Kotlin中如何约束泛型？限定PublishSubject接收可食用水果

Great question! Let's break this down and find a clean, type-safe solution without subclassing PublishSubject.

First, your initial attempt PublishSubject<Fruit : Edible> uses invalid Kotlin syntax for generic constraints. What you need is a way to enforce that the stream only accepts types that are both Fruit and Edible. Kotlin supports this with intersection types, denoted by the & operator.

Correct Implementation #

Here's how to create your stream with the exact constraint you want:

// Create a PublishSubject that only accepts types which are Fruit AND Edible
val stream = PublishSubject.create<Fruit & Edible>()

This works because:

• The Fruit & Edible intersection type tells the compiler that only objects implementing both Fruit (or its subclasses) and Edible are allowed in the stream.
• Your Apple class (which extends Fruit and implements Edible) will be accepted without issue.
• Non-Fruit Edible types (like IceCream or Potato) will be rejected at compile time, as will Fruit subclasses that don't implement Edible.

Simplify with a Type Alias #

If you need to reuse this constrained type across multiple places, define a type alias to keep code clean:

// Define a reusable alias for types that are both Fruit and Edible
typealias EdibleFruit = Fruit & Edible

val stream = PublishSubject.create<EdibleFruit>()

Why Avoid the Filter Approach? #

Your initial idea of PublishSubject<Edible>.create().filter { it is Fruit } is not ideal because:

• It relies on runtime checks instead of compile-time type safety. Invalid objects can still be sent to the stream (they'll just be filtered out later), which hides potential bugs.
• It's less efficient, as it requires processing every Edible object even if it's not a Fruit.

Key Takeaway #

Using Kotlin's intersection types gives you compile-time enforcement of your dual constraint, no subclassing required. This is the most idiomatic and safe way to achieve your goal with RxJava in Kotlin.

内容的提问来源于stack exchange，提问作者Arturs Vancans