diff --git a/src/main/java/com/github/nkzawa/emitter/Emitter.java b/src/main/java/com/github/nkzawa/emitter/Emitter.java
index e61681a..0aa2039 100644
--- a/src/main/java/com/github/nkzawa/emitter/Emitter.java
+++ b/src/main/java/com/github/nkzawa/emitter/Emitter.java
@@ -80,7 +80,8 @@ public class Emitter {
 
     public List<Listener> listeners(String event) {
         ConcurrentLinkedQueue<Listener> callbacks = this.callbacks.get(event);
-        return new ArrayList<Listener>(callbacks);
+        return callbacks != null ?
+                new ArrayList<Listener>(callbacks) : new ArrayList<Listener>();
     }
 
     public boolean hasListeners(String event) {
diff --git a/src/test/java/com/github/nkzawa/emitter/EmitterTest.java b/src/test/java/com/github/nkzawa/emitter/EmitterTest.java
new file mode 100644
index 0000000..8a83358
--- /dev/null
+++ b/src/test/java/com/github/nkzawa/emitter/EmitterTest.java
@@ -0,0 +1,228 @@
+package com.github.nkzawa.emitter;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import static org.hamcrest.CoreMatchers.is;
+import static org.junit.Assert.assertThat;
+
+@RunWith(JUnit4.class)
+public class EmitterTest {
+
+    @Test
+    public void on() {
+        Emitter emitter = new Emitter();
+        final List<Object> calls = new ArrayList<Object>();
+
+        emitter.on("foo", new Emitter.Listener() {
+            @Override
+            public void call(Object... args) {
+                calls.add("one");
+                calls.add(args[0]);
+            }
+        });
+
+        emitter.on("foo", new Emitter.Listener() {
+            @Override
+            public void call(Object... args) {
+                calls.add("two");
+                calls.add(args[0]);
+            }
+        });
+
+        emitter.emit("foo", 1);
+        emitter.emit("bar", 1);
+        emitter.emit("foo", 2);
+
+        assertThat(calls.toArray(), is(new Object[] {"one", 1, "two", 1, "one", 2, "two", 2}));
+    }
+
+    @Test
+    public void once() {
+        Emitter emitter = new Emitter();
+        final List<Object> calls = new ArrayList<Object>();
+
+        emitter.once("foo", new Emitter.Listener() {
+            @Override
+            public void call(Object... args) {
+                calls.add("one");
+                calls.add(args[0]);
+            }
+        });
+
+        emitter.emit("foo", 1);
+        emitter.emit("foo", 2);
+        emitter.emit("foo", 3);
+        emitter.emit("bar", 1);
+
+        assertThat(calls.toArray(), is(new Object[] {"one", 1}));
+    }
+
+    @Test
+    public void off() {
+        Emitter emitter = new Emitter();
+        final List<Object> calls = new ArrayList<Object>();
+
+        Emitter.Listener one = new Emitter.Listener() {
+            @Override
+            public void call(Object... args) {
+                calls.add("one");
+            }
+        };
+        Emitter.Listener two = new Emitter.Listener() {
+            @Override
+            public void call(Object... args) {
+                calls.add("two");
+            }
+        };
+
+        emitter.on("foo", one);
+        emitter.on("foo", two);
+        emitter.off("foo", two);
+
+        emitter.emit("foo");
+
+        assertThat(calls.toArray(), is(new Object[] {"one"}));
+    }
+
+    @Test
+    public void offWithOnce() {
+        Emitter emitter = new Emitter();
+        final List<Object> calls = new ArrayList<Object>();
+
+        Emitter.Listener one = new Emitter.Listener() {
+            @Override
+            public void call(Object... args) {
+                calls.add("one");
+            }
+        };
+
+        emitter.once("foo", one);
+        emitter.off("foo", one);
+
+        emitter.emit("foo");
+
+        assertThat(calls.toArray(), is(new Object[] {}));
+    }
+
+    @Test
+    public void offWhenCalledfromEvent() {
+        final Emitter emitter = new Emitter();
+        final boolean[] called = new boolean[] {false};
+        final Emitter.Listener b = new Emitter.Listener() {
+            @Override
+            public void call(Object... args) {
+                called[0] = true;
+            }
+        };
+        emitter.on("tobi", new Emitter.Listener() {
+            @Override
+            public void call(Object... args) {
+                emitter.off("tobi", b);
+            }
+        });
+        emitter.once("tobi", b);
+        emitter.emit("tobi");
+        assertThat(called[0], is(true));
+        called[0] = false;
+        emitter.emit("tobi");
+        assertThat(called[0], is(false));
+    }
+
+    @Test
+    public void offEvent() {
+        Emitter emitter = new Emitter();
+        final List<Object> calls = new ArrayList<Object>();
+
+        Emitter.Listener one = new Emitter.Listener() {
+            @Override
+            public void call(Object... args) {
+                calls.add("one");
+            }
+        };
+        Emitter.Listener two = new Emitter.Listener() {
+            @Override
+            public void call(Object... args) {
+                calls.add("two");
+            }
+        };
+
+        emitter.on("foo", one);
+        emitter.on("foo", two);
+        emitter.off("foo");
+
+        emitter.emit("foo");
+        emitter.emit("foo");
+
+        assertThat(calls.toArray(), is(new Object[] {}));
+    }
+
+    @Test
+    public void offAll() {
+        Emitter emitter = new Emitter();
+        final List<Object> calls = new ArrayList<Object>();
+
+        Emitter.Listener one = new Emitter.Listener() {
+            @Override
+            public void call(Object... args) {
+                calls.add("one");
+            }
+        };
+        Emitter.Listener two = new Emitter.Listener() {
+            @Override
+            public void call(Object... args) {
+                calls.add("two");
+            }
+        };
+
+        emitter.on("foo", one);
+        emitter.on("bar", two);
+
+        emitter.emit("foo");
+        emitter.emit("bar");
+
+        emitter.off();
+
+        emitter.emit("foo");
+        emitter.emit("bar");
+
+        assertThat(calls.toArray(), is(new Object[]{"one", "two"}));
+    }
+
+    @Test
+    public void listeners() {
+        Emitter emitter = new Emitter();
+        Emitter.Listener foo = new Emitter.Listener() {
+            @Override
+            public void call(Object... args) {}
+        };
+        emitter.on("foo", foo);
+        assertThat(emitter.listeners("foo").toArray(), is(new Object[] {foo}));
+    }
+
+    @Test
+    public void listenersWithoutHandlers() {
+        Emitter emitter = new Emitter();
+        assertThat(emitter.listeners("foo").toArray(), is(new Object[] {}));
+    }
+
+    @Test
+    public void hasListeners() {
+        Emitter emitter = new Emitter();
+        emitter.on("foo", new Emitter.Listener() {
+            @Override
+            public void call(Object... args) {}
+        });
+        assertThat(emitter.hasListeners("foo"), is(true));
+    }
+
+    @Test
+    public void hasListenersWithoutHandlers() {
+        Emitter emitter = new Emitter();
+        assertThat(emitter.hasListeners("foo"), is(false));
+    }
+}