Java enum inheritance special case

The Java enum class is a special class that allows you to create a group of constant values (also known as enumerations).

The enum class is compiled as a final class in Java, so you can’t extend an enum class in your code:

enum Colors {
    RED, BLUE;
}

enum NewColors extends Colors {}

// ERROR: Cannot inherit from enum 'Colors'

The enum class itself is also an extension of the abstract class Enum, so you can’t create a class that inherits an enum as shown below:

class Levels {}

enum NewLevels extends Levels {}
// No extends clause allowed for enum

An enum class represents a complete enumeration of possible values, so you can’t extend an enum class to add new possible values to it.

What you can do, however, is to create a separate enum class and use it whenever necessary:

enum Colors {
    RED, BLUE;
}

enum NewColors {
    GREEN, YELLOW;
}

class Main {
    public static void main(String[] args) {

        System.out.println(Colors.RED);
        System.out.println(NewColors.GREEN);
    }
}

But what if your enum types have some methods defined in the class? The best way to share an enum class method is to create an interface that defines those methods.

For example, both Colors and NewColors enum can have a shared describeColor() method as shown below:

interface DescribeColor {
    String describeColor();
}

enum Colors implements DescribeColor {
    RED, BLUE;

    @Override
    public String describeColor() {
        return "The color is " + this;
    }
}

enum NewColors implements DescribeColor {
    GREEN, YELLOW;

    @Override
    public String describeColor() {
        return "The color is " + this;
    }
}

class Main {
    public static void main(String[] args) {
        System.out.println(Colors.RED.describeColor());
        System.out.println(NewColors.GREEN.describeColor());
    }
}

By providing an implementation of the describeColor() method in each enum class, you would be able to access the same method from those enums.

The enum inheritance is intentionally made unavailable in Java because an enumeration should contain a complete list of possible values.

That being said, you can still share methods by implementing one or more interface types to an enum class.