To compare UUID, can I use == or have to use UUID.equals(UUID)?

Question

Just start using java.util.UUID. My question is if I have two UUID variables, say u1 and u2, and I would like to check if they are equal, can I safely use expression u1 == u2 or have to write u1.equals(u2)? assuming both are not null.

BTW, I am using its randomUUID method to create new UUID values, but I think this should not be matter. I wonder as UUID is unique, each value could be a singleton, then it is safe to use u1 == u2.

void method1(UUID u1, UUID u2) {

   // I know it is always safe to use equal method
   if (u1.equals(u2)){ 
     // do something
   }

   // is it safe to use  ==
   if (u1 == u2) {
     // do something
   }
}

Answer 1

It depends: which type of equality do you want?

UUID a = new UUID(12345678, 87654321);
UUID b = new UUID(12345678, 87654321);
UUID c = new UUID(11111111, 22222222);

System.out.println(a == a); // returns true
System.out.println(a.equals(a)); // returns true

System.out.println(a == b); // returns false
System.out.println(a.equals(b)); // returns true

System.out.println(a == c); // returns false
System.out.println(a.equals(c)); // returns false

a == b is true only if a and b are the same object. If they are two identical objects, it will still be false.

a.equals(b) is true if a and b are the same UUID value - if their two parts are the same.

It's a rhetorical question, by the way. Almost always you want .equals. There isn't much use for == with UUIDs.

Answer 2

Well...no.

== against an object checks for reference equality. That is, it checks to see if these two objects are literally the same spot in memory.

.equals() will check for actual object equivalence. And, the Javadoc for UUID goes into great detail to explain when two UUID instances are equivalent.