Why shouldn't Java enum literals be able to have generic type parameters?

This has been discussed as of JEP-301 Enhanced Enums, which was withdrawn, regrettably. The example given in the JEP is, which is precisely what I was looking for:

enum Argument<X> { // declares generic enum
   STRING<String>(String.class), 
   INTEGER<Integer>(Integer.class), ... ;

   Class<X> clazz;

   Argument(Class<X> clazz) { this.clazz = clazz; }

   Class<X> getClazz() { return clazz; }
}

Class<String> cs = Argument.STRING.getClazz(); //uses sharper typing of enum constant

Unfortunately, the JEP was struggling with significant issues, which couldn't be resolved: http://mail.openjdk.java.net/pipermail/amber-spec-experts/2017-May/000041.html

The answer is in the question:

because of type erasure

None of these two methods are possible, since the argument type is erased.

public <T> T getValue(MyEnum<T> param);
public T convert(Object);

To realise those methods you could however construct your enum as:

public enum MyEnum {
    LITERAL1(String.class),
    LITERAL2(Integer.class),
    LITERAL3(Object.class);

    private Class<?> clazz;

    private MyEnum(Class<?> clazz) {
      this.clazz = clazz;
    }

    ...

}

Because you can't. Seriously. That could be added to the language spec. It hasn't been. It would add some complexity. That benefit to cost means it isn't a high priority.

Update: Currently being added to the language under JEP 301: Enhanced Enums.

There are other methods in ENUM that wouldn't work. What would MyEnum.values() return?

What about MyEnum.valueOf(String name)?

For the valueOf if you think that compiler could make generic method like

public static MyEnum valueOf(String name);

in order to call it like MyEnum<String> myStringEnum = MyEnum.value("some string property"), that wouldn't work either. For example what if you call MyEnum<Int> myIntEnum = MyEnum.<Int>value("some string property") ? It is not possible to implement that method to work correctly, for example to throw exception or return null when you call it like MyEnum.<Int>value("some double property") because of type erasure.

Frankly this seems like more of a solution in search of a problem than anything.

The entire purpose of the java enum is to model a enumeration of type instances that share similiar properties in a way that provides consistency and richness beyond that of comparable String or Integer representations.

Take an example of a text book enum. This is not very useful or consistent:

public enum Planet<T>{
    Earth<Planet>,
    Venus<String>,
    Mars<Long>
    ...etc.
}

Why would I want my different planets to have different generic type conversions? What problem does it solve? Does it justify complicating the language semantics? If I do need this behavior is an enum the best tool to achieve it?

Additionally how would you manage complex conversions?

for Instance

public enum BadIdea<T>{
   INSTANCE1<Long>,
   INSTANCE2<MyComplexClass>;
}

Its easy enough with String Integer to supply the name or ordinal. But generics would allow you to supply any type. How would you manage the conversion to MyComplexClass? Now your mucking up two constructs by forcing the compiler to know that there are a limited subset of types that can be supplied to generic enums and introducing additional confusion to concept(Generics) that already seems elude a lot of programmers.

By using this Java annotation processor https://github.com/cmoine/generic-enums, you can write something like (the convert method were implemented as an example):

import org.cmoine.genericEnums.GenericEnum;
import org.cmoine.genericEnums.GenericEnumParam;

@GenericEnum
public enum MyEnum {
    LITERAL1(String.class) {
        @Override
        @GenericEnumParam
        public Object convert(Object o) {
            return o.toString(); // example
        }
    },
    LITERAL2(Integer.class) {
        @Override
        @GenericEnumParam
        public Object convert(Object o) {
            return o.hashCode(); // example
        }
    },
    LITERAL3(Object.class) {
        @Override
        @GenericEnumParam
        public Object convert(Object o) {
            return o; // example
        }
    };

    MyEnum(Class<?> clazz) {
    }

    @GenericEnumParam
    public abstract Object convert(Object o);
}

The annotation processor will generate an enum MyEnumExt (customizable) which overcomes the limitation of java enums. Instead, it generates a Java class usable exactly as an enum (in the end, an enum is compiled into a Java class implementing Enum !).

Becasue "enum" is the abbreviation for enumeration. It's just a set of named constants that stand in place of ordinal numbers to make the code better readable.

I don't see what the intended meaning of a type-parameterized constant could be.

I think because basically Enums can't be instanced

Where would you set the T class, if JVM allowed you to do so?

Enumeration is data that is supposed to be always the same, or at least, that it won't change dinamically.

new MyEnum<>()?

Still the following approach may be useful

public enum MyEnum{

    LITERAL1("s"),
    LITERAL2("a"),
    LITERAL3(2);

    private Object o;

    private MyEnum(Object o) {
        this.o = o;
    }

    public Object getO() {
        return o;
    }

    public void setO(Object o) {
        this.o = o;
    }   
}