Lambda expressions are just (anonymous) functions. f(x) = x can be expressed as x→x f(x, y) = x*x + y*y can be expressed as x, y→x*x + y*y In simple Java terms, 1. Return an int 23 with a void argument int j = { => 23 }.invoke(); 2. Closure with 1 argument, convert 23 kilograms to pounds double j = { double kilograms => kilograms*2.2D }.invoke(23); 3. Closure with 2 arguments, get the sum of the 2 squares int squareSum = { int x, int y => (x*x) + (y*y) }.invoke(2,3); 4. Closure returning an instance and invoking an instance method { new RocketLauncher() }.invoke().fire(); Every time a closure is created at compile time, an interface is automatically created for each expression having an invoke method with a signature depending on the number of arguments, then an anonymous subclass of this interface is constructed. Enjoy!