Total number of object string creates?

[Sharad Kharya]

Please confirm me, that how many string objects gets created totally for below code -

String str = "a"+"b"+"c";

As per my understanding there will be two options:-
a) str, a,b,ab,c,abc (i.e. 6)
b) str, a,b,c,abc (i.e. 5)

Please confirm which one is correct and also let me know if i missed any other object.

[Ankit Garg]

I think only 1 object will be created and that is "abc" as a b and c are all literals, so the compiler will join them together at compilation...

[Duc Vo]

Sharad Golu wrote:Please confirm me, that how many string objects gets created totally for below code -

String str = "a"+"b"+"c";

As per my understanding there will be two options:-
a) str, a,b,ab,c,abc (i.e. 6)
b) str, a,b,c,abc (i.e. 5)

Please confirm which one is correct and also let me know if i missed any other object.

I would say it will be "a", "b", "c" and "abc". The variable str is just a reference to "abc", won't be another object.

[Sharad Kharya]

Duc,
Agree that str is just a reference not any object..Thanks for making me clear.

but ideally a, b, c these 3 object gets created at compile time only in the string constant pool.
But at run time whether only abc gets created or ab will also gets created.

Is there is any way to check the total nos of objects created in heap?

[Henry Wong]

I think Ankit is correct. The sum of three literals can be determined at compile time, hence, "abc" is in the string pool. And "a", "b", and "c", is not needed (and should not be created) at all.

Henry

[Duc Vo]

(1) There is no object got created at compile time, it just compiles java code to bytecode.
(2) The three object "a", "b", and "c" should be created at the class loading time, i.e. when JVM loads the class
(3) There should be no interim "ab" object in the modern JVMs, they should use some other less expensive way to concatenate the string with the + operator. So it should depend on specific JVM implementation.

My opinion is still the same: there will be only 4 string objects created.

[Duc Vo]

Duc Vo wrote:(1) There is no object got created at compile time, it just compiles java code to bytecode.
(2) The three object "a", "b", and "c" should be created at the class loading time, i.e. when JVM loads the class
(3) There should be no interim "ab" object in the modern JVMs, they should use some other less expensive way to concatenate the string with the + operator. So it should depend on specific JVM implementation.

My opinion is still the same: there will be only 4 string objects created.

My bad, it does seem the compiler (at least with my eclipse 3.4 compiler running under JDK6) concatenates the string at compile time, hence there will be only one object.

[Ankit Garg]

Duc, the compiler is very smart, it looks for every opportunity to save memory and JVM's time. So when you write this

String str = "a" + "b" + "c";

The compiler sees that unnecessary strings are being created and thus to save memory and JVM's time, the compiler would convert the statement to

String str = "abc";

And thus only one string will be created...

[Sharad Kharya]

Thanks Duc & Ankit for such a good explanation.

But say if i modify the code
String str = "a"+"b";
str = str+"c";

In this case i hope ab will also be created (i.e total 5 objects)
Correct me if i am wrong.

[Ankit Garg]

String str = "a"+"b";
str = str+"c"; 

This code will create 3 objects i.e. "ab", "c" and "abc"...

[Sharad Kharya]

Hi Ankit,

In this case i completely got for ab & abc, but didn't understand this time why "c" is also created as object in heap.

[Harshana Dias]

Ankit Garg wrote:

String str = "a"+"b";
str = str+"c"; 

This code will create 3 objects i.e. "ab", "c" and "abc"...

Based on the earlier explanation its two right? "ab" and "abc"

[Harshana Dias]

Harshana Dias wrote:

Ankit Garg wrote:

String str = "a"+"b";
str = str+"c"; 

This code will create 3 objects i.e. "ab", "c" and "abc"...

Based on the earlier explanation its two right? "ab" and "abc"

Well I later read that, "Generally using JDK 1.6 and above the compiler will automatically join strings together using StringBuilder."

[Henry Wong]

Harshana Dias wrote:

Ankit Garg wrote:
This code will create 3 objects i.e. "ab", "c" and "abc"...

Based on the earlier explanation its two right? "ab" and "abc"

It's three. Ankit is correct.

Basically, the second line of code is executed at runtime. The strings "ab" and "c" are concatenated to form the "abc" string at runtime.

Henry

[Coder Rk]

String Str = "a" +"b"+"c";

Only one object will be created in scp area.

Bcs all are constants , at compile time all three joins together with help of '+' operator , it's will become "abc" so now it's consider as one string and a single object will be created in scp area ..