Merge pull request #311 from habibasorour/issue_302_comment

Issue 302 fixed added comments to code in 6.4

Author: pearcej

source/ch6_definingclasses.ptx

@@ -568,7 +568,7 @@ Fraction@6ff3c5b5
             <program language="java">
                 <code>
 public String toString() {
-    return numerator.toString() + "/" + denominator.toString();
+    return numerator.toString() + "/" + denominator.toString(); // convert to a string
 }
                 </code>
             </program>
@@ -703,16 +703,16 @@ public class Fraction extends Number { // fraction class is a child of Number
             <listing xml:id="java-object-type-conversions">
             <program language="java">
                 <code>
-public double doubleValue() {
+public double doubleValue() { // convert to a double
     return numerator.doubleValue() / denominator.doubleValue();
 }
-public float floatValue() {
+public float floatValue() { // convert to a float
     return numerator.floatValue() / denominator.floatValue();
 }
-public int intValue() {
+public int intValue() {  // convert to an int
     return numerator.intValue() / denominator.intValue();
 }
-public long longValue() {
+public long longValue() { // convert to a long
     return numerator.longValue() / denominator.longValue();
 }
                 </code>
@@ -740,7 +740,7 @@ public long longValue() {
             <program language="java">
                 <code>
 public void test(Number a, Number b) {
-    a.add(b);
+    a.add(b); // this is a bad idea
 }
                 </code>
             </program>