Why is javac failing on @Override annotation

Question

Eclipse is adding @Override annotations when I implement methods of an interface. Eclipse seems to have no problem with this. And our automated build process from Cruise Control seems to have no problem with this. But when I build from the command-line, with ant running javac, I get this error:

[javac] C:\path\project\src\com\us\MyClass.java:70: method does not override a method from its superclass
[javac]     @Override
[javac]      ^
[javac] 1 error

Eclipse is running under Java 1.6. Cruise Control is running Java 1.5. My ant build fails regardless of which version of Java I use.

Answer 1

I can't really explain the problem you're seeing but it seems to be related to the fact that JDK 5 will not allow @Override on implemented methods of an interface, only on overridden methods present in a super class.

JDK 6 will allow @Override on any of them.

If your ant build fails it may be passing a source parameter to javac, asking for JDK 5 compliance.

Answer 2

@Override tags for implemented methods are new to Java 1.6. In Java 1.5 @Override is only correct when overriding a method in a base class. Read more here and here.

Answer 3

The @Override annotation spec changed in Java 1.6. In Java 1.5, the compiler did not allow the @Override annotation on implemented interface methods, but in 1.6 it does. First search result I found is a blog post here.. It may not have been well documented, but it did change.

Eclipse is adding it because your Eclipse is set for 1.6 compliance...you should try to keep your build and eclipse environments on the same version of Java. It's unclear to me by your specifying Cruise Control is running Java 5 on whether or not it is compiling using a separate JDK6 or not.

Separate from the above 1.5 vs 1.6 @Override annotation rules, remember that Eclipse has its own compiler implementation (not javac) and will occasionally have different behavior. Whenever something compiles in Eclipse, but not Ant or Maven, you will need to find a way to make both compilers happy.

Answer 4

Eclipse would be pointing to 1.6 version of Java rather than 1.5. See here for configuring java version in eclipse.

Answer 5

The direct answer to the question "Why" an error is raised by javac when @Override is used in the context of a method implementation is actually in the java specifications:

"The rationale for this is that a concrete class that implements an interface will necessarily override all the interface's methods irrespective of the @Override annotation, and so it would be confusing to have the semantics of this annotation interact with the rules for implementing interfaces."

See http://java.sun.com/docs/books/jls/third_edition/html/interfaces.html#9.6.1.4

But apparently someone changed his mind for java 1.6 and 1.5 u21...