Covariant Parameter Types

Read - What is covariance?
Covariant Return type

Now we have a look at covariant method parameters, which are considered unsound.


Consider the following code:

public interface TestInterface { }

public class TestClass implements TestInterface { }

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

public class Test {
 private List<testclass> list;

 public TestInterface test() {
   list = new ArrayList<testclass>();
   list.add(new TestClass());

   return covariant(list);
 }

 public TestInterface covariant(List<testinterface> ilist) {
   return ilist.remove(0);
 }
}

Now there is absolutely no reason why this should not work. It is trivially inferable that the above code treats ilist as covariant in the list-type - and that therefore this code is statically correct.

Of course Java's typing has never been particularly smart. List.add(T1) is contra-variant in t1, and T2 List.get(int) is co-variant in t2; so the Java compiler is correct to infer that in the general case List and List are substitutable iff t1 == t2.

If we can't declare a generic parameter to be covariant in its type parameter we have a serious problem - it means that any non-trivial algorithm involving collections is going to run afoul of this. You might consider trying to cast your way around it:

public TestInterface test() {
   list = new ArrayList<testclass>();
   list.add(new TestClass());

   return covariant((List<testinterface>)list);
 }

but not surprisingly that didn't work.

Test.java:11: inconvertible types
found   : java.util.List<testclass>
required: java.util.List<testinterface>
 return convariant((List<testinterface>)list);
                                       ^
1 error

If you continue to hack at it you might try a double cast via a non-generic List.

public TestInterface test() {
   list = new ArrayList<testclass>();
   list.add(new TestClass());

   return covariant((List<testinterface>)((List)list));
 }

This works but leaves us with the unchecked/unsafe operation warning:

Note: Test.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.

Now this is a perfectly reasonable warning - it is unchecked; it is unsafe; and more importantly it does violate encapsulation. The problem here is that the caller should not be defining the type invariants of the callee - that's the job of the method signature!

The correct solution is to allow us to declare covariant() to be covariant in its argument; and fortunately Java does support this.

To declare an argument to be covariant in its type parameter you can use the extends keyword:

public TestInterface covariant(List<? extends TestInterface> ilist) {
   return ilist.remove(0);
 }

To declare an argument to be contravariant in its type parameter you use the super keyword:

public void contravariant(List<? super TestClass> clist, TestClass c) {
   clist.add(c);
 }