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python_newbie34
·
What does the out keyword mean in Kotlin and how to use it with generics?
Hi! I started learning Kotlin after Java and encountered a construct I don't understand. In Java, I'm used to generics with wildcards like `? extends T` and `? super T`. But in Kotlin, I see the `in` and `out` keywords directly in the class declaration.
For example, I came across this code:
```kotlin
interface Source {
fun get(): T
}
```
What does `out` before `T` mean? I tried writing my own class without `out`:
```kotlin
class Box(private val item: T) {
fun get(): T = item
fun set(newItem: T) { /* ... */ }
}
```
And it works. But if I add `out`, the compiler complains about the `set` method. Why? And what is `out` even needed for? In what situations should it be used? How is this related to the PECS principle (Producer Extends, Consumer Super) from Java? Please explain.
