A stupid question?

sharkbc

Hi all, Near one or two years (version Java at that time is 1.5x), I have a short code to compare 2 Strings in java like bellow: String a = "abc"; String b ="abc";//this is load from the database. CODE 1

if(a==b){
System.out.println("equal");
}else {
System.out.println("dif");
}

and I got the result is dif I remember because after I change the code to String a = "abc"; String b ="abc";//this is load from the database. CODE 2

if(a.equals(b)){
System.out.println("equal");
}else {
System.out.println("dif");
}

It give me the result is equal Is it true or my eyes is wrong? Now the CODE 1 always gives the result is equal

BobJanova

This is as expected, in Java there is no operator overloading and == always does a reference equality check. To do a value equality check, you need to use .equals (after doing a null check).

TorstenH

Code1 compares the objects and if they are equal, also both Objects have the same value they are still different objects. Code2 checks character wise for the value. It's a common failure.[^].

regards Torsten When I'm not working

jschell

sharkbc wrote:

Now the CODE 1 always gives the result is equal

Is it true or my eyes is wrong?

Java, regardless of version, has always operated in that way (equals is required.) So something is wrong in how you are validating this. Perhaps because despite what your comment says you are not in fact loading it from the database. (An very odd corner case would involve how you are 'loading' it from the database and would indicate an error in the load process.)

sharkbc

hi Jschell, I've just modify my question, it must be ... Is it true or my eyes is wrong? Now the CODE 1 always gives the result is equal Im sure at that time, the database is not change and i do not change any other code in the programe, because after we got the mistake, we debug the code and get it. We just only change

if(a==b)

to

if(a.equals(b))

TorstenH

what JRE are you using now? is it the new Java 7? There might be an improvment as Strings are now also allowed in switch-conditions: http://docs.oracle.com/javase/7/docs/technotes/guides/language/strings-switch.html[^] But in general I would say that there must be something else wrong.

regards Torsten When I'm not working

jschell

The real problem is that we do not know what you are actually doing. Again the following has always been true regardless of java version for the major java vendors... Equality compares references The method equals() compares value (content of string.) As I said there are ways which MIGHT present seemingly different results dependent on the specifics of what is going on. But you haven't presented the specifics. And based ONLY on the code you posted you are comparing references for what you refer to "CODE 1". And reference equality will be equal. So what are some possible causes for some other difference for some OTHER code? 1. You were not using a real version of java, it was some odd variant. 2. There was something odd about the way in which your database code worked. 3. There was something odd about your data 4. You misinterpreted what was happening before. 5. The real code would make it explicitly obvious why a difference was occurring. 6. The method intern() was being called somewhere (incorrectly called.)

Nagy Vilmos

Where is a initialised from? It is possible that the two objects are the same and then == will return true. For future reference, if you want to compare two objects always use the equals() method.

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