Java：使用泛型指定类型的单个子类型

Question

I have an interface called DrawableSegment and multiple classes which implement that interface, for example LineSegment and PercentageSegment .

I also have a another class called BarChart which makes use of these classes. For instance BarChart has a method called Add(DrawableSegment segment) which accepts any object which implements that interface.

I would like to restrict this to be objects of the same type which implements that interface. So I do not want to mix LineSegments with PercentageSegments , if I add in a LineSegment I want the rest of the additions to also be LineSegments . If I add in PercentageSegments , I would like the rest of them to be PercentageSegments , if I did attempt to add in a LineSegment I would like it to be an type error.

Is there a way I can express this?

Answer 1

Is this what you mean?

import java.util.ArrayList;
import java.util.List;

interface DrawableSegment {}

class LineSegment implements  DrawableSegment {}

class PercentageSegment implements  DrawableSegment {}

class BarChart<T extends DrawableSegment> {
    private List<T> drawableSegments = new ArrayList<>();

    public void add(T drawableSegment) {
        this.drawableSegments.add(drawableSegment);
    }

    public List<T> getDrawableSegments() {
        return this.drawableSegments;
    }
}

public static void main(String[] args) {
    BarChart<LineSegment> barCharLineSegment = new BarChart<LineSegment>();
    barCharLineSegment.add(new LineSegment());
    barCharLineSegment.add(new PercentageSegment()); // Compiler Error: cannot be applied
}

Answer 2

If I understood your question correctly, you want to check the type of the object at runtime , meaning if a LineSegment is added then the subsequent aditions would only allow LineSegment and not PercentageSegment . In that case, at runtime we can check what is the class of the first item and compare it with the next items. Please see if my code helps:

interface DrawableSegment {}

class LineSegment implements  DrawableSegment {}

class PercentageSegment implements  DrawableSegment {}

class Barchart<T extends DrawableSegment> {
    List<DrawableSegment> drawableSegList = new ArrayList<>();

    public static void main(String[] args) throws IOException  {
       Barchart main = new Barchart();
       main.add(new LineSegment());
       System.out.println(main.getListSize());
       main.add(new LineSegment());
       System.out.println(main.getListSize());
       main.add(new PercentageSegment());
       System.out.println(main.getListSize());
    }
    public void add(T t){
        if(!drawableSegList.isEmpty() && drawableSegList.get(0).getClass() != t.getClass()){
            System.out.println("First element :"+ drawableSegList.get(0).getClass()+" Does not match next Element "+t.getClass());
            throw new RuntimeException();
        }
        drawableSegList.add(t);
    }
    public int getListSize(){
        return this.drawableSegList.size();
    }
}