diff -r 000000000000 -r fb9019fb1bf7 src/net/fortuna/ical4j/util/ParameterValidator.java
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/net/fortuna/ical4j/util/ParameterValidator.java	Tue Feb 10 18:12:00 2015 +0100
@@ -0,0 +1,131 @@
+/**
+ * Copyright (c) 2012, Ben Fortuna
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ *  o Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *
+ *  o Redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution.
+ *
+ *  o Neither the name of Ben Fortuna nor the names of any other contributors
+ * may be used to endorse or promote products derived from this software
+ * without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+ * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+ * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+ * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
+ * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
+ * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+ * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+package net.fortuna.ical4j.util;
+
+import net.fortuna.ical4j.model.Parameter;
+import net.fortuna.ical4j.model.ParameterList;
+import net.fortuna.ical4j.model.ValidationException;
+
+/**
+ * $Id$ [15-May-2004]
+ *
+ * Defines methods for validating parameters and parameter
+ * lists.
+ * 
+ * @author Ben Fortuna
+ */
+public final class ParameterValidator {
+
+    private static final String ASSERT_NONE_MESSAGE = "Parameter [{0}] is not applicable";
+
+    private static final String ASSERT_ONE_OR_LESS_MESSAGE = "Parameter [{0}] must only be specified once";
+
+    private static final String ASSERT_ONE_MESSAGE = "Parameter [{0}] must be specified once";
+
+    private static final String ASSERT_NULL_OR_EQUAL_MESSAGE = "Parameter [{0}] is invalid";
+
+    private static ParameterValidator instance = new ParameterValidator();
+
+    /**
+     * Constructor made private to enforce singleton.
+     */
+    private ParameterValidator() {
+    }
+
+    /**
+     * Ensure a parameter occurs no more than once.
+     *
+     * @param paramName
+     *            the parameter name
+     * @param parameters
+     *            a list of parameters to query
+     * @throws ValidationException
+     *             when the specified parameter occurs more than once
+     */
+    public void assertOneOrLess(final String paramName,
+            final ParameterList parameters) throws ValidationException {
+
+        if (parameters.getParameters(paramName).size() > 1) {
+            throw new ValidationException(ASSERT_ONE_OR_LESS_MESSAGE, new Object[] {paramName});
+        }
+    }
+
+    /**
+     * Ensure a parameter occurs once.
+     *
+     * @param paramName
+     *            the parameter name
+     * @param parameters
+     *            a list of parameters to query
+     * @throws ValidationException
+     *             when the specified parameter does not occur once
+     */
+    public void assertOne(final String paramName,
+            final ParameterList parameters) throws ValidationException {
+
+        if (parameters.getParameters(paramName).size() != 1) {
+            throw new ValidationException(ASSERT_ONE_MESSAGE, new Object[] {paramName});
+        }
+    }
+    
+    /**
+     * Ensure a parameter doesn't occur in the specified list.
+     * @param paramName the name of a parameter
+     * @param parameters a list of parameters
+     * @throws ValidationException thrown when the specified property
+     * is found in the list of properties
+     */
+    public void assertNone(final String paramName, final ParameterList parameters) throws ValidationException {
+        if (parameters.getParameter(paramName) != null) {
+            throw new ValidationException(ASSERT_NONE_MESSAGE, new Object[] {paramName});
+        }
+    }
+
+    /**
+     * @param param a parameter instance
+     * @param parameters a list of parameters
+     * @throws ValidationException where the assertion fails
+     */
+    public void assertNullOrEqual(final Parameter param, final ParameterList parameters) throws ValidationException {
+        final Parameter p = parameters.getParameter(param.getName());
+        if (p != null && !param.equals(p)) {
+            throw new ValidationException(ASSERT_NULL_OR_EQUAL_MESSAGE, new Object[] {p});
+        }
+    }
+    
+    /**
+     * @return Returns the instance.
+     */
+    public static ParameterValidator getInstance() {
+        return instance;
+    }
+}