Using @experimental, covariance and final class

Author: akarnokd

src/main/java/rx/Observable.java

@@ -3752,8 +3752,8 @@ public final Observable<T> defaultIfEmpty(T defaultValue) {
      * @return  an Observable that emits the items emitted by the source Observable or the items of an alternate Observable if the source Observable
      *          is empty.
      */
-    @Beta
-    public final Observable<T> switchIfEmpty(Observable<T> alternate) {
+    @Experimental
+    public final Observable<T> switchIfEmpty(Observable<? extends T> alternate) {
         return lift(new OperatorSwitchIfEmpty<T>(alternate));
     }
 

src/main/java/rx/internal/operators/OperatorSwitchIfEmpty.java

@@ -25,10 +25,10 @@
  * functionality to {@link rx.internal.operators.OperatorDefaultIfEmpty} except instead of one item being emitted when
  * empty, the results of the given Observable will be emitted.
  */
-public class OperatorSwitchIfEmpty<T> implements Observable.Operator<T, T> {
-    private final Observable<T> alternate;
+public final class OperatorSwitchIfEmpty<T> implements Observable.Operator<T, T> {
+    private final Observable<? extends T> alternate;
 
-    public OperatorSwitchIfEmpty(Observable<T> alternate) {
+    public OperatorSwitchIfEmpty(Observable<? extends T> alternate) {
         this.alternate = alternate;
     }