From a48071b0d489420bfdc0c2ed7254d1dba243f700 Mon Sep 17 00:00:00 2001
From: Matthew Aguirre <matt.aguirre@gmail.com>
Date: Mon, 11 May 2015 13:22:50 -0400
Subject: [PATCH] Added new addListener() method that takes a boolean for if
 duplicates are allowed. Added test method to make sure that duplicate
 listeners are not added.

---
 .../lang3/event/EventListenerSupport.java     | 21 ++++++++++++++++++-
 .../lang3/event/EventListenerSupportTest.java | 20 ++++++++++++++++++
 2 files changed, 40 insertions(+), 1 deletion(-)

diff --git a/src/main/java/org/apache/commons/lang3/event/EventListenerSupport.java b/src/main/java/org/apache/commons/lang3/event/EventListenerSupport.java
index 0f51796fbaf..4f3a9a77ac3 100644
--- a/src/main/java/org/apache/commons/lang3/event/EventListenerSupport.java
+++ b/src/main/java/org/apache/commons/lang3/event/EventListenerSupport.java
@@ -179,8 +179,27 @@ public L fire() {
      *         <code>null</code>.
      */
     public void addListener(final L listener) {
+        addListener(listener, true);
+    }
+
+    /**
+     * Registers an event listener.  Will not add a pre-existing listener
+     * object to the list if <code>allowDuplicate</code> is false.
+     *
+     * @param listener the event listener (may not be <code>null</code>).
+     * @param allowDuplicate the flag for determining if duplicate listener
+     * objects are allowed to be registered.
+     *
+     * @throws NullPointerException if <code>listener</code> is
+     * <code>null</code>.
+     */
+    public void addListener(final L listener, boolean allowDuplicate) {
         Validate.notNull(listener, "Listener object cannot be null.");
-        listeners.add(listener);
+        if (allowDuplicate) {
+            listeners.add(listener);
+        } else if (!allowDuplicate && !listeners.contains(listener)) {
+            listeners.add(listener);
+        }
     }
 
     /**
diff --git a/src/test/java/org/apache/commons/lang3/event/EventListenerSupportTest.java b/src/test/java/org/apache/commons/lang3/event/EventListenerSupportTest.java
index 5f65125f610..d8772a30908 100644
--- a/src/test/java/org/apache/commons/lang3/event/EventListenerSupportTest.java
+++ b/src/test/java/org/apache/commons/lang3/event/EventListenerSupportTest.java
@@ -41,6 +41,26 @@
  */
 public class EventListenerSupportTest 
 {
+    @Test
+    public void testAddListenerNoDuplicates() {
+        final EventListenerSupport<VetoableChangeListener> listenerSupport = EventListenerSupport.create(VetoableChangeListener.class);
+
+        final VetoableChangeListener[] listeners = listenerSupport.getListeners();
+        assertEquals(0, listeners.length);
+        assertEquals(VetoableChangeListener.class, listeners.getClass().getComponentType());
+        final VetoableChangeListener[] empty = listeners;
+        //for fun, show that the same empty instance is used 
+        assertSame(empty, listenerSupport.getListeners());
+
+        final VetoableChangeListener listener1 = EasyMock.createNiceMock(VetoableChangeListener.class);
+        listenerSupport.addListener(listener1);
+        assertEquals(1, listenerSupport.getListeners().length);
+        listenerSupport.addListener(listener1, false);
+        assertEquals(1, listenerSupport.getListeners().length);
+        listenerSupport.removeListener(listener1);
+        assertSame(empty, listenerSupport.getListeners());
+    }
+
     @Test(expected=NullPointerException.class)
     public void testAddNullListener()
     {