Question

I want to pass any enum value to method in utility class and get another enum value of same enum type. Something like this:

public class XMLUtils {

    public static Enum<?> getEnumAttribute(Element element, String name, 
            Enum<?> defaultValue) {

        if (element.hasAttribute(name)) {
            String valueName = element.getAttribute(name);
            // search for value 
            for (Enum<?> value: defaultValue.getClass().getEnumConstants())
                if (value.toString().equalsIgnoreCase(valueName))
                    return value;
        }
        // not found, return default value
        return defaultValue;
    } 
}

Use of method getEnumAttribute():

// simple enum
public enum EUploadMethod {
    INSERT, UPDATE, DELETE
}

// read enum value from XML config file
EUploadMethod method = XMLUtils.getEnumAttribute(element, "method",
        EUploadMethod.INSERT);

This code is fully functional, Eclipse compiles and runs it without warnings or errors and it works like a charm.

But when I clean and compile project from command line by Maven2, it fails with error on line where is getEnumAttribute called:

$ mvn clean compile
....
[ERROR] /home/.... DataUploader.java:[173,53] inconvertible types
found   : java.lang.Enum<capture#414 of ?>
required: .....DataUploader.EUploadMethod

I am using Sun JDK 1.6 in either Eclipse and Maven:

$ mvn -version
Apache Maven 2.2.1 (r801777; 2009-08-06 21:16:01+0200)
Java version: 1.6.0_14
Java home: /usr/lib/jvm/java-6-sun-1.6.0.14/jre
Default locale: en_US, platform encoding: UTF-8
OS name: "linux" version: "2.6.27-17-generic" arch: "i386" Family: "unix"

Questions:

1. Why this code is compilable and functional in Eclipse, and compile fails in Maven which using as far as I know same javac compiler?

2. What’s wrong with passing specific enums to generic Enum<?> parameters?

Thanks,

Martin Schayna

Answer 1

1. Eclipse compiler and javac have some differences, especially when it comes to generics. It is believed that eclipse is correct, but that doesn’t matter 🙂

2. Try

   public static <T extends Enum<T>> Enum<T> getEnumAttribute(Element element, String name, 
       Enum<T> defaultValue) {
      ...
   }