Covariant return types - JLS to support but JVM will not?
Could you please confirm the following? My understanding is that covariant return types will be supported by the JLS but the JVM will know nothing about them. That is, the invokevirtual instruction will not change to support covariant return types but rather the compiler will use bridge methods to simulate support. Is this correct?
Right: but is there support for this in 1.5?
javac -verbose -source 1.5 -target 1.5 *java
was run on a .java file with the following methods and it failed "getX already defined"
public Integer getX() {
return(null);
public String getX() {
return(null);
The above would be overloading by return type and fails to compile.
================
class A {
Exception f() {}
class B extends A {
BindException f() {} // overload or override?
The above would be covariant returns and works just fine right?
Similar Messages
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Covariant return types over Generic return types.
Feeling rather slow on the uptake having only just discovered covariant return types (thanks again cowwoc), I've been looking over one of our soon to be released APIs and think using covariants makes muchos sense for it but there's one issue:
public interface Foo {
Foo get();
public interface Bar {
Bar get();
public interface FooBar
extends Foo, Bar {
FooBar get();
}Which isn't allowed, as the compiler states the return types are incompatible (I think incorrectly as it's been further overridden). Its generic equivalent is:
public interface FooX<T> {
T get();
public interface BarX<T> {
T get();
public interface FooBarX
extends FooX<FooBarX>, BarX<FooBarX> {
FooBarX get();
}Which works, and well, but the extra hassle of declaring/using generics is turning out to be a pain for our users (this pattern is used extensively). Does anyone have any simple suggestions/recommendations to resolve either the covariant issue or simplify the generics one? Also does anyone have any preferences to which one they prefer (and why)?
Incidentally does anyone know the the rationale behind not allowing the first example, when clearly it can't be abused (can it)?BobDavies wrote:
. it is because you can not override a method or its definition with the same signatur, which does not include return type.What do you mean "does not include return type"?
method signature does not count the return type. i think java does allow you to repeat definitions in a sub class but when you put up a different retyrn type, it sees it as an attempt to do overriding, so it flags it out.
the fact that it works is becuase you are not extending anything but itsself:I am aware of the reasons for the Generic version working, it achieves our aim, I just don't like it when in use. And was wondering why the joining covariants do not work. I still can't see the reason for the FooBar not being allowed when you are allowed to return subtypes. You cannot break the inheritance/return types expectations like this can you? E.g. if I write a bit of code using Foo, it will still work with FooBar, and the same with Bar using FooBar, no? Can you show me an example of why this isn't allowed?your generic version should NOT work either; it is working becuase you are confused:
public interface FooBarX extends FooX<FooBarX>, BarX<FooBarX> only equals to
public interface FooBarX extends FooBarX, FooBarX -
Covariant return types without "-source 1.5"
Hello all,
I've just noticed that one can compile code that makes use of covariant return types even without specifying the flag -source 1.5.
Just out of curiosity: Was that behvaiour of javac intentional?
What's more, if you compile it with target 1.4
you can even run it with a 1.4 JRE!
Great, ain't it?I've just noticed that one can compile code thatmakes use of covariant return types even without
specifying
the flag -source 1.5.
Very interesting, actually it runs on jdk 1.3.1 too.
Actually using -target 1.4 or -target 1.3 generated the sameclass file in a small example (with the exception of the version number). And this class file contains 2 method entry per covariant(ed) method. One method with the return type specified in the source code and a second method with the return type of the base class. The second method is of synthetic and has a virtual call to the the this method.
with -target 1.5 you also get both method entries, but the second one is not synthetic anymore (??). -
Covariant Return Types - J2SE 5.0
I just read the tech tip regarding the new covariant return types in j2se 5.0
In the doc, it said that you can now compile two methods with the same signature but with different return types. I was wondering if anyone knew how this would affect what the following expression returns: anObject.getClass().getMethod(aMethodNameString, aClassArray)
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What would be involved in getting to gether to submit a formal JCP to enable covariant return types?
AFAIK, the changes are:
1) the language spec
2) the JVM Spec
3) changes to the compiler code
4) changes to the class file verifier in the JVM code
And that should be it. Who wants to organise this?My understanding is that the current generics proposal already includes covariant return types.
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