Understanding Java generics. Type parameter conventions

Question

The most commonly used type parameter names are:

E - Element (used extensively by the Java Collections Framework)

K - Key

N - Number

T - Type

V - Value

S,U,V etc. - 2nd, 3rd, 4th types

I don't seem to quite understand what exactly does every letter correspond to. I understand that each letter represents just a convention but what does 2nd, 3rd and 4th type mean exactly? When should I use what? On their official tutorials website it doesn't give further information.

Answer 1

Some examples:

• Map<K, V>: A map usually assigns Values to Keys. These are special kinds of types, so they are used here.
• List<E>: A list contains Elements. It's a convention that they are called elements. On the other hand, T would also be acceptable here.
• Formatter<T>: A formatter can format any Type. It's not really an element, nor a key, nor a value, so T is the correct letter here.
• Triplet<T, U, V>: A triplet for arbitrary types. Since the type definition does not know anything about the types that will be filled in later, it uses just the T for the first type, followed by the next letters in alphabetical order.

Answer 2

As you know, the letters don't mean anything in themselves and are just conventions to make reading the code a bit easier. The "2nd, 3rd and 4th types" bit just means when you have a parameterised type with multiple arguments you should call those arguments S, T, U, V etc. For example

class MyClass<S, T, U> { } 

is a class with three type parameters.

Obviously you don't have to use S, T and U. If a more meaningful convention can be used then you should do so, for example

class Car<W, E, P> { } 

could be a Car class parameterised on wheel, engine and paint type.

EDIT: As the other answer pointed out, even better would be:

class Car<WheelType, EngineType, PaintType> { }