Java abstract class, abstract constructor

Question

I'm trying to find a way to force the user that will implement an interface or will extend a class to create several constructors. I thought that a way to do that would be to create an abstract constructor in an abstract class, but I figured out that I can't do that.

How can I solve this issue? Is there some method to force the user to create a specific constructor?

Thanks!

Answer 1

You can't create an abstract constructor, but here's a workaround that you can do:

Implement an abstract class with all the constructors you want the user to implement. In each of them, you will have a single statement that refers to an abstract method from this class, which implementations will be in the subclass. This way you will force the user to provide a body for the constructor, via the abstract method implementation. For example:

public abstract class MyAbstractClass {
    public MyAbstractClass(int param) {
        method1(param);
    }

    public MyAbstractClass(int param, int anotherParam) {
        method2(param, anotherParam);
    }

    protected abstract void method1(int param);

    protected abstract void method2(int param, int anotherParam);
}

In the implementation, you will be forced to provide implementation of these two methods, which can represent the bodies of the two constructors:

public class MyClass extends MyAbstractClass {
    public MyClass(int param) {
        super(param);
    }

    public MyClass(int param, int anotherParam) {
        super(param, anotherParam);
    }

    public void method1(int param) {
        //do something with the parameter
    }

    public void method2(int param, int anotherParam) {
        //do something with the parameters 
    }
}

Answer 2

No. But, as Constructors are often substituted by factorymethods anyway, you could implement abstract factorymethods or just other patterns like the builderpattern.