Adds Comparable to allow compile under JDK7

git-svn-id: https://svn.apache.org/repos/asf/struts/struts2/trunk@1417321 13f79535-47bb-0310-9956-ffa450edef68

plugins/sitegraph/src/main/java/org/apache/struts2/sitegraph/entities/Target.java

@@ -24,7 +24,8 @@ package org.apache.struts2.sitegraph.entities;
 
 /**
  */
-public class Target {
+public class Target implements Comparable<Target> {
+
     private String target;
     private int type;
 
@@ -59,4 +60,13 @@ public class Target {
         result = 29 * result + type;
         return result;
     }
+
+    public int compareTo(Target o) {
+        if (o.target != null && target != null) {
+            return o.target.compareTo(target);
+        } else {
+            return 0;
+        }
+    }
+
 }