Java enum attributes returning null based on order of access

Question

I was exploring enums in java to see how they could be abused and I came across a behaviour I couldn\'t explain. Consider the following class:

public class PROGR         


        

Answer 1

This has to do with enums and class initialization.

First, enum is just a fancy class with constant fields. That is, the enum constants you declare are in reality just static fields. So

enum SomeEnum {
    CONSTANT;
}

compiles to something similar to

final class SomeEnum extends Enum<SomeEnum> {
    public static final SomeEnum CONSTANT = new SomeEnum();
}

Second, static fields are initialized in the left to right order they appear in the source code.

Next, execute either the class variable initializers and static initializers of the class, or the field initializers of the interface, in textual order, as though they were a single block.

In the following

final class SomeEnum extends Enum<SomeEnum> {
    public static final SomeEnum CONSTANT = new SomeEnum();
    public static final SomeEnum CONSTANT_2 = new SomeEnum();
}

CONSTANT would be initialized first, and CONSTANT_2 second.

Third, an enum type will be [initialized][3] when you access one of its constants (which is really just a static field).

Fourth, if a class is currently being initialized by the current thread, you proceed normally.

If the Class object for C indicates that initialization is in progress for C by the current thread, then this must be a recursive request for initialization. Release LC and complete normally.

How does this all come together?

This

ENUM.ANIMALS.CATS.GARFIELD.RIVAL

is evaluated like

CATS cat = ENUM.ANIMALS.CATS.GARFIELD;
DOGS rvial = cat.RIVAL;

The first access to GARFIELD forces the initialization of the enum type CATS. That begins initializing the enum constants in CATS. Compiled, those appear like

private static final CATS FELIX = new CATS(DOGS.AKAME);
private static final CATS GARFIELD = new CATS(DOGS.WEED);
private static final CATS BUBSY = new CATS(DOGS.GIN);

These get initialized in order. So FELIX goes first. As part of its new instance creation expression, it accesses DOGS.AKAME, where the type DOGS is not yet initialized, so Java starts initializing it. The DOGS enum type, compiled, looks like

private static final DOGS GIN = new DOGS(CATS.FELIX);
private static final DOGS WEED = new DOGS(CATS.BUBSY);
private static final DOGS AKAME = new DOGS(CATS.GARFIELD);

So we start with GIN. In its new instance creation expression, it tries to access CATS.FELIX. CATS is current being initialized, so we just continue. CATS.FELIX hasn't been assigned a value yet. It's currently in construction lower on the stack. So its value is null. So GIN.RIVALS gets a reference to null. The same happens to all DOGS' RIVAL.

When all of the DOGS are initialized, execution returns to

private static final CATS FELIX = new CATS(DOGS.AKAME);

where DOGS.AKAME now refers to a fully initialize DOGS object. That gets assigned to its CATS#RIVAL field. Same for each of the CATS. In other words, all the CATS' RIVAL field are assigned a DOGS reference, but not the other way around.

Reordering the statements simply determines which enum type gets initialized first.

Answer 2

When you call ENUM.ANIMALS.CATS.GARFIELD.RIVAL, it will start by creating the CATS enum. When processing the first element, FELIX, it needs to create the DOGS enum so that DOGS.AKAME can be passed as a parameter to the CATS constructor.

The DOGS constructor receives a parameter of type CATS, but since CATS was not yet initialized all CATS.something will return null, thus setting the RIVAL attribute to null for all elements in the DOGS enum.

When all DOGS elements are created, it goes back to CATS and resumes the creation of its elements, passing the just created DOGS elements as parameters.

Similarly, when you invert the order of the calls it starts by creating the DOGS enum which causes the CATS elements RIVAL attribute to be set as null.

If this is not clear, try to run your code with breakpoints set at the enum elements' declarations and at the constructors to understand it better.