string concatenation

Why is string concatenation considered to be expensive operation.

String is immutable. When you concat another string, it is created new object instead of manipulating the same string object. That's why is expensive when appending more string objects using +=.

If i try str = str1 + str2, then how many objects get created.

Naresh Chaurasia wrote:If i try str = str1 + str2, then how many objects get created.

I believe ....

str = str1 + str2;

is converted to something like this ....

str = new StringBuilder().append(str1).append(str2).toString();

If i try str = str1 + str2, then how many objects get created.

there are three objects ...... two already exists str1,str2 and new str

my doubt is that str =str + str2 ............ how many objects created

i think 3 objects are old : str , str2 and new : str
two object has reference and one object without reference
correct me if not

yes you are right.

Henry Wong wrote:

Naresh Chaurasia wrote:If i try str = str1 + str2, then how many objects get created.

I believe ....

str = str1 + str2;

is converted to something like this ....

str = new StringBuilder().append(str1).append(str2).toString();

Looks like you are right. Quote from the API:

The Java language provides special support for the string concatenation operator ( + ), and for conversion of other objects to strings. String concatenation is implemented through the StringBuilder(or StringBuffer) class and its append method.

So, if you use the '+' sign in any case a new StirngBuilder or StringBuffer object will be created.

cheers
Bob