diff --git a/src/org/eidecker/oca8lernen/general/DoubleImplementsClash.java b/src/org/eidecker/oca8lernen/general/DoubleImplementsClash.java
new file mode 100644
index 0000000..a95704c
--- /dev/null
+++ b/src/org/eidecker/oca8lernen/general/DoubleImplementsClash.java
@@ -0,0 +1,38 @@
+package org.eidecker.oca8lernen.general;
+
+import java.io.IOException;
+import java.nio.file.AccessDeniedException;
+import org.junit.jupiter.api.Test;
+
+/**
+ * @author seidecke.
+ */
+public class DoubleImplementsClash {
+
+	@Test
+	public void testSomeTHings() {
+
+	}
+
+}
+
+class WorkingClass extends AbstractClass implements AnInterface {
+
+	@Override
+	public void method1() throws AccessDeniedException {
+
+	}
+}
+
+
+abstract class AbstractClass implements AnInterface {
+
+	@Override
+	public abstract void method1() throws AccessDeniedException;
+}
+
+interface AnInterface {
+	default void method1() throws IOException {
+
+	}
+}
diff --git a/src/org/eidecker/oca8lernen/general/Interfaces.java b/src/org/eidecker/oca8lernen/general/Interfaces.java
index 7d7db10..45de9be 100644
--- a/src/org/eidecker/oca8lernen/general/Interfaces.java
+++ b/src/org/eidecker/oca8lernen/general/Interfaces.java
@@ -14,7 +14,7 @@ class TestInterfaces {
 
 }
 
-public class Interfaces extends AbstractClass implements I1 {
+public class Interfaces extends SomeAbstractClass implements I1 {
 
 	public void method1() {
 
@@ -30,7 +30,7 @@ public class Interfaces extends AbstractClass implements I1 {
 }
 
 
-abstract class AbstractClass {
+abstract class SomeAbstractClass {
 
 	public abstract void method1() throws IOException;
 
diff --git a/src/org/eidecker/oca8lernen/general/StringIndex.java b/src/org/eidecker/oca8lernen/general/StringIndex.java
index 5df9bde..9a22e63 100644
--- a/src/org/eidecker/oca8lernen/general/StringIndex.java
+++ b/src/org/eidecker/oca8lernen/general/StringIndex.java
@@ -61,6 +61,9 @@ public class StringIndex {
 		assertEquals("71234567890", testStringBuilder.insert(0,7).toString());
 		testStringBuilder = new StringBuilder("1234567890");
 		assertEquals("1true234567890", testStringBuilder.insert(1,true).toString());
+
+		String s = null;
+		assertEquals("Hallonull", new StringBuilder("Hallo").append(s).toString());
 	}