Java Generics强制执行兼容的通配符

Question

我有这些课程.

class RedSocket {}
class GreenSocket {}
class RedWire {}
class GreenWire {}

我有一个使用2种泛型的类

public class Connection<W, S> {}

其中W是线型,S是套接字类型.

我正在尝试强制执行编译时检查以确保套接字和线具有相同的颜色.

我试过这样做:

public class Connection<W extends Wire & Color, S extends Socket & Color> {}

interface Color {}

interface Red extends Color {}
interface Green extends Color {}

interface Socket {}
interface Wire {}

class RedSocket implements Socket, Red {}
class GreenSocket implements Socket, Green {}
class RedWire implements Wire,  Red {}
class GreenWire implements Wire, Green {}

但这并不能确保Color两种泛型类型的使用相同,但仍然允许我这样做:

public class Connection<W extends Wire & Color, S extends Socket & Color> {
    public static void main(String[] args) {
        new Connection<RedWire, GreenSocket>();
        new Connection<GreenWire, RedSocket>();
    }
}

(Radiodef 在这里解释了为什么会发生这种情况)

如何强制执行编译时检查以确保套接字和电线具有相同的颜色？

Answer 1

似乎参数化Socket和Wire颜色更好:

interface Socket<C extends Color> {}
interface Wire<C extends Color> {}

class RedSocket implements Socket<Red> {}
class GreenSocket implements Socket<Green> {}
class RedWire implements Wire<Red> {}
class GreenWire implements Wire<Green> {}

这样,您可以将以下一个通用参数引入Connection:

public class Connection<C extends Color, M extends Wire<C>, Q extends Socket<C>> {...}

并像这样使用它:

new Connection<Red, RedWire, RedSocket>(); // ok
new Connection<Green, GreenWire, GreenSocket>(); // ok
new Connection<Green, GreenWire, RedSocket>(); // error

Answer 2

作为Tagir Valeev答案的一个小变化:你可以Connection通过使它的构造函数private(或者包可见)来摆脱类的第三个泛型参数,并提供一个工厂方法来创建Connection确保Color类型相同的实例对于给定的Wire- 和Socket类型:

class Connection<
    W extends Wire<? extends Color>, 
    S extends Socket<? extends Color>> 
{
    static <C extends Color, 
        W extends Wire<C>, 
        S extends Socket<C>> Connection<W, S> create()
    {
        return new Connection<W, S>();        
    }

    // Private constructor
    private Connection() {}
}

interface Color {}

interface Red extends Color {}
interface Green extends Color {}

interface Socket<C extends Color> {}
interface Wire<C extends Color> {}

class RedSocket implements Socket<Red> {}
class GreenSocket implements Socket<Green> {}
class RedWire implements Wire<Red> {}
class GreenWire implements Wire<Green> {}

public class CompatibleGenericsTest
{
    public static void main(String[] args)
    {
        Connection<RedWire, RedSocket> c0 = Connection.create(); // ok
        Connection<GreenWire, GreenSocket> c1 = Connection.create(); // ok
        Connection<GreenWire, RedSocket> c2 = Connection.create(); // error
    }
}