I have Decorator interface:

fun interface Decorator {
    decorate(text: String): String
}

And I want to implement it for class Padded by lambda like this:

class Padded(padding: String) : Decorator {
    text -> "$padding $text $padding"
}

I don't want to write method signature, introduce another class wrapper, or replace class with higher-order function. How can I do it?

CodePudding user response：

The point of using functional interfaces is to be able to provide a lambda in places where an instance of such interface is required, instead of instantiating a class that implements that interface (or creating an anonymous object implementing it). Functional interfaces don't help with implementing interfaces in class declarations.

What you could do is declare a Padded function, which returns an instance of Decorator using a lambda:

fun Padded(padding: String) = Decorator { "$padding $it $padding" }

But if you want a class, you'll have to implement the function explicitly:

class Padded(padding: String) : Decorator {
    override fun decorate(text: String) = "$padding $text $padding"
}

which is honestly the same number of lines as the syntax you were looking for, and very clear to everyone.

CodePudding user response：

It can be implemented with delegate pattern.

class Padded(padding: String) : Decorator by (Decorator {
    text -> "$padding $text $padding"
})