How many Java Strings Created?

Question

public static void main(String [] args){ 
    String s = "java"; //line 1
    s.concat(" SE 6"); //line 2
    s.toLowerCase();  //line 3
    System.out.print(s); //line 4
 }

The answer to this question is "4". I thought it would be "3". My confusion is line 3, which creates "java" string again, but doesn't java know that the "java" string already exist in the string constant pool, so why create it again?

Strings are immutable in Java, so s.toLowerCase() returns a new String object. — Ocracoke
– Ocracoke, Commented Feb 24, 2015 at 16:14
@Ocracoke - Nope. He is not re-assigning the value returned by concat, so s will be "java" and then "java",toLowerCase() will return the same String "java" — TheLostMind
– TheLostMind, Commented Feb 24, 2015 at 16:23
@TheLostMind Ah, just looked at the code for toLowerCase() and seen that there is a return this line in it when it finds nothing to do. My bad. — Ocracoke
– Ocracoke, Commented Feb 24, 2015 at 16:37
@TheLostMind However, a new string object will still be created. The string itself, OTOH, should have the same address in the constant pool — Cole Tobin
– Cole Tobin, Commented Feb 24, 2015 at 17:37

TheLostMind · Accepted Answer · 2015-02-25 06:02:22Z

5

3 Java Strings are created.

1. "java"  -> Goes into String constants pool // will be added if no already present
2.  " SE 6" --> Goes into String constants pool?
3. java SE 6 --> Goes on heap (call to concat)// Note : You are not re-assinging the value returned from concat() So s will still be "java"
** toLowerCase() \\ does nothing in your case since "java" is already present. toLowerCase retruns the same "java" object ( as there is no modification required to turn it into lowercase)

edited Feb 25, 2015 at 6:02

answered Feb 24, 2015 at 16:16

TheLostMind

36.3k12 gold badges72 silver badges109 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Luke SpringWalker Over a year ago

Does the object returned in step 3 has references to both strings, so that is why it doesn't count as a string itself?

TheLostMind Over a year ago

@SotiriosDelimanolis - I meant the same thing. The same String will be returned as there are no uppercase chars in the string. Edited the answre to make it clearer. :)

Sergey Kalinichenko · Accepted Answer · 2015-02-24 16:24:54Z

5

Java know that the "java" string already exists in constant pool, so it needs not to create the object again?

Actually, it's not "java" string that exists in the pool, but "Java" (in uppercase). If it were indeed "java", toLowerCase() would have recognized it, and returned the original string. But since the return value (i.e. "java" in all lowercase) does not match the original string (i.e. "Java" in mixed case) a new String object needs to be created, bringing the count to 4.

Edit: After the edit to the question the answer changes: now that you've changed "Java" to "java", the number of objects that get created is three, because Java String has an optimization that returns the original string from toLowerCase when the string is already in lowercase. So line 1 creates one string object "java", line 2 creates two string objects " SE 6" and "java SE 6", and lines 3 and 4 do not create any additional objects.

edited Feb 24, 2015 at 16:24

answered Feb 24, 2015 at 16:14

Sergey Kalinichenko

729k85 gold badges1.2k silver badges1.6k bronze badges

1 Comment

user1529412 Over a year ago

These are line numbers not how I counted the objects!

Collectives™ on Stack Overflow

How many Java Strings Created?

2 Answers 2

2 Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related