RxJava Maybe samples added. (#3646)

* RxJava Maybe samples added.

* update rxjava maybe test

* Update rxjava maybe tests

rxjava/src/test/java/com/baeldung/rxjava/MaybeTest.java

@@ -0,0 +1,40 @@
+package com.baeldung.rxjava;
+
+import org.junit.Test;
+
+import io.reactivex.Flowable;
+import io.reactivex.Maybe;
+
+public class MaybeTest {
+    @Test
+    public void whenEmitsSingleValue_thenItIsObserved() {
+        Maybe<Integer> maybe = Flowable.just(1, 2, 3, 4, 5)
+            .firstElement();
+
+        maybe.map(x -> x + 7)
+            .filter(x -> x > 0)
+            .test()
+            .assertResult(8)
+            .assertComplete();
+    }
+
+    @Test
+    public void whenEmitsNoValue_thenSignalsCompletionAndNoValueObserved() {
+        Maybe<Integer> maybe = Flowable.just(1, 2, 3, 4, 5)
+            .skip(5)
+            .firstElement();
+
+        maybe.test()
+            .assertComplete()
+            .assertNoValues();
+    }
+
+    @Test
+    public void whenThrowsError_thenErrorIsRaised() {
+        Maybe<Integer> maybe = Flowable.<Integer> error(new Exception("msg"))
+            .firstElement();
+
+        maybe.test()
+            .assertErrorMessage("msg");
+    }
+}