Java:特定枚举和通用 Enum<?>参数 [英] Java: specific enums and generic Enum&lt;?&gt; parameters

问题描述

我想将任何枚举值传递给实用程序类中的方法，并获得另一个相同枚举类型的枚举值.像这样:

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;
    } 
}

方法getEnumAttribute()的使用:

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

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

此代码功能齐全，Eclipse 编译并运行它而不会出现警告或错误，而且它的工作原理非常棒.

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

但是，当我从命令行通过 Maven2 清理和编译项目时，它在 getEnumAttribute 调用的地方失败并出现错误:

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

我在 Eclipse 和 Maven 中使用 Sun JDK 1.6:

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"

问题:

1. 为什么这段代码在 Eclipse 中是可编译和功能的，而编译在 Maven 中失败，据我所知使用相同的 javac 编译器?

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?

将特定枚举传递给通用 Enum 参数有什么问题?

What's wrong with passing specific enums to generic Enum<?> parameters?

谢谢，

马丁·沙伊纳

推荐答案

1. Eclipse 编译器和 javac 有一些区别，尤其是在泛型方面.人们认为 eclipse 是正确的，但这没关系:)

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 :)

试试

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

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