### Question:
MainClassResolver { @VisibleForTesting static boolean isValidJavaClass(String className) { for (String part : Splitter.on('.').split(className)) { if (!SourceVersion.isIdentifier(part)) { return false; } } return true; } private  MainClassResolver(); static String resolveMainClass(
      @Nullable String configuredMainClass, ProjectProperties projectProperties);  }### Answer:
@Test public void testValidJavaClassRegex() { Assert.assertTrue(MainClassResolver.isValidJavaClass("my.Class")); Assert.assertTrue(MainClassResolver.isValidJavaClass("my.java_Class$valid")); Assert.assertTrue(MainClassResolver.isValidJavaClass("multiple.package.items")); Assert.assertTrue(MainClassResolver.isValidJavaClass("is123.valid")); Assert.assertFalse(MainClassResolver.isValidJavaClass("${start-class}")); Assert.assertFalse(MainClassResolver.isValidJavaClass("123not.Valid")); Assert.assertFalse(MainClassResolver.isValidJavaClass("{class}")); Assert.assertFalse(MainClassResolver.isValidJavaClass("not valid")); }