added an idx tag

Author: habibasorour

source/ch6_definingclasses.ptx

@@ -196,6 +196,7 @@ public void setDenominator(Integer denominator) {
         <title>Writing a constructor</title>
 
         <p>
+            <idx>constructor</idx>
             Once you have identified the instance variables for your class the next thing to consider is the constructor.
             In Java, <term>constructors</term> have the same name as the class and are declared public.
             They are declared without a return type.