implemented change req. #46 propertyNames()

in interface Accessible

    Set<String> propertyNames();

This differs from the correspondent method in java.util.Properties
since Properties.propertyNames returns an Enumeration<?> but property
names are required to be Strings. In JDK 1.6 the method
`Set<String> stringPropertyNames()` has been introduced but since
we require JDK 1.5+ support I couldn't just delegate and I had to
reimplement it. So I chosed to implement it with the proper method
signature, even though it doesn't match java.util.Properties
interface.

src/main/java/org/aeonbits/owner/Accessible.java

@@ -12,6 +12,7 @@
 import java.io.OutputStream;
 import java.io.PrintStream;
 import java.io.PrintWriter;
+import java.util.Set;
 
 /**
  * <p>Allows a <tt>Config</tt> object to access the contents of the properties, providing utility methods to perform
@@ -98,4 +99,7 @@ public interface Accessible {
 
     //TODO: javadocs
     void storeToXML(OutputStream os, String comment) throws IOException;
+
+    //TODO: javadocs
+    Set<String> propertyNames();
 }

src/main/java/org/aeonbits/owner/PropertiesManager.java

@@ -26,18 +26,21 @@
 import java.net.MalformedURLException;
 import java.net.URL;
 import java.util.ArrayList;
-import java.util.Collections;
+import java.util.Enumeration;
+import java.util.LinkedHashSet;
 import java.util.LinkedList;
 import java.util.List;
 import java.util.Map;
 import java.util.Properties;
+import java.util.Set;
 import java.util.concurrent.ScheduledExecutorService;
 import java.util.concurrent.locks.ReentrantReadWriteLock;
 import java.util.concurrent.locks.ReentrantReadWriteLock.ReadLock;
 import java.util.concurrent.locks.ReentrantReadWriteLock.WriteLock;
 
 import static java.lang.annotation.ElementType.METHOD;
 import static java.lang.annotation.RetentionPolicy.RUNTIME;
+import static java.util.Collections.synchronizedList;
 import static org.aeonbits.owner.Config.LoadType;
 import static org.aeonbits.owner.Config.LoadType.FIRST;
 import static org.aeonbits.owner.PropertiesMapper.defaults;
@@ -65,7 +68,7 @@ class PropertiesManager implements Reloadable, Accessible, Mutable {
 
     private volatile boolean loading = false;
 
-    private final List<ReloadListener> reloadListeners = Collections.synchronizedList(new LinkedList<ReloadListener>());
+    private final List<ReloadListener> reloadListeners = synchronizedList(new LinkedList<ReloadListener>());
     private Object proxy;
     private final LoadersManager loaders;
 
@@ -211,6 +214,19 @@ public void storeToXML(OutputStream os, String comment) throws IOException {
         }
     }
 
+    @Delegate
+    public Set<String> propertyNames() {
+        readLock.lock();
+        try {
+            LinkedHashSet<String> result = new LinkedHashSet<String>();
+            for (Enumeration<?> propertyNames = properties.propertyNames(); propertyNames.hasMoreElements();)
+                result.add((String)propertyNames.nextElement());
+            return result;
+        } finally {
+            readLock.unlock();
+        }
+    }
+
     @Delegate
     public void list(PrintStream out) {
         readLock.lock();

src/test/java/org/aeonbits/owner/interfaces/AccessibleConfigTest.java

@@ -27,10 +27,14 @@
 import java.io.PrintWriter;
 import java.io.StringWriter;
 import java.util.Properties;
+import java.util.Set;
 import java.util.concurrent.ScheduledExecutorService;
 
+import static org.hamcrest.CoreMatchers.is;
+import static org.hamcrest.Matchers.containsInAnyOrder;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertThat;
 import static org.junit.Assert.assertTrue;
 
 /**
@@ -57,7 +61,7 @@ public static interface AccessibleConfig extends Config, Accessible {
     @Test
     public void testListPrintStream() throws IOException {
         ByteArrayOutputStream expected = new ByteArrayOutputStream();
-        PropertiesManagerForTest manager = 
+        PropertiesManagerForTest manager =
                 new PropertiesManagerForTest(AccessibleConfig.class, new Properties(), scheduler, expander, loaders);
         manager.load().list(new PrintStream(expected, true));
 
@@ -71,7 +75,7 @@ public void testListPrintStream() throws IOException {
     @Test
     public void testListPrintWriter() throws IOException {
         StringWriter expected = new StringWriter();
-        PropertiesManagerForTest manager = 
+        PropertiesManagerForTest manager =
                 new PropertiesManagerForTest(AccessibleConfig.class, new Properties(), scheduler, expander, loaders);
         manager.load().list(new PrintWriter(expected, true));
 
@@ -115,4 +119,12 @@ public void testGetPropertyWithDefaultThatDoesNotExists() throws IOException {
         assertEquals("Hello", cfg.getProperty("salutation.text.nonexistent", "Hello"));
     }
 
+    @Test
+    public void testStringPropertyNames() throws Throwable {
+        AccessibleConfig cfg = ConfigFactory.create(AccessibleConfig.class);
+        Set<String> propNames = cfg.propertyNames();
+        assertThat(propNames.size(), is(2));
+        assertThat(propNames, containsInAnyOrder("favoriteSong", "salutation.text"));
+    }
+
 }