A DESCRIPTION OF THE REQUEST :
Currently, when javac encounters a String concatenation, it converts the code to use a StringBuilder. For example:
   String a = String.valueOf(System.currentTimeMillis());
    String b = String.valueOf(System.currentTimeMillis());
    String c = String.valueOf(System.currentTimeMillis());
    String d = a + b + c + "_US";

becomes something like

String d = new StringBuilder().append(a).append(b).append(c).append("_US");

Since the StringBuilder is not explicitly sized, the default size is used and often results in expandCapacity calls at runtime when the default size is too small.

Many developers uses String concatenation for many operations e.g. build keys for various HashMaps, build unique key for each element in JSF etc



JUSTIFICATION :
When profiling our application, we often see AbstractStringBuilder.expandCapacity shows up at the top, and they are almost always the result of String concatenation.

This is especially apparent in JSF applications, when each component needs a distinct ID, and for things like looking up a translated value with a key in the form of key + locale.

A simple change listed below could potentially avoid most of the expandCapacity calls.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
 javac should consider constructing the StringBuilder with the correct size:

String d = new StringBuilder(a.length() + b.length() + c.length() + "_US".length() ).append(a).append(b).append(c).append("_US");

---------- BEGIN SOURCE ----------
public class SB {
  public static void main(String [] args) {
    String a = String.valueOf(System.currentTimeMillis());
    String b = String.valueOf(System.currentTimeMillis());
    String c = String.valueOf(System.currentTimeMillis());
    String d = a + b + c + "_US";
    System.out.println(d);
  }
}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Developers have to explicitly use StringBuilder. For the above, the workaround would be to write this explicitly:
String d = new StringBuilder(a.length() + b.length() + c.length() + "_US".length() ).append(a).append(b).append(c).append("_US");