standard way to work with Stream, Comparator, Runnable, and the java.util.function interfaces in modern Java (verified on JDK 25).

The syntax is (parameters) -> expression for a single expression, or (parameters) -> { statements; return value; } for a block body. Parentheses are optional with a single inferred-type parameter (x -> x * 2); they are required for zero parameters (() -> 42) or multiple parameters ((a, b) -> a + b). The compiler infers parameter types from the target functional interface.

A lambda is more concise and is treated by the JVM as an invokedynamic call — no extra .class file is generated, unlike an anonymous class. Lambdas can only target a functional interface (one abstract method); anonymous classes can extend any class or implement any interface. Inside a lambda, this refers to the enclosing instance; inside an anonymous class, this refers to the anonymous class itself. Use lambdas for single-method behaviour; use anonymous classes when you need state, multiple methods, or to override equals/hashCode.