Add Event and EventTest

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 |v| is true, and returns its value. Throws

[Lambros, 2016/05/27 00:38:52]

s/v/value

Why bother returning the value if it's always true?

[Hzj_jie, 2016/05/27 02:55:20]

I think it's more helpful in the following scenario,
bool do_something_else(...) {
  if (...) {
    // parameters are valid combinations.
    return true;

  return false;
}

bool do_something(...) {
  if (...) {
    return false;
  }
  // ... do something
  // the parameters we have sent to do_something_else should always valid.
  return Preconditions.isTrue(do_something_else(...));
}

+    // {@link IllegalStateException} if |v| is false.
+    public static final boolean isTrue(boolean v) {
+        if (!v) {
+            throw new IllegalStateException();
+        }
+        return v;
+    }
+
+    // Checks whether input |ref| is not a null reference, and return 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;
+    }
+}