Logo Questions Linux Laravel Mysql Ubuntu Git Menu
 

Is it possible to extend enum in Java 8?

Just playing and came up with a sweet way to add functionality to enums in Java Enum toString() method with this.

Some further tinkering allowed me to nearly also add a tidy (i.e. not throwing an exception) reverse look-up but there's a problem. It's reporting:

error: valueOf(String) in X cannot implement valueOf(String) in HasValue
public enum X implements PoliteEnum, ReverseLookup {
overriding method is static

Is there a way?

The aim here is to silently add (via an interface implementation with a default method like I added politeName in the linked answer) a lookup method that does the valueOf function without throwing an exception. Is it possible? It is clearly now possible to extend enum - one of my major problems with Java until now.

Here's my failed attempt:

public interface HasName {

    public String name();
}

public interface PoliteEnum extends HasName {

    default String politeName() {
        return name().replace("_", " ");
    }
}

public interface Lookup<P, Q> {

    public Q lookup(P p);
}

public interface HasValue {
    HasValue valueOf(String name);
}

public interface ReverseLookup extends HasValue, Lookup<String, HasValue> {

    @Override
    default HasValue lookup(String from) {
        try {
            return valueOf(from);
        } catch (IllegalArgumentException e) {
            return null;
        }
    }

}

public enum X implements PoliteEnum/* NOT ALLOWED :( , ReverseLookup*/ {

    A_For_Ism, B_For_Mutton, C_Forth_Highlanders;
}

public void test() {
    // Test the politeName
    for (X x : X.values()) {
        System.out.println(x.politeName());
    }
    // ToDo: Test lookup
}
like image 942
OldCurmudgeon Avatar asked Feb 28 '14 00:02

OldCurmudgeon


People also ask

Can you extend enum class?

We cannot extend enum classes in Java. It is because all enums in Java are inherited from java. lang. Enum .

Can enums extend each other?

Enum and has several static members. Therefore enum cannot extend any other class or enum: there is no multiple inheritance. Class cannot extend enum as well. This limitation is enforced by compiler.

Can enum extend interface?

Enum, it can not extend any other class or enum and also any class can not extend enum. So it's clear that enum can not extend or can not be extended. But when there is a need to achieve multiple inheritance enum can implement any interface and in java, it is possible that an enum can implement an interface.

Can we override enum?

You cannot extend, override or inherit an enum .


1 Answers

You are over-complicating your design. If you are willing to accept that you can invoke a default method on an instance only, there entire code may look like this:

interface ReverseLookupSupport<E extends Enum<E>> {
    Class<E> getDeclaringClass();
    default E lookup(String name) {
        try {
            return Enum.valueOf(getDeclaringClass(), name);
        } catch(IllegalArgumentException ex) { return null; }
    }
}
enum Test implements ReverseLookupSupport<Test> {
    FOO, BAR
}

You can test it with:

Test foo=Test.FOO;
Test bar=foo.lookup("BAR"), baz=foo.lookup("BAZ");
System.out.println(bar+"  "+baz);

An non-throwing/catching alternative would be:

interface ReverseLookupSupport<E extends Enum<E>> {
    Class<E> getDeclaringClass();
    default Optional<E> lookup(String name) {
        return Stream.of(getDeclaringClass().getEnumConstants())
          .filter(e->e.name().equals(name)).findFirst();
}

to use like:

Test foo=Test.FOO;
Test bar=foo.lookup("BAR").orElse(null), baz=foo.lookup("BAZ").orElse(null);
System.out.println(bar+"  "+baz);
like image 101
Holger Avatar answered Oct 07 '22 18:10

Holger