Reversing String Not Working...

[Somnath Mallick]

Hi Everyone,

StringBuilder sb=new StringBuilder("mom");

StringBuilder sbr=new StringBuilder(sb.reverse());

System.out.format("sbr);

System.out.println(sb.equals(sbr));

This is giving the output as false! Could you please tell me why? I dont think there is anything wrong with the code!

[Christophe Verré]

I dont think there is anything wrong with the code!

Think twice. Does StringBuilder override the Object#equals method ? If no, what are the consequences ?

[Somnath Mallick]

So could you please tell me how to check it? I checked the Java API for the StringBuilder class and I found that there is mention of:

Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

[Seetharaman Venkatasamy]

Object's equals method below

    

public boolean equals(Object obj) {
	return (this == obj);
    }

== check whether two references point to the same Object or not

[Christophe Verré]

So could you please tell me how to check it?

That's a good opportunity to learn how to read the API. Check the java.lang.StringBuilder API again. In the "Method Summary", you'll see all the methods implemented in StringBuilder. These can be StringBuilder's own methods, or some overriden methods. The equals method is not in there, so StringBuilder does not override it. You can see in the "Methods inherited from class java.lang.Object" part that it actually inherits the equals method from Object. Now, look at the API for java.lang.Object#equals(). What does it tell you ?


Furthermore, if you take a look at the java.lang.String API, you'll see that in the "Method Summary", the equals method is there. It's overridden, and it does something special, that StringBuilder does not. Try to find out what.

[Somnath Mallick]

Ok! Thanks Christophe! So does this mean that the check in the above code is not is that the objects are pointing to the same location or not and not the contents of the objects?

[nandini lagunia]

Christophe Verré wrote:

So could you please tell me how to check it?

That's a good opportunity to learn how to read the API. Check the java.lang.StringBuilder API again. In the "Method Summary", you'll see all the methods implemented in StringBuilder. These can be StringBuilder's own methods, or some overriden methods. The equals method is not in there, so StringBuilder does not override it. You can see in the "Methods inherited from class java.lang.Object" part that it actually inherits the equals method from Object. Now, look at the API for java.lang.Object#equals(). What does it tell you ?


Furthermore, if you take a look at the java.lang.String API, you'll see that in the "Method Summary", the equals method is there. It's overridden, and it does something special, that StringBuilder does not. Try to find out what.

Oh man!! You must be a very good tutor.

[Somnath Mallick]

Oh man!! You must be a very good tutor.

Yeah! Agree with that part!!!

[Christophe Verré]

So does this mean that the check in the above code is not is that the objects are pointing to the same location or not and not the contents of the objects?

Yes, you're right, the equals() implementation of the Object class compares references:

API wrote:The equals method for class Object implements the most discriminating possible equivalence relation on objects; that is, for any non-null reference values x and y, this method returns true if and only if x and y refer to the same object (x == y has the value true).

So, in your case, sb.equals(sbr) compares both StringBuilder's reference, not their content.

[netali joshef]

Is this post closed..?

[Rob Spoor]

If you need to compare StringBuilders or StringBuffers on content, call toString() on each and compare those:

System.out.println(sb.toString().equals(sbr.toString()));

[Somnath Mallick]

Yes! That's what i finally did! Thanks everyone for their comments!