Fixed Maven build with experimental java 9 Maven compiler plug-in. Minor

code changes

core-java-9/src/test/java/com/baeldung/java9/language/PrivateInterface.java

@@ -1,23 +0,0 @@
-package com.baeldung.java9.language;
-
-public interface PrivateInterface {
-    
-    private static String staticPrivate() {
-        return "static private";
-    }
-    
-    private String instancePrivate() {
-        return "instance private";
-    }
-    
-    public default void check(){
-        String result = staticPrivate();
-        if (!result.equals("static private"))
-            throw new AssertionError("Incorrect result for static private interface method");
-        PrivateInterface pvt = new PrivateInterface() {
-        };
-        result = pvt.instancePrivate();
-        if (!result.equals("instance private"))
-            throw new AssertionError("Incorrect result for instance private interface method");
-    }
-}

pom.xml

@@ -18,6 +18,7 @@
         <module>apache-fop</module>
         <module>core-java</module>
         <module>core-java-8</module>
+        <module>core-java-9</module>
         <module>couchbase-sdk-intro</module>
         <module>couchbase-sdk-spring-service</module>