What is the reason for using single-character generic type names in java?

Question

In java most (or all?) generic classes in the JDK have single-digit generic type names. For example HashMap's definition looks like this:

public class HashMap<K,V> extends AbstractMap<K,V>
implements Map<K,V>, Cloneable, Serializable {

Why is this the convention instead of more descriptive type names like HashMap<KEY,VALUE> ?

Answer 1

I think the main point here is simple convention. If you use single-letter for generic types and multiple-letter names for class names it comes obvious what you are dealing with. Also, if you used KEY or VALUE as you point, these names would look as constant names. Convention says:

UPPERCASE_AND_UNDERSCORE -> CONSTANTS

UpperCamelCase -> ClassNames

lowerCamelCase -> attributes

T,S,V -> genericTypes

Check out the official documentation

Type Parameter Naming Conventions

By convention, type parameter names are single, uppercase letters. This stands in sharp contrast to the variable naming conventions that you already know about, and with good reason: Without this convention, it would be difficult to tell the difference between a type variable and an ordinary class or interface name.

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

You'll see these names used throughout the Java SE API and the rest of this lesson.