== String comparison [modified]

[AnnnS]

Hi All, I have a doubt. I was just trying out some basic things in Core Java. Here is the code public class Test { public static void main(String args[]) { String a = "Hai"; String b = "Hai"; if(a == b) { System.out.println("Equal"); } else { System.out.println("Not Equal"); } } } I Assumed that the result would be "Not Equal" as == does reference comparison. But I got the answer as "Equal". Then I tried the second code public class Test { public static void main(String args[]) { String a = "Hai"; a += ""; String b = "Hai"; if(a == b) { System.out.println("Equal"); } else { System.out.println("Not Equal"); } } } Now i could get "Not equal". But logically both codes are same. Why is this so?? Does Java share similar Objects?? Can anyone give an explanation on this?? Thanks, Annns...:confused:

modified on Thursday, February 11, 2010 8:38 AM

[Lost User]

As you guess the objects are the same, as they both point to the exact same constant; this is the Java compiler optimising your code. Then when you add another character to a it becomes a new object and is no longer the same as b, even though, in human terms it has not changed.

txtspeak is the realm of 9 year old children, not developers. Christian Graus

[AnnnS]

Thanks a lot !!

[Nagy Vilmos]

To add to Richards earlier answer, == checks that the objects are the same and equals(Object) should be overriden for ALL classes and checks that the contents are the same [this is the case with String]. Try this to see:

public class TestEquals
{
public static void main(String args[])
{
// Make sure we have two DIFFERENT objects:
String a = new String("Hai");
String b = new String("Hai");

if(a == b)
{
  System.out.println("a == b true"); 
} else {
  System.out.println("a == b false");
}

if(a.equals(b))
{
  System.out.println("a.equals(b) true"); 
} else {
  System.out.println("a.equals(b) false");
}

}
}

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Panic, Chaos, Destruction. My work here is done. or "Drink. Get drunk. Fall over." - P O'H

[AmbiguousName]

Nagy Vilmos wrote:

// Make sure we have two DIFFERENT objects: String a = new String("Hai"); String b = new String("Hai");

Thanx...You are right. We should make sure that we make two different objects. But I have a doubt in my mind....both a and b should refer to same objects if we write b = a...why they are refering to same object if we say a="Hai" and b="Hai"??

[Nagy Vilmos]

What I think you are asking is why, when we write...

String a="Hai";
String b="Hai";

...are a and b the same object and not just the same value? When you use a literal string, there will only be one created unless you explicitly create a new instance. m Think of the literal as a constant referencing an object.

---

Panic, Chaos, Destruction. My work here is done. or "Drink. Get drunk. Fall over." - P O'H