为什么不能从定义了该类的实例的实例访问枚举？

Question

In the example below, if I try to access the enum through an instance of Class1 from Class2, I get the error: "unexpected type":

public class Class1 {
    public enum EnumExample {
        ONE;
    }
}

public class Class2 {
    public Class2 {
        Class1 class1 = new Class1();
        Class1.EnumExample enumEx = class1.EnumExample.ONE;
    }
}

I can access it using Class1.EnumExample.ONE which led me to believe that I can't access it using an instance because the enum is static, but I can access other static variables through the instance, so there must be more to it than that.

So why does this happen?

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Also, I know this probably should be another question, but I'm including this here as an aside in case it's an easy answer: Is there some way for me to access the enum from an instance and still maintain the convenience of the enum's auto-completion in an IDE (Netbeans in this case).

I tried to return the enum in a static method from Class1 in hopes of exploiting the fact that I can access static methods from its instances:

public static Class<EnumExample> getEnumExample() {
    return EnumExample.class;
}

But when I access it on Class2, Netbeans fills the options with methods from Class , and not specifically from EnumExample .

I could just use Class1.EnumExample , but I'm trying to avoid using the class' name in here for convenience and type-safety, because this code will be reused in different places.

Answer 1

From the specification

A nested enum type is implicitly static. It is permitted for the declaration of a nested enum type to redundantly specify the static modifier.

Regardless, a type is not a member of an instance. It is (possibly) a member of a type.

As the specification says, a field access expression takes the following form

FieldAccess :

• Primary . Identifier . Identifier
• super . Identifier
• TypeName . super . Identifier

We care about the first case, with a Primary expression. You have to access the enum constant through its type name, through the name of its enclosing type.

You say

I could just use Class1.EnumExample, but I'm trying to avoid using the class' name in here for convenience and type-safety,

There's not much more type safe than using the type name in source code.

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A Class instance and a type name are two completely different things. The type java.lang.Class is a type like any other ( String , List , ExecutorService , etc.). It has its fields and methods. If you have a value of type Class , you can use it to access those members. With a type name, you can declare variables, you can declare type arguments, create new instances (with new ), and invoke static methods.