Rule Definition
String concatenation is not efficient because it creates a StringBuffer for each concatenation. When placed in a loop, String concatenation results in the creation and garbage collection of large numbers of temporary objects. This both consumes memory and can dramatically slow the program execution. It is recommended to create a StringBuffer before entering the loop, and append to it within the loop, thus reducing the overhead. When the artifacts have a high fan-in the risk is highly increased.

Remediation
It is recommended to create a StringBuilder (if JDK >= 1.5 and not in thread environment) or StringBuffer before entering the loop, and append to it within the loop, thus reducing the overhead.

Violation Code Sample

String result = "hello";
for (int i = 0; i < 1500; i++) {
   result += "hello";  // VIOLATION
}

Fixed Code Sample

StringBuffer result = new StringBuffer("hello");
for (int i = 0; i < 1500; i++) {
   result.append("hello");  // FIXED
}

http://www.precisejava.com/javaperf/j2se/StringAndStringBuffer.htm#Strings104