Concrete classes are used to implement the properties and behaviors of a domain and create instances out of it. The following Lion concrete class will inherit from the Animal abstract class:

class Lion : Animal {
    var noise: String

    constructor(noise: String) {
        this.noise = noise;
    }

    override fun name(): String? {
        return "Lion";
    }

    override fun makeNoise(): String? {
        return noise;
    }

}

The : symbol is used to specify in the class declaration that one class extends from another.