Iterable<? extends File> getDefaultBootclasspath() {
    List<File> files = new ArrayList<>();
    String javaversion = System.getProperty("java.version"); // $NON-NLS-1$
    if (javaversion.length() > 3) javaversion = javaversion.substring(0, 3);
    long jdkLevel = CompilerOptions.versionToJdkLevel(javaversion);
    if (jdkLevel < ClassFileConstants.JDK1_6) {
      // wrong jdk - 1.6 or above is required
      return null;
    }

    for (String fileName : org.eclipse.jdt.internal.compiler.util.Util.collectFilesNames()) {
      files.add(new File(fileName));
    }
    return files;
  }