Lambda Expressions

The Big Picture

Lambdas are anonymous functions - concise syntax for implementing functional interfaces.

• Syntax: (params) -> expression
• Or block form: (params) -> { body; }
• Essential for streams: list.forEach(x -> print(x))

Replaced bulky anonymous classes for single-method callbacks.

How It Works

Lambdas capture variables from enclosing scope (closure). Captured variables must be effectively final.

• invokedynamic call site; runtime bootstrap links the lambda impl
• Non-capturing lambdas are often cached (no per-call allocation)
• Capturing lambdas create lightweight objects
• Compiler infers types from context (target typing)

In the Java Landscape

Java 8's flagship feature alongside streams.

• Work with Stream API: map, filter, reduce
• Replace Comparator boilerplate: Comparator.comparingInt(p -> p.age)
• Method references shorthand: System.out::println
• Scala, Groovy, C# had closures before Java
• Modern libraries designed around lambda parameters

Pitfalls and Best Practices

• Can't reassign captured locals - must be effectively final
• Type inference can fail - add explicit types
• Debugging harder - stack traces show synthetic names
• Prefer expression lambdas (x -> x * 2) over blocks
• Don't return from lambda to exit enclosing method