Kotlin - no_image

What is the difference between “*” and “Any” in generics

The type MutableList<*> represents the list of something (you don’t know what exactly). So if you try to add something to this list, you won’t succeed. It may be a list of Strings, or a list of Ints, or a list of something else. The compiler won’t allow to put any object in this list at all because it cannot verify that the list accepts objects of this type. However, if you attempt to get an element out of such list, you’ll surely get an object of type Any?, because all objects in Kotlin inherit from Any.

Additionally List<*> can contain objects of any type, but only that type, so it can contain Strings (but only Strings), while List<Any> can contain Strings and Integers and whatnot, all in the same list.

Example

Let’s say that you have this generic class Crate that you intend to use for storing fruits. This class is invariant in T. This means, this class can consume as well as produce T (fruits). In other words, this class has functions that take T as argument(consume) as well as return T(produce). The size() function is T-independent, it neither takes T nor does it return T:

class Crate<T> {
    private val items = mutableListOf<T>()
    fun produce(): T = items.last()
    fun consume(item: T) = items.add(item)
    fun size(): Int = items.size
}

Star projection is no producer, no consumer of T

By projecting the Crate as *, you are telling the compiler: “give me an error when I accidentally use the Crate class as a producer or consumer of T because I don’t want to use the functions or properties that consume and produce T. I just want to safely use the T-independent functions and properties like size()”:

fun useAsStar(star: Crate<*>) {

    // T is unknown, so the star produces the default supertype Any?.
    val anyNullable = star.produce()         // Not useful

    // T is unknown, cannot access its properties and functions.
    anyNullable.getColor()                   // Error

    // Cannot consume because you don't know the type of Crate.
    star.consume(Fruit())                    // Error

    // Only use the T-independent functions and properties.
    star.size()                              // OK
}

Any is not a projection

When you say Crate<Any>, you are not projecting, you are simply using the original invariant class Crate<T> as it is, which can produce as well as consume Any:

fun useAsAny(any: Crate<Any>) {

    // T is known to be Any. So, an invariant produces Any.
    val anyNonNull = any.produce()           // OK

    // T is known to be Any. So, an invariant consumes Any.
    any.consume(Fruit())                     // OK

    // Can use the T-independent functions and properties, of course.
    any.size()                               // OK
}

Links

Difference between “*” and “Any” in Kotlin generics

Further reading

Generics: in, out, where

credit goes to @swayangjit