Lambda syntax, method references and effectively final variables

A lambda expression is a compact implementation of a functional interface's single abstract method. Its shape is parameters, an arrow and a body. Parentheses are optional only for a single parameter with no declared type: x -> x * 2. Zero or several parameters need parentheses: () -> 42, (a, b) -> a + b. You may declare types, (String s) -> s.length(), or use var, (var s) -> s.length(), but you must be consistent: all parameters typed, all var or all untyped. Mixing, as in (var a, b), does not compile.