Java Enum is abstract class, how it create own instance

Question

As I know, Enum is abstract class, I write an Enum, then I do javap command, I got:

public abstract class com.tonyzhai.homework.TestEnum extends java.lang.Enum<com.tonyzhai.homework.TestEnum> {
  public static final com.tonyzhai.homework.TestEnum a;

  public static com.tonyzhai.homework.TestEnum[] values();
    Code:
       0: getstatic     #2                  // Field $VALUES:[Lcom/tonyzhai/homework/TestEnum;
       3: invokevirtual #3                  // Method "[Lcom/tonyzhai/homework/TestEnum;".clone:()Ljava/lang/Object;
       6: checkcast     #4                  // class "[Lcom/tonyzhai/homework/TestEnum;"
       9: areturn

  public static com.tonyzhai.homework.TestEnum valueOf(java.lang.String);

so, it cannot instantiation, my question is that abstract must be has no instance, so if use Enum implement Singleton pattern, how it create its own instance?

It would help if you'd show the Java source code... A simple enum doesn't end up generating an abstract class, as far as I'm aware. — Jon Skeet
– Jon Skeet, Commented Sep 24, 2015 at 18:56
Why are you looking at the java disassembler output? What is the source code? — Colonel Thirty Two
– Colonel Thirty Two, Commented Sep 24, 2015 at 18:57
Enum instances in Java are always instantiated when you declare them (that's when the constructor is called). — Mick Mnemonic
– Mick Mnemonic, Commented Sep 24, 2015 at 19:02
@dasblinkenlight: I believe the implementation of enum classes is abstract if and only if the individual enum constants override behavior from the top-level enum type. — Louis Wasserman
– Louis Wasserman, Commented Sep 24, 2015 at 19:21

Patrick Ziegler · Accepted Answer · 2015-09-24 19:06:56Z

3

Yes Enum is abstract but you cannot actually extend it, except for some special circumstances, see here for more information.

answered Sep 24, 2015 at 19:06

Patrick Ziegler

7961 gold badge7 silver badges22 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Mick Mnemonic Over a year ago

All enums implicitly extend java.lang.Enum.

Patrick Ziegler Over a year ago

Yes you're right, but that's a little bit different from actually making a class that extends Enum.

Holger · Accepted Answer · 2015-09-25 13:55:44Z

1

When you declare an enum, the compiler will fill in certain aspects to fulfill the contract. E.g. it will add a YourEnumType[] values() and a YourEnumType valueOf(String) method.

It will also make your constructor private, regardless of whether you declared it, and it will add implicit int and String parameters whose arguments get passed to the super constructor Enum(String, int). These match the name and ordinal and the compiler will also insert the appropriate arguments when the constants are created.

For example:

public enum Axis {
    HORIZONTAL(true), VERTICAL(false);

    private boolean isHorizontal;

    Axis(boolean horizontal) {
        isHorizontal=horizontal;
    }
    public Axis getOther() {
        return isHorizontal? VERTICAL: HORIZONTAL;
    }
    public static void main(String... arg) {
        for(Method m: Axis.class.getDeclaredMethods())
            System.out.println(m);
        System.out.println("constructor:");
        System.out.println(Axis.class.getDeclaredConstructors()[0]);
    }
}

will print

public static void Axis.main(java.lang.String[])
public static Axis[] Axis.values()
public static Axis Axis.valueOf(java.lang.String)
public Axis Axis.getOther()
constructor:
private Axis(java.lang.String,int,boolean)

So you see the compiler added methods as well as the constructor being private and having the additional parameters. Note, however, that it is up to the compiler, where and in which order it adds these two parameters. Since the constructor is private and only used within the class itself, there is no compatibility requirement about how the initialization exactly works.

Further, getting the Constructor reflectively doesn’t allow you to construct additional instances. The Reflection implementation recognizes that this is an enum and rejects such attempts. You would have to hack deeper into the implementation to make such things possible, but that’s counteracting the intention of enums anyway.

answered Sep 25, 2015 at 13:55

Holger

301k43 gold badges481 silver badges827 bronze badges

2 Comments

Tony Over a year ago

I add empty constructor in my Enum, then I do reflect and call newInstance() method, but it throws NoSuchMethodException, it said I have only enmu(String, int) constructor, but no my empty constructor

Holger Over a year ago

That’s the point. You will always have the two additional parameters, regardless of how you defined your constructor. But in principle, there is no standard regarding the resulting signature. In theory, there are other possibilities to implement it on binary level. You can’t invoke the constructor reflectively, anyway.

Collectives™ on Stack Overflow

Java Enum is abstract class, how it create own instance

2 Answers 2

2 Comments

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related