Is it OK to use == on enums in Java?

Question

Is it OK to use == on enums in Java, or do I need to use .equals()? In my testing, == always works, but I\'m not sure if I\'m guarant

Answer 1

Yes, it is as if you had created singleton instances for each value in the enum:

public abstract class RoundingMode {
  public static final RoundingMode HALF_UP = new RoundingMode();
  public static final RoundingMode HALF_EVEN = new RoundingMode();

  private RoundingMode() {
    // private scope prevents any subtypes outside of this class
  }
}

However, the enum construct gives you various benefits:

• Each instance's toString() prints the name given in code.
• (As mentioned in another post,) a variable of the enum type can be compared against constants using the switch-case control structure.
• All the values in the enumeration can be queried using the values field that is 'generated' for each enum type
• Here's the big one w.r.t identity comparisons: enum values survive serialization without cloning.

The serialization is a big gotchya. If I were to use the code above instead of an enum, here's how identity equality would behave:

RoundingMode original = RoundingMode.HALF_UP;
assert (RoundingMode.HALF_UP == original); // passes

ByteArrayOutputStream baos = new ByteArrayOutputStream();
ObjectOutputStream oos = new ObjectOutputStream(baos);
oos.writeObject(original);
oos.flush();

ByteArrayInputStream bais = new ByteArrayInputStream(baos.toByteArray());
ObjectInputStream ois = new ObjectInputStream(bais);
RoundingMode deserialized = (RoundingMode) ois.readObject();

assert (RoundingMode.HALF_UP == deserialized); // fails
assert (RoundingMode.HALF_EVEN == deserialized); // fails

You can address this issue without enum, using a technique that involves writeReplace and readResolve, (see http://java.sun.com/j2se/1.4.2/docs/api/java/io/Serializable.html)...

I guess the point is -- Java goes out of its way to allow you use enum values' identities for testing equality; it is an encouraged practice.