How do I return a class' enums from its own class in Java?

Question

I want to enforce enum implementation in Java but I found out that I couldn't do it. So I decided to define an abstract function that returns the enums of subclasses. But don't know how to do it.

Here is the code:

EnumInterface

public interface EnumInterface 
{
    public String getString();
}

ParentClass

public abstract class ParentClass {
    public abstract Enum<?> getEnums();
}

ChildClass

public class ChildClass extends ParentClass{
    public enum EnumImplementation implements EnumInterface
    {
        FOO("foo"), 
        BAR("bar");

        String string;

        EnumImplementation(String field)
        {
            this.string = string;
        }

        @Override
        public String getString() {
            return string;
        }
    }

    @Override
    public Enum<?> getEnums() {
        return ?;
    }    
}

The code above doesn't work, I'm just trying to describe my problem. I also want to enforce the enum return type to EnumInterface if possible. If you know how to enforce specific enum implementation that would be better as I don't even have to define the function in ParentClass .

So, how do I return the enum so that I can just do this instanceOfParentClass.getEnums().FOO ?

Answer 1

What about this:

public <T extends Enum<T> & EnumInterface> T[] getEnums() {
    return (T[])EnumImplementation.values();
}  

Note the cast, which might result in a ClassCastException . To prevent this you could pass the enum class or just return an array of EnumInterface :

public <T extends Enum<T> & EnumInterface> T[] getEnums(Class<T> enumType) 
public <T extends Enum<T> & EnumInterface> EnumInterface[] getEnums() 

Note that this doesn't enable you to call the method like getEnums().FOO , but you could pass the class and the name, eg

public <T extends Enum<T> & EnumInterface> T[] getEnum(Class<T> enumType, String enumName) {
    return Enum.valueOf( enumType, enumName);
}

However, just as Tim B said, there might be a better option for what you're trying to achieve.

Answer 2

Try this.

EnumInterface

public interface EnumInterface {
    public String getString();
}

ParentClass

public abstract class ParentClass<ENUM_TYPE extends Enum<ENUM_TYPE> & EnumInterface> {
    public abstract ENUM_TYPE getEnums();
}

ChildClass

public class ChildClass extends ParentClass<ChildClass.EnumImplementation> {
    public static enum EnumImplementation implements EnumInterface {
        FOO("foo"), 
        BAR("bar");

        String string;

        EnumImplementation(String field) {
            this.string = field;
        }

        @Override
        public String getString() {
            return string;
        }
    }

    public EnumImplementation getEnums() {
        return EnumImplementation.values()[0];
    }

    public static void test() {
        Object result = new ChildClass().getEnums().FOO;
    }
}

Note that getEnums() returns an enum value. It's not possible to return the enum container itself, but you can call other enum values from any value (fe EnumImplementation.FOO.BAR.FOO )

Answer 3

In order to map property names to database field you can use the enum valueOf method.

You need to do the mapping in your class as you cannot pass a reference to the static enumeration:

http://www.tryjava8.com/app/snippets/52b86150e4b0f5090255bc45

 import java.util.*;
 public class Main{

     static enum TestE {
         FOO,
         BAR
     }

   static class TestC {
     TestE getEnum(String name) { 
       return TestE.valueOf(name);
     }
   }

   public static void main(String[] args){
     System.out.println(TestE.FOO);
     System.out.println(new TestC().getEnum("BAR"));
   }
 }