This rule is defined by the following Java class: net.sourceforge.pmd.rules.UselessOperationOnImmutable
Example:
import java.math.*; class Test { void method1() { BigDecimal bd=new BigDecimal(10); bd.add(new BigDecimal(5)); // this will trigger the rule } void method2() { BigDecimal bd=new BigDecimal(10); bd = bd.add(new BigDecimal(5)); // this won't trigger the rule } }