[Chromoting] Decouple DesktopView and TouchInputHandler

remoting/android/java/src/org/chromium/chromoting/Preconditions.java

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package org.chromium.chromoting;
 
 /**
  * A set of helper functions to write assertions. These functions are always being executed, and
  * will not ignored in release build.
  */
 
 public final class Preconditions {
     // This class contains only static functions, so it should not be instantiated.
     private Preconditions() {}
 
     /**
      * Checks whether input |value| is true, and returns its value. Throws
      * {@link IllegalStateException} if |value| is false.
      */
     public static final boolean isTrue(boolean value) {
         if (!value) {
             throw new IllegalStateException();
         }
         return value;
     }
 
     /**
-     * Checks whether input |ref| is not a null reference, and return its value. Throws
+     * Checks whether input |ref| is not a null reference, and returns its value. Throws
      * {@link NullPointerException} if |ref| is null.
      */
     public static final <T> T notNull(T ref) {
         if (ref == null) {
             throw new NullPointerException();
         }
         return ref;
     }
+
+    /**
+     * Checks whether input |ref| is a null reference, and returns its value. Throws
+     * {@link IllegalArgumentException} if |ref| is not null.
+     */
+    public static final <T> T isNull(T ref) {
+        if (ref != null) {
+            throw new IllegalArgumentException();
+        }
+        return ref;
+    }
 }