Java Generics enforcing compatible wildcards

Question

I've these classes.

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

I've a class which uses 2 generic types

public class Connection<W, S> {} 

where W is Wire type & S is Socket type.

I'm trying to enforce compile time check to ensure that socket & wire have the same color.

I tried doing this:

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 {} 

But this doesn't really ensure that the Color used is same for both generic types and still lets me do this:

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>();     } } 

(Why this happens has been explained brilliantly by Radiodef here)

How can I enforce compile time check to ensure that socket & wire have the same color?

Answer 1

Seems that it's better to parameterize Socket and Wire with 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> {} 

This way you can introduce one more generic parameter to the Connection:

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

And use it like this:

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