Java：如何使用超类的静态方法从其创建子类的实例

Question

My specific problem is that I want to create a static method in a base class which saves away the instances of the subclass in a file. And another static method in the base class which retrieves the data and recreates the instances of the subclass.

I have got all of it working except where in my strategy I need to create an instance of the subclass within the static method of the base class. Please see what I mean below.

And so the general question is in Java how do you Create an instance of a subclass from within a Static method of the superclass.

class Base

    public static Void saveAllInstances(){
       saveAttributesofAllInstancesInFile();  // this works
    }

    public static Void retrieveAllInstances(){
       destroyAllInstances(); // this works
       getInstanceAttributesFromFile();  // this works and returns e.g. ArrayList<HashMap>

       for( instanceAttributes : retrievedInstances ){

         // Here is the part I cant figure out: I want to create an instance of 
         // the subclass here 
         sub = new Sub();

         sub.setAttributes(instanceAttributes); //this works
       }
}

class Sub extends Base{
}

Answer 1

Apart from the fact that your code isn't valid Java syntax, exactly the way you've done. However as others have suggested what you're doing is actually probably better suited for a (Pure Fabrication) Factory Pattern.

Base.java

public class Base {
    public static void retrieveAllInstances() {
        Sub sub = new Sub();
    }
}

Sub.java

public class Sub extends Base{
}

Works fine. Apart from the cyclic reference, but if you don't care about maintainability, knock yourself out.

Answer 2

Using the factory pattern works fine. And looks something like this in pseudocode:

class MainFactory{

    protected Worker makeWorker(){
        return new Worker();
    }

    public createIntanceOfSubworker(){
       Worker i = makeWorker()
    }
}

class SubFactory extends MainFactory{
    @override
    protected SubWorker makeWorker(){
        return new SubWorker();
    }
}

class Worker{
}

class SubWorker extends Worker{
}