泛型类型的 Java 枚举

Java enums in generic type

提问by Marcin Cylke

I'd like to create a generic enum-based mapper for IBatis. I'm doing this with the below code. This does have compile time errors, which I don't know how to fix. Maybe my solution is just plain wrong (keep in mind the use of IBatis), in such case please suggest something better.

我想为 IBatis 创建一个通用的基于枚举的映射器。我正在使用以下代码执行此操作。这确实有编译时错误，我不知道如何解决。也许我的解决方案完全错误（请记住使用 IBatis），在这种情况下，请提出更好的建议。

Any help appreciated.

任何帮助表示赞赏。

What I want to achieve is to define subsequent mappers as:

我想要实现的是将后续映射器定义为：

public class XEnumTypeHandler extends CommonEnumTypeHandler<X> {
}

The current code:

当前代码：

public class CommonEnumTypeHandler<T extends Enum> implements TypeHandlerCallback {

 public void setParameter(ParameterSetter ps, Object o) throws SQLException {
  if (o.getClass().isAssignableFrom(**T**)) { 
   ps.setString(((**T**) o).value().toUpperCase());
  } else
   throw new SQLException("Excpected ParameterType object than: " + o);
 }

 public Object getResult(ResultGetter rs) throws SQLException {
  Object o = valueOf(rs.getString());
  if (o == null)
   throw new SQLException("Unknown parameter type: " + rs.getString());
  return o;
 }

 public Object valueOf(String s) {
  for (T pt : T.**values()**) {
   if (pt.**value()**.equalsIgnoreCase(s))
    return pt;
  }
  return null;
 }
}

I've added error markings to the above code, the error messages are in order:

我在上面的代码中添加了错误标记，错误信息按顺序排列：

• T cannot be resolved
• The method value() is undefined for the type T
• The method values() is undefined for the type T
• The method values() is undefined for the type T

• T无法解析
• 未定义类型 T 的方法 value()
• 方法 values() 对于类型 T 是未定义的
• 方法 values() 对于类型 T 是未定义的

I've solved this issue with the following code:

我已经用以下代码解决了这个问题：

public class CommonEnumTypeHandler<T extends Enum> implements TypeHandlerCallback {

    Class<T> clazz;

    public CommonEnumTypeHandler(Class<T> clazz) {
        this.clazz = clazz;
    }

    public void setParameter(ParameterSetter ps, Object o) throws SQLException {
        if (o.getClass().isAssignableFrom(clazz)) {
            ps.setString(((T) o).name().toUpperCase());
        } else
            throw new SQLException("Excpected " + clazz + " object than: " + o);
    }

    public Object getResult(ResultGetter rs) throws SQLException {
        Object o = valueOf(rs.getString());
        if (o == null)
            throw new SQLException("Unknown parameter type: " + rs.getString());
        return o;
    }

    public Object valueOf(String s) {
        return Enum.valueOf(clazz, s);
    }
}

Inheriting from this class I do:

我从这个类继承：

public class SalesChannelTypeHandler extends CommonEnumTypeHandler<SalesChannel> {

    public SalesChannelTypeHandler() {
        super(SalesChannel.class);
    }

    public SalesChannelTypeHandler(Class<SalesChannel> clazz) {
        super(clazz);
    }

}

采纳答案by polygenelubricants

I'm not sure what you're doing (a general overview in words would be nice), but:

我不确定你在做什么（用文字概括一下会很好），但是：

• You can't do isAssignableFrom(T)(you need a Classobject), and you can't do instanceof Teither (generics are non-reified). You maywant to pass Class<T>type tokens instead.
• Have you looked at EnumMap?

• 你不能做isAssignableFrom(T)（你需要一个Class对象），你也不能做instanceof T（泛型是非具体化的）。您可能希望改为传递Class<T>类型标记。
• 你看了EnumMap吗？

See also

也可以看看

• Java Tutorials/Runtime Type Tokens
• Neal Gafter's Blog - Super Type Tokens
• Josh Bloch - Typesafe Heterogenous Container (THC) pattern (PDF)

• Java 教程/运行时类型标记
• Neal Gafter 的博客 - 超级类型令牌
• Josh Bloch - 类型安全的异构容器 (THC) 模式 (PDF)

---

---

It's still not clear what is desired, but perhaps it's something along the lines of this:

目前还不清楚需要什么，但也许是这样的：

enum Color { BLACK, WHITE; }

public static void main(String[] args) {
    Color c = Enum.valueOf(Color.class, "black".toUpperCase());
    System.out.println(c); // prints "BLACK"
}

So we use Enum.valueOfthat takes a type token Class<T extends Enum<T>>, and ask it for the enumconstant with a given name. valueOfis NOTcase-insensitive, but by conventions, allconstants should be in uppercase, so we simply take the query string and turn it .toUpperCase().

所以我们使用Enum.valueOf它接受一个类型 token Class<T extends Enum<T>>，并要求它enum提供具有给定名称的常量。valueOf是不区分大小写的，而是由惯例，所有的常量应该是大写的，所以我们干脆把查询字符串，并把它.toUpperCase()。

回答by BalusC

As pointed by Polygenelubricants, you need to pass concrete runtime types around, e.g. Class<?>instead of syntactic compiletime types like generic parameters. Here's a rewrite how you could use it:

正如 Polygenelubricants 所指出的，您需要传递具体的运行时类型，例如，Class<?>而不是像泛型参数这样的语法编译时类型。这是重写如何使用它：

public abstract class CommonEnumTypeHandler<E extends Enum<E>> implements TypeHandlerCallback {
    private Class<E> enumClass;

    public CommonEnumTypeHandler(Class<E> enumClass) {
        this.enumClass = enumClass;
    }

    public void setParameter(ParameterSetter ps, Object o) throws SQLException {
        if (enumClass.isInstance(o)) {
            ps.setString((enumClass.cast(o)).name().toUpperCase());
        } else {
            throw new SQLException("Excpected ParameterType object than: " + o);
        }
    }

    public Object getResult(ResultGetter rs) throws SQLException {
        try {
            return Enum.valueOf(enumClass, rs.getString());
        } catch (IllegalArgumentException e) {
            throw new SQLException("Unknown parameter type: " + rs.getString(), e);
        }
    }
}

Which you can then use as follows:

然后您可以按如下方式使用它：

public class XEnumTypeHandler extends CommonEnumTypeHandler<X> {
    public XEnumTypeHandler() {
        super(X.class);
    }
}