A type system's main purpose is to reduce software bugs. Types are assigned to each data value. Rules define what operations each data type supports. Checking determines whether our code follows those rules. Type safety means that the code follows all the type rules.

Kinds of Types

Static vs Dynamic

Static types are known and checked at compile-time.

Dynamic types are known and checked at run-time.

In other words, if the compiler knows the type, it's static. If the compiler cannot know the type, it's dynamic. The actual run-time value of a dynamic type determines the operations it supports.

Strong vs Weak

strong types provide less freedom of type conversion

weak types provide more freedom of type conversion

Explicit vs Implicit

explicit our code tells the compiler the data type

implicit the compiler figures out the data type

Safe vs Unsafe

safe we must treat the raw-bytes as a compatible type

unsafe we are able to treat the raw-bytes as whatever type we want

Compile-time vs Run-time

compile-time the time at which we build the code

run-time the time at which we execute the code (aka execution-time)

Variance

covariance a more specific type of data ("cat") can go into a more general type of data ("animal") Animal a = new Cat();

contravariance a more general type of data ("animal") can go into a more specific type of data ("cat") CatComparer cc = new AnimalComparer(); cast, changing the compile-time type to fit the actual run-time type convert, changing data from one type into another