Introduce Self interface

[jenetics]

Some classes/interface in the library are using recursive generic types, as a possibility to model a self type. Similar to the abstract Enum<E extends Enum<E>> type in the JDK. The Gene type is such a type.

public interface Gene<A, G extends Gene<A, G>> {
    G newInstance(final A value);
}

The purpose of this, as illustrated in the code snippet above, is to avoid casts when working with the interface definition alone and creating new instances of the concrete implemented type G. In a proper implementation, it is save to cast this to G. But this is a very implicit convention. Nobody prevents you from creating the following class.

class FooGene implements NumericGene<Double, DoubleGene> {
    ...
}

Instead of the correct declaration of

class FooGene implements NumericGene<Double, FooGene> {
    ...
}

To make the intention more explicit, the following interface is added

interface Self<S extends Self<S>> {
    @SuppressWarnings("unchecked")
    default S self() {
        return (S)this;
    }
}

It is still possible to create invalid implementations, but the interface allows to make the intended purpose more explicit, also via the Javadoc of the interface.

[jenetics]

Merged into r7.0.0 branch.