string problem

[Rippon Jalali]

the below given code gives "Not Equal". But as trim() returns a string whose value is string used to invoke the method but with any leading or trailing blank spaces removed. So isn't the condition result in true and output be "Equal"?

if(" String ".trim() == "String")

System.out.println("Equal");

else

System.out.println("Not Equal");

[Sundeep Gupta]

When you try to compare two strings using ==, Java compares thier object references and result will be false, even if lexically the strings are equal. This is because Strings in Java are immutable and hence separate object is created for each string.
To compare two strings lexically, use equals() method.

[Rippon Jalali]

but when we are comparing string literals there is no need of equals method.As when we run the code below.it gives output "Equal".
if("String".toString() == "String")

System.out.println("Equal");

else

System.out.println("Not Equal");

[Jesper de Jong]

But " String ".trim() is not a String literal. You call a method on a String literal and it returns a new String object, of which the value is "String" (without the leading and trailing white space).

As to why == normally does work with string literals, that's a more complicated story. It is because of an optimization that the Java compiler has for strings, the string pool.

Never rely on == for comparing strings. Always use equals(), also for string literals.

[Muhammad Saifuddin]

See java Doc for the mehtod String.intern()

[H. J. Yoshi]

Hi,
I agree with this.. but then I tried out this:

   if ("STRING".toUpperCase()=="STRING")
System.out.println("Equal");
 else
System.out.println("Unequal"); 

This prints "Equal".Again, .toUpperCase() returns a String object then how is it returning true..

The following though prints "Unequal"..

   if ("STRING ".toUpperCase()=="STRING")
System.out.println("Equal");
 else
System.out.println("Unequal"); 

[Anton Uwe]

toUpperCase() will return the reference of the String it was called on when it sees that no changes are needed.

[Rippon Jalali]

As the calls to "STRING ".toUpperCase() == "STRING" does not change anything, the reference to the orignal unchanged string in the pool is returned.

[Muhammad Saifuddin]

Originally posted by Himadri Joshi:

   if ("STRING".toUpperCase()=="STRING")
System.out.println("Equal");
 else
System.out.println("Unequal"); 

This prints "Equal".Again, .toUpperCase() returns a String object then how is it returning true..

toUpperCase() methods Converts all of the characters in this String to upper case.

in your case "STRING" and comparative STRING is already defined in uppercase try this toLowerCase() then you would fine you answer..
[ January 16, 2007: Message edited by: Saif uddin ]

[H. J. Yoshi]

Yes, got it! Thanks a lot, everybody.