Java How To Program, Late Objects, 11th Edition

... multiplies its value by 2 and returns the result. In this case, the body is a statement block that may contain statements enclosed in curly braces. The compiler infers from the lambda that it returns an int, because the parameter x is an int and the literal 2 is an int—multiplying an int by an int yields an int result. As in a method declaration, lambdas specify parameters in a comma-separated list. The preceding lambda is similar to the method

int multiplyBy2(int x) {
   return x * 2;
}

but the lambda does not have a name and the compiler infers its return type. There are several variations of the lambda syntax.

Eliminating a Lambda’s Parameter Type(s)

A lambda’s parameter type(s) usually may be omitted, as in:

(x) -> {return x * 2;}

in which ...