This rule is defined by the following Java class: net.sourceforge.pmd.rules.strings.InefficientStringBuffering
Example:
public class Foo { void bar() { // Avoid this StringBuffer sb=new StringBuffer("tmp = "+System.getProperty("java.io.tmpdir")); // use instead something like this StringBuffer sb = new StringBuffer("tmp = "); sb.append(System.getProperty("java.io.tmpdir")); } }