String builder vs string concatenation

Question

What is the benefit and trade-off of using a string builder over pure string concatenation?

new StringBuilder(32).append(str1)
                     .append(" test: ")
                     .append(val)
                     .append(" is changed")
                     .toString();

vs say

str1 + " test: " + val + " is changed".

str1 is a random 10 character string. str2 is a random 8 character string.

Answer 1

In your particular example, none because the compiler internally uses StringBuilders to do String concatenation. If the concatenation occurred in a loop, however, the compiler could create several StringBuilder and String objects. For example:

String s= "" ;
for(int i= 0 ; i < 10 ; i++ )
    s+= "a" ;

Each time line 3 above is executed, a new StringBuilder object is created, the contents of s appended, "a" appended, and then the StringBuilder is converted into a String to be assigned back to s. A total of 10 StringBuilders and 10 Strings.

Conversely, in

StringBuilder sb= new StringBuilder() ;
for(int i= 0 ; i < 10 ; i++ )
    sb.append( "a" );
String s= sb.toString() ;

Only 1 StringBuilder and 1 String are created.

The main reason for this is that the compiler could not be smart enough to understand that the first loop is equivalent to the second and generate more efficient (byte) code. In more complex cases, it's impossible even for the smartest compiler to know. If you absolutely need this optimization, you have to introduce it manually by using StringBuilders explicitly.