java force an extending class

Question

In Java, can I somehow force a class that extends an abstract class to implement its constructor with a Object as a parameter?

Something like

public abstract class Points {

    //add some abstract method to force constructor to have object.
}

public class ExtendPoints extends Points {

    /**
     * I want the abstract class to force this implementation to have
     *  a constructor with an object in it?
     * @param o
     */
    public ExtendPoints(Object o){

    }
}

Answer 1

You can use a constructor with a parameter in your abstract class (make it protected if you want to dis-allow anonymous subclasses).

public abstract class Points{
    protected Points(Something parameter){
        // do something with parameter
    }
}

Doing that, you force the implementing class to have an explicit constructor, as it must call the super constructor with one parameter.

However, you cannot force the overriding class to have a constructor with parameters. It can always fake the parameter like this:

public class ExtendPoints extends Points{
    public ExtendPoints(){
        super(something);
    }
}

Answer 2

As said by others before, the signatue of Constructors cvannot be enforced, but you could enforce a particular set of arguments by using the AbstractFactory pattern instead. Then you can define the create methods of your factory interface to have a particular signature.

Answer 3

Probably there it's not possible at compile time, but you can use reflection to check at run time if the desired constructor was declared:

public abstract class Points {

    protected Points() {
        try {

            Constructor<? extends Points> constructor = 
                getClass().getDeclaredConstructor(Object.class);
            if (!Modifier.isPublic(constructor.getModifiers()))
                throw new NoSuchMethodError("constructor not public");

        } catch (SecurityException ex) {
            throw new RuntimeException(ex);
        } catch (NoSuchMethodException ex) {
            throw (NoSuchMethodError) new NoSuchMethodError().initCause(ex);
        }
    }
}

Answer 4

没有构造函数不会被继承，因此除非您没有指定构造函数并获取默认的无参数构造函数，否则每个类都需要提供它自己的构造函数。

Answer 5

If you add a public Points(Object o) {} constructor to Points, you force any subclass constructors to call that super constructor. However I don't think there's no way of ensuring that subclasses use that exact constructor signature.

Answer 6

EDIT

Well, no, its not possible to force the implementation of a constructor with argument.