Enum and their super-class

All java enum E implicitly extends java.lang.Enum. Since java doesn't allow multiple inheritance, enum types can't have superclass. They can't even extend from java.lang.Enum, nor java.lang.Object. It also means enum A can't inherit or extend enum B.

For example, the following is an invalid enum declaration:

public enum MyType extends Object {
ONE, TWO
}

Compiler error:

MyType.java:3: '{' expected

public enum MyType extends Object {
MyType.java:6:  expected
2 errors

The correct form should be:
public enum MyType {
ONE, TWO
}