Question

I have an abstract super class and a few derived class.
I want to create a method

createNew()   

when invoked on a derived class it will create a new instance of the derived with a smart copy of all of its members.
All the members that are important for the creation are in the abstract class so the code should be the same.
can I write the implementation of createNew() in the super class?

something like that

SonA sonA2 = sonA1.createNew()   
SonB sonB2 = sonB1.createNew()  

the abs superclass should do the implementation because the code is identical .
thanks.

Answer 1

The implementation should be split between the abstract and the concrete classes. You can do it using the Template Method pattern:

public abstract class AbstractSon {
    protected abstract AbstractSon createNewImpl();
    public AbstractSon createNew() {
        AbstractSon res = createNewImpl();
        res.name = "default name";
        return res;
    }
}
public class Son1 extends AbstractSon {
    protected AbstractSon createNewImpl() {
        return new Son1();
    }
}
public class Son2 extends AbstractSon {
    protected AbstractSon createNewImpl() {
        return new Son2();
    }
}

You can split the responsibility differently to get the precise return type:

public abstract class AbstractSon {
    protected void prepare(AbstractSon toPrepare) {
        toPrepare.name = "default name";
    }
}
public class Son1 extends AbstractSon {
    public Son1 createNew() {
        Son1 res = new Son1();
        prepare(res);
        return res;
    }
}
public class Son2 extends AbstractSon {
    public Son2 createNew() {
        Son2 res = new Son2();
        prepare(res);
        return res;
    }
}