类型参数名称P和R是什么意思?它们是如何使用的?

我最近在SO上发布了一个问题( How do I use double dispatch to analyze intersection of graphic primitives? )其中一个答案(我已经接受)使用了泛型,包括 <P> 和 <R> 。它们不在Oracle文档中 Java generics list 但我在其他地方看到过它们的使用(例如。 Generics overkill in visitor pattern )-它们是特定于访客模式的吗?为什么两者都是 super 和 extends 习惯于

代码为:

public interface ShapeVisitor<P, R> { 
    R visitRect(Rect rect, P param);
    R visitLine(Line line, P param);
    R visitText(Text text, P param);
}

public interface Shape {
    <P, R> R accept(P param, ShapeVisitor<? super P, ? extends R> visitor);
    Shape intersectionWith(Shape shape);
}

public class Rect implements Shape {

    public <P, R> R accept(P param, ShapeVisitor<? super P, ? extends R> visitor) {
        return visitor.visitRect(this, param);
    }

    public Shape intersectionWith(Shape shape) {
        return shape.accept(this, RectIntersection);
    }

    public static ShapeVisitor<Rect, Shape> RectIntersection = new ShapeVisitor<Rect, Shape>() {
        public Shape visitRect(Rect otherShape, Rect thisShape) {
            // TODO...
        }
        public Shape visitLine(Line otherShape, Rect thisShape) {
            // TODO...
        }
        public Shape visitText(Text otherShape, Rect thisShape) {
            // TODO...
        }
    };
}

我会很感激