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.