Comparing Strings

[souFrag]

Hi everyone,

Why does this return true?

Code:

String s1 = "hola";
String s2 = "hola";

if(s1 == s2) return true;

By the way... I'm teaching an introductory java course and I know everything about objects and memory allocation... I also know C, PHP, Assembly, Java, etc. I mention this so you don't start explaining me about objects and stuff like that :-)

I just want to know why the code above returns true even though s1 and s2 are supposed to be 2 different String objects... I just don't get it. Is it a Java bug??

Thx.

[souFrag]

Never mind... I've found the answer:
When the java compiler finds two or more string "constants" that are equals, it points them to the same reference.

[sanjeevtarar]

Quote:

Originally Posted by souFrag

Hi everyone,

Why does this return true?

Code:

String s1 = "hola";
String s2 = "hola";

if(s1 == s2) return true;

By the way... I'm teaching an introductory java course and I know everything about objects and memory allocation... I also know C, PHP, Assembly, Java, etc. I mention this so you don't start explaining me about objects and stuff like that :-)

I just want to know why the code above returns true even though s1 and s2 are supposed to be 2 different String objects... I just don't get it. Is it a Java bug??

Thx.

When you are creating String object like this.....String s1 = "hello";......then the compiler first checks in HEAP ....if same object is already there it points new to the existing one. But if you create like .... String s1 = new String("hello");.....it will always create a new String object.

intern() method is also there to check for existing Objects(new String("hello"))....in HEAP....

String s = "helo";
String s1 = new String("hello");

if(s == s1).......gives false

s1.intern();

if(s == s1).......gives true


Compiler does this to save memory.

and == operator checks only references(two objects are pointing to the same Object) so it returns true.

[danielstoner]

This question was answered a few times before. It is always a good idea to search the forums for answers before you ask.

[Eranga]

And the Google too. You can found lots of information regarding this. Most people confused with this each day.

[souFrag]

Of course I tried searching :-)

But the only answers I found were "you must use equals() to compare Strings" and that was not my question... I wanted to know why sometimes using "==" to compare to String objects would return true even though they seemed to be different objects.

sanjeevtarar, thanks for your answer.