Java 中枚举内的 EnumSet 修饰符

Question

It makes sense declaring an EnumSet within a enum with the modifiers static and final ? Or are redundant?

Example:

public enum Guitar {
    STRATOCASTER,
    LES_PAUL,
    CLASSIC;

    private static final EnumSet electrics = EnumSet.of(STRATOCASTER, LES_PAUL);
}

Thank you!

Answer 1

It makes sense declaring an EnumSet within a enum with the modifiers static and final? Or are redundant?

No, they are not redundant. Consider the following example in which nonElectrics can be set because it's not final .

import java.util.EnumSet;

enum Guitar {
    STRATOCASTER, LES_PAUL, CLASSIC;

    private static final EnumSet<Guitar> electrics = EnumSet.of(STRATOCASTER, LES_PAUL);
    static EnumSet<Guitar> nonElectrics = EnumSet.of(CLASSIC);

    public static EnumSet<Guitar> getNonElectrics() {
        return nonElectrics;
    }

    public static void setNonElectrics(EnumSet<Guitar> nonElectrics) {
        Guitar.nonElectrics = nonElectrics;
    }

    public static EnumSet<Guitar> getElectrics() {
        return electrics;
    }

    public boolean isElectronics() {
        return electrics.contains(this);
    }
}

public class Main {
    public static void main(String[] args) {
        System.out.println(Guitar.STRATOCASTER.isElectronics());
        System.out.println(Guitar.CLASSIC.isElectronics());
        EnumSet<Guitar> electrics = EnumSet.of(Guitar.STRATOCASTER, Guitar.LES_PAUL);
        Guitar.setNonElectrics(electrics);
        System.out.println(Guitar.getNonElectrics());
    }
}

Answer 2

This is not redundant, fields in enum class not static or final. But, you can't invoke not static enum's fields from other classes.

Answer 3

tl;dr

static final public Set ELECTRICS = Set.of( STRATOCASTER , LES_PAUL ) ;

Details

Regarding your code:

private static final EnumSet electrics = EnumSet.of(STRATOCASTER, LES_PAUL);

private

As written, no need for private .

If marked private no other code would have access to this set. So it would serve no purpose.

static

Yes, static is appropriate. An enum defines a specific number of named instances of its class. The instantiation happens automatically when the class first loads at runtime. So each enum object is a separate object. No need for each of them to have their own set. The set is unchanging, so just define a single instance of that set by marking it static .

This makes the syntax more clear as we do not involve any of the enum objects by name. Example code:

Set < Guitar > guitars = Guitar.ELECTRICS ; 

Or:

Guitar.ELECTRICS.stream.forEach( System.out::println ) ;

final

Yes, mark it as final if this set will be unchanging during the entire run of this app. Marking as final prevents any other set from being assigned to that field.

EnumSet ➠ Set

Regarding the first EnumSet , make that simply Set .

Generally better to promise a more general superclass or interface rather than lock yourself into a specific concrete class.

electrics ➠ ELECTRICS

The name electrics should be in all uppercase, if you intend this set to be fixed, unchanging. Constants in Java are named in all uppercase, by convention. So, ELECTRICS .

Plural naming of collection

Good that you named ELECTRICS in plural. This name suggests the fact that is a collection rather than a singular enum object.

Set.of for unmodifiable collection

As for the second EnumSet , generally using EnumSet is the right choice for handling enum objects. The EnumSet class is highly optimized for enums, resulting in very fast performance while using very little memory.

But the problem here is that EnumSet is mutable. I expect you do not want any errant code to be adding or removing elements from your set.

So we want an immutable collection there. Java provides an unmodifiable set via the Set.of… methods.

Under the covers, those methods are free to use any concrete class they want, and so are free to optimize in the choice of underlying class. So you might end up with an efficient concrete class like EnumSet . But that is not paramount, as being immuable trumps performance in this situation.

Solution code

So I would write that code as:

static final public Set ELECTRICS = Set.of( STRATOCASTER , LES_PAUL ) ;

Explicitly immutable collections in third-party library

An alternative to using Set.of is to add a library to your project that provides immutability as an explicit part of its collection data types. Perhaps Eclipse Collections or Google Guava .

As for redundancy, see correct Answer by Live and Let Live .