diff --git a/src/test/kotlin/com/github/daggerok/callbackhell/CallbackHellTests.java b/src/test/kotlin/com/github/daggerok/callbackhell/CallbackHellTests.java
new file mode 100644
index 0000000..bfd4832
--- /dev/null
+++ b/src/test/kotlin/com/github/daggerok/callbackhell/CallbackHellTests.java
@@ -0,0 +1,84 @@
+package com.github.daggerok.callbackhell;
+
+import java.util.Objects;
+import java.util.concurrent.atomic.AtomicLong;
+import lombok.Data;
+import org.hamcrest.core.IsEqual;
+import org.junit.jupiter.api.DisplayNameGeneration;
+import org.junit.jupiter.api.DisplayNameGenerator;
+import org.junit.jupiter.api.Test;
+
+import static org.hamcrest.MatcherAssert.assertThat;
+
+@Data
+class Ad {
+
+    private final String content;
+}
+
+interface Promise {
+    void complete(Ad ad);
+}
+
+@Data
+class MyPromise implements Promise {
+
+    private final Ad value;
+
+    @Override
+    public void complete(Ad ad) {
+        System.out.printf("Completing Ad(%s)...%n", ad);
+        if (Objects.isNull(ad)) return;
+        System.out.printf("Ad(%s) completed.%n", ad);
+    }
+}
+
+interface Mediation {
+    <P extends Promise> void mediate(P promise);
+}
+
+@Data
+class MyMediation implements Mediation {
+
+    private final AtomicLong mediateCounter = new AtomicLong();
+    private final AtomicLong myPromiseCounter = new AtomicLong();
+
+    @Override
+    public <P extends Promise> void mediate(P promise) {
+        System.out.printf("Start Promise(%s) mediation...%n", promise);
+        if (Objects.isNull(promise)) return;
+        doMediation(promise);
+        System.out.printf("Promise(%s) mediation completed.%n", promise);
+    }
+
+    private <P extends Promise> void doMediation(Promise promise) {
+        System.out.printf("Process mediation...%n");
+        if (Objects.isNull(promise)) return;
+        mediateCounter.incrementAndGet();
+        System.out.printf("Process %s mediation...%n", promise);
+        if (promise instanceof MyPromise) {
+            Ad value = ((MyPromise) promise).getValue();
+            myPromiseCounter.incrementAndGet();
+            System.out.printf("MyPromise mediation content: %s%n", value.getContent());
+        }
+    }
+}
+
+@DisplayNameGeneration(DisplayNameGenerator.ReplaceUnderscores.class)
+class CallbackHellTests {
+
+    @Test
+    void should_test() {
+        // given
+        Ad ad = new Ad("My ad");
+        MyPromise myPromise = new MyPromise(ad);
+        MyMediation myMediation = new MyMediation();
+
+        // when
+        myMediation.mediate(myPromise);
+
+        // then
+        assertThat(myMediation.getMediateCounter().get(), IsEqual.equalTo(1L));
+        assertThat(myMediation.getMyPromiseCounter().get(), IsEqual.equalTo(1L));
+    }
+}