实例化另一个抽象类的内部抽象类

Question

I have an abstract class that has another inner abstract class like following:

public abstract class TemplateFace {

    public abstract Layout generateFace();

    public abstract class PopulateGridHandler implements ClickHandler {

        @Override
        public abstract void onClick(ClickEvent event);
    }
}

And I have another class which extends this TemplateFace class as the following:

public class AdminFace extends TemplateFace {
    private ListGrid adminListGrid;

    public Layout generateFace() {
        adminListGrid = GridBuilder.buildFullGrid(this);
    }

    public class PopulateGridHandler implements ClickHandler {

        @Override
        public void onClick(ClickEvent event) {
            // some implementations...
        }
    }
}

And Finally I am trying to use the inner PopulateGridHandler class of the AdminFace class (the class that extended the abstract one) inside another class like the following:

public class GridBuilder {

    // Here i get the argument as abstract class
    public static ListGrid buildFullGrid(TemplateFace templateFace)
    {
        ToolStripButton addRowButton = new ToolStripButton();
        addRowButton.setPrompt("Add new record");

        // I get the error here saying: Cannot instantiate the type 
        // TemplateFace.PopulateGridHandler
        addRowButton.addClickHandler(  templateFace.new PopulateGridHandler() );
    }
}

ALL IN ALL , I am just trying to utilize an abstract class which has another abstract class in it. However the java compiler thinks that I am trying to instantiate an abstract class. How can I workaround the problem?

Thanks for any help, recommendation, comments...

Answer 1

The Java compiler only knows about the declared type of your parameter templateFace that is - in this case - TemplateFace . So the expression

templateFace.new PopulateGridHandler()

does exactly mean that: Instantiate the abstract class PopulateGridHandler (which isn't allowed).

Actually it seems that you tried to override an inner abstract class definition in a subclass. That won't work. Only methods can be overridden. And this is also the way to go:

public abstract class TemplateFace {
    public abstract Layout generateFace();
    public abstract PopulateGridHandler createPopulateGridHandler();
    protected abstract class PopulateGridHandler implements ClickHandler{
        @Override
        public abstract void onClick(ClickEvent event); 
    }
}

Then declare your subclass as following:

public class AdminFace extends TemplateFace{
    private ListGrid adminListGrid;
    public Layout generateFace() {
        adminListGrid = GridBuilder.buildFullGrid(this);       
    }
    public PopulateGridHandler createPopulateGridHandler() {
        return new MyPopulateGridHandler();
    }
    private class MyPopulateGridHandler extends PopulateGridHandler {
        @Override
        public void onClick(ClickEvent event) { // some implementations ... }
    }
}

And then simply use the method createPopulateGridHandler for creating such a class:

public static ListGrid buildFullGrid(TemplateFace templateFace) {
    ToolStripButton addRowButton = new ToolStripButton();
    addRowButton.setPrompt("Add new record");
    addRowButton.addClickHandler(templateFace.createPopulateGridHandler());
}