TYPES, VALUES AND VARIABLES(6)

TYPES, VALUES AND VARIABLES(6)

TYPE VARIABLES:

(Note before: this sectiont contains components that will be expanded on in later blogs. If you have questions about any statement particularly the fact that I decided to produce a skeletal illustration, feel free. )

A type variable is an unqualified identifier. They are introduced by generic class declarations, generic interface declarations, generic method declarations and generic constructor declarations^[1].

Let’s think of typical declarations of type variables assuming our type variable is T and T may have superclass S and there are interfaces I₁, I₂.

a. where T is declaring a class.

class T extends S implements I₁, I₂ { }

Notes

1. a class can extend only one class, that is a class can have only one superclass otherwise compile time error.

2. extending and implementing are optional contracts T can chose to embark upon, if there is none, then superclass is Object.

3. the order of the bounds S, I₁, I₂ are important. A class implements only interfaces never another class.

4. if I₁, I₂ are parameterized types, they should not be the same erasures[2].

b. where T is declaring an interface.

Interface T extends I₁, I₂ { }

1. (2) above applies here.

2. (4) above also applies here.

3. an interface can extend any number of interfaces.

On the jls: the discussion on §4.4 is quite illustrative but for beginners, might be somewhat confusing, keep this for reference after parameterized types and modifiers are discussed.

Footers:

^[1]. Genericity: I can best try to define genericity in this terms, as the ability of a type to define what types it can accept as parameters within its “type entity” (what I can think of here is xml’s namespace).

For example, if a house is built with the stipulation that: “N has the first flat and D has the second flat and that is how it should be”, then we can call this house generic and declare it as,

class house {} //house is the type variable.

Generics will be dwelt with in the next blog and later, we’ll discuss the scope of generics.

^[2]. Erasures. Erasures are mappings from a generic type to a non-generic one. Assume a parameterized type P and P, both correspond to the same erasure |P|. Both M and P have different erasures, |M| and |P|.