A lambda expression is a concise, anonymous way to represent an implementation of a functional interface (an interface with exactly one abstract method) — `(parameters) -> expression/body` — removing the boilerplate of anonymous inner classes.

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#Lambda Expressions

What Makes a Lambda Valid

A lambda can only be used where the compiler can infer a target functional interface type from context — assigning to a variable of that interface type, or passing as an argument to a parameter of that type. This is why lambdas can't exist standalone; they always need a functional interface 'shape' to match against.

Anonymous Class (7 lines of boilerplate) → Lambda Expression: (a, b) -> a + b (1 line)

Interview Answer

A lambda expression is Java's concise syntax for implementing a functional interface inline — it captures the target interface's single abstract method as `(params) -> body`, replacing verbose anonymous inner classes and enabling the functional style used throughout the Stream API.

Anonymous Class vs Lambda

java

// Before Java 8
Runnable r1 = new Runnable() {
    public void run() { System.out.println("Running"); }
};

// With a lambda
Runnable r2 = () -> System.out.println("Running");