final String

[Indraneel Das]

Hi all
i came accross the following question in one of the mock tests of Dan Chisholm:

class test{
public static void main(String args[]) {
String a = "A";
String b = "B";
final String c = a+b;
final String d = a+b;
System.out.print((c==c) + ",");
System.out.print(((a+b)==(a+b)) + ",");
System.out.print(c==d);
}
}
Prints: true,false,false
Now if both the variables a & b are declared as final, then why the output changes to
true,true,true

Thanx and Regards
Indraneel

[Arpana Rai]

Originally posted by Indraneel Das:
Hi all
class test{
public static void main(String args[]) {
String a = "A";
String b = "B";
final String c = a+b;
final String d = a+b;
System.out.print((c==c) + ",");
System.out.print(((a+b)==(a+b)) + ",");
System.out.print(c==d);
}
}
Indraneel

a+b evaluates at runtime.so, everytime u execute a+b it returns new string object. that is why (a+b)=(a+b) prints false.
But, in the below given code a and b are declared final.So, a+b evaluates at compile time and hence (a+b)=(a+b) prints true.

 
class test{
public static void main(String args[]) {
final String a = "A";
final String b = "B";
final String c = a+b;
final String d = a+b;
System.out.print((c==c) + ",");
System.out.print(((a+b)==(a+b)) + ",");
System.out.print(c==d);
}
}
Prints: true,true,true
 

hope it helps.
regds
Arpana

[Indraneel Das]

Thx Arpana for clarifying my doubt. Now its clear to me.
Regards
Indraneel

[John Pritchard]

Given that a+b evaluates at runtime, does the JVM check the String pool to see if the result of the runtime calculation already exists or does it always return a new String?

[Arpana Rai]

Originally posted by John Pritchard:
Given that a+b evaluates at runtime, does the JVM check the String pool to see if the result of the runtime calculation already exists or does it always return a new String?

At runtime a+b (if a and b are not final ) returns new String object and the String constructor does not check the String pool when it creates a new String object.
regds
arpana