Seems that you forgot to copy splitter.jar and fastutil.jar.
Sorry about that. You are right. Building mkgmap runs now without errors. But cd mkgmap/dist java -Xmx5000m -cp mkgmap.jar:../lib/optional/* uk.me.parabola.mkgmap.sea.optional.PrecompSeaGenerator ../../land-polygons-split-4326/land_polygons.shp WGS84 ../../sea still produces this error: Exception in thread "main" java.lang.UnsupportedClassVersionError: uk/me/parabol a/mkgmap/sea/optional/PrecompSeaGenerator : Unsupported major.minor version 51.0 at java.lang.ClassLoader.defineClass1(Native Method) at java.lang.ClassLoader.defineClass(ClassLoader.java:634) at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:14 2) at java.net.URLClassLoader.defineClass(URLClassLoader.java:277) at java.net.URLClassLoader.access$000(URLClassLoader.java:73) at java.net.URLClassLoader$1.run(URLClassLoader.java:212) at java.security.AccessController.doPrivileged(Native Method) at java.net.URLClassLoader.findClass(URLClassLoader.java:205) at java.lang.ClassLoader.loadClass(ClassLoader.java:321) at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:294) at java.lang.ClassLoader.loadClass(ClassLoader.java:266) Could not find the main class: uk.me.parabola.mkgmap.sea.optional.PrecompSeaGene rator. Program will exit.