Add methods to FieldMap API to make it usable in for-each loop (#347)

* Add methods to FieldMap API to make it usable in for-each loop

FieldMap already provides iterator() to inspect all fields, however does not implement Iterable - and so can't be used in for-each loop. This PR adds Iterable interface, as well as groupKeys() which exposes group keys as Iterable.
 The motivation for the change is that in some cases it is useful to traverse message as a tree of key-value pairs - e.g. when visualising message structure in UI, or transforming it to another intermediate representation. Being able to iterate in a more idiomatic Collection-like way improves readability of such code.

* Deprecate groupKeyIterator() in favour of groupKeys(). Add javadoc.

Author: AndreyNudko

quickfixj-core/src/main/java/quickfix/DataDictionary.java

@@ -43,7 +43,6 @@
 import java.util.Collection;
 import java.util.HashMap;
 import java.util.HashSet;
-import java.util.Iterator;
 import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Map;
@@ -751,9 +750,8 @@ private static boolean isVersionSpecified(DataDictionary dd) {
 
     private void iterate(FieldMap map, String msgType, DataDictionary dd) throws IncorrectTagValue,
             IncorrectDataFormat {
-        final Iterator<Field<?>> iterator = map.iterator();
-        while (iterator.hasNext()) {
-            final StringField field = (StringField) iterator.next();
+        for (final Field<?> f : map) {
+            final StringField field = (StringField) f;
 
             checkHasValue(field);
 

quickfixj-core/src/main/java/quickfix/FieldMap.java

@@ -45,7 +45,7 @@
 /**
  * Field container used by messages, groups, and composites.
  */
-public abstract class FieldMap implements Serializable {
+public abstract class FieldMap implements Serializable, Iterable<Field<?>> {
 
     static final long serialVersionUID = -3193357271891865972L;
 
@@ -448,6 +448,7 @@ public void removeField(int field) {
         fields.remove(field);
     }
 
+    @Override
     public Iterator<Field<?>> iterator() {
         return fields.values().iterator();
     }
@@ -601,8 +602,21 @@ public int getGroupCount(int tag) {
         return getGroups(tag).size();
     }
 
+    /**
+     * @deprecated use {@linkplain #groupKeys()} instead
+     */
+    @Deprecated
     public Iterator<Integer> groupKeyIterator() {
-        return groups.keySet().iterator();
+        return groupKeys().iterator();
+    }
+
+    /**
+     * Returns tags which are repeating group counters as {@code Iterable}
+     *
+     * @return tags which are repeating group counters
+     */
+    public Iterable<Integer> groupKeys() {
+        return groups.keySet();
     }
 
     Map<Integer, List<Group>> getGroups() {

quickfixj-core/src/main/java/quickfix/Message.java

@@ -66,7 +66,6 @@
 import javax.xml.transform.stream.StreamResult;
 import java.io.ByteArrayOutputStream;
 import java.text.DecimalFormat;
-import java.util.Iterator;
 import java.util.List;
 
 /**
@@ -392,9 +391,7 @@ private void toXMLFields(Element message, String section, FieldMap fieldMap,
         final Document document = message.getOwnerDocument();
         final Element fields = document.createElement(section);
         message.appendChild(fields);
-        final Iterator<Field<?>> fieldItr = fieldMap.iterator();
-        while (fieldItr.hasNext()) {
-            final Field<?> field = fieldItr.next();
+        for (final Field<?> field : fieldMap) {
             final Element fieldElement = document.createElement("field");
             if (dataDictionary != null) {
                 final String name = dataDictionary.getFieldName(field.getTag());
@@ -412,9 +409,7 @@ private void toXMLFields(Element message, String section, FieldMap fieldMap,
             fieldElement.appendChild(value);
             fields.appendChild(fieldElement);
         }
-        final Iterator<Integer> groupKeyItr = fieldMap.groupKeyIterator();
-        while (groupKeyItr.hasNext()) {
-            final int groupKey = groupKeyItr.next();
+        for (final int groupKey : fieldMap.groupKeys()) {
             final Element groupsElement = document.createElement("groups");
             fields.appendChild(groupsElement);
             if (dataDictionary != null) {