code: public static boolean checkUnsupportedJava() {
      if (Boolean.getBoolean(H2O.OptArgs.SYSTEM_PROP_PREFIX + "debug.noJavaVersionCheck")) {
        return false;
      }
  
      // Notes: 
      // - make sure that the following whitelist is logically consistent with whitelist in R code - see function .h2o.check_java_version in connection.R
      // - upgrade of the javassist library should be considered when adding support for a new java version
      if (JAVA_VERSION.isKnown() && !isUserEnabledJavaVersion() && (JAVA_VERSION.getMajor()<7 || JAVA_VERSION.getMajor()>12)) {
        System.err.println("Only Java 7, 8, 9, 10, 11 and 12 are supported, system version is " + System.getProperty("java.version"));
        return true;
      }
      String vmName = System.getProperty("java.vm.name");
      if (vmName != null && vmName.equals("GNU libgcj")) {
        System.err.println("GNU gcj is not supported");
        return true;
      }
      return false;
    }
docs: Check if the Java version is not supported
  
  @return true if not supported