Subtyping

Abstract

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• A data type $T$ is a subtype of a data type $S$ if a piece of code written for a variable of data type $S$ can also be safely used on a variable of data type $T$
• The relationship can be represented with $T<:S$, where $S$ is also known as the super type of $T$

Java Primitive Subtyping

Subtyping among Java Primitive Datatype

• byte <: short <: int <: long <: float <: double
• char <: int <: long <: float <: double

Important

Subtyping is Reflexive, $T<:T$

Subtyping is Transitive, if $S<:T$ and $T<:U$, then $S<:U$

OOP

It can be a type that inherits from a OOP Class or Interface. Less specific compared to Sub-class

Compile-time Type vs Run-time Type

• We can assign a subtype instance to a supertype variable during compilation
• At runtime, we can call all methods with matching signatures declared in the supertype on the subtype instance, including those overridden in the subtype

Important

The runtime type of an object is always a subtype of its compile-time type.

At compile time, the compile-time type is identified, and the method descriptor in the compile-time type is stored in the generated bytecode. When the runtime type is determined during runtime, Java then looks for an accessible method with a matching descriptor in that runtime type.

Check if the run-time type of obj is a subtype of Type in Java

obj instanceof Type