Add nullPointerException comments and ObjectHelper test code. (#5255)

* Add NullPointerException comments for doc.

* Add test code for ObjectHelper.java

src/main/java/io/reactivex/Observable.java

@@ -3595,6 +3595,9 @@ public static Observable<Long> timer(long delay, TimeUnit unit) {
      *            time units to use for {@code delay}
      * @param scheduler
      *            the {@link Scheduler} to use for scheduling the item
+     * @throws NullPointerException
+     *             if {@code unit} is null, or
+     *             if {@code scheduler} is null
      * @return an Observable that emits {@code 0L} after a specified delay, on a specified Scheduler, and then
      *         completes
      * @see <a href="http://reactivex.io/documentation/operators/timer.html">ReactiveX operators documentation: Timer</a>

src/test/java/io/reactivex/internal/functions/ObjectHelperTest.java

@@ -33,8 +33,27 @@ public void hashCodeOf() {
     }
 
     @Test
-    public void compare() {
+    public void verifyPositiveInt() throws Exception{
+        assertEquals(1, ObjectHelper.verifyPositive(1, "param"));
+    }
+
+    @Test
+    public void verifyPositiveLong() throws Exception{
+        assertEquals(1L, ObjectHelper.verifyPositive(1L, "param"));
+    }
 
+    @Test(expected = IllegalArgumentException.class)
+    public void verifyPositiveIntFail() throws Exception{
+        assertEquals(-1, ObjectHelper.verifyPositive(-1, "param"));
+    }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void verifyPositiveLongFail() throws Exception{
+        assertEquals(-1L, ObjectHelper.verifyPositive(-1L, "param"));
+    }
+
+    @Test
+    public void compare() {
         assertEquals(-1, ObjectHelper.compare(0, 2));
         assertEquals(0, ObjectHelper.compare(0, 0));
         assertEquals(1, ObjectHelper.compare(2, 0));