Lambdas: the syntax that started it

Key points

• A lambda is not "an anonymous class with less typing" - it compiles to an invokedynamic call site, not a new class file.
• It only works where the target type is an interface with one abstract method.
• The parameter types are inferred from that target type, which is why (a, b) -> a - b needs no declarations.
• this inside a lambda means the enclosing object - in an anonymous class it meant the anonymous instance.
• A non-capturing lambda can be reused as a single instance, so it is often cheaper than the class it replaced.

Example

import java.util.*;

public class Main {
    public static void main(String[] args) {
        List<String> names = new ArrayList<>(List.of("Charlie", "ada", "Bob"));

        // Java 7 style
        names.sort(new Comparator<String>() {
            @Override public int compare(String a, String b) {
                return a.compareToIgnoreCase(b);
            }
        });
        System.out.println("anonymous class : " + names);

        // Java 8 style - same behaviour, one line
        names.sort((a, b) -> a.compareToIgnoreCase(b));
        System.out.println("lambda          : " + names);

        // 'this' differs: a lambda sees the enclosing instance
        Runnable r = () -> System.out.println("lambdas are not inner classes");
        r.run();
    }
}

A lambda is a target-typed function, linked at runtime - not sugar for an inner class.