### Question:
PropertyUtils { public static String getString(String key) { if (StringUtils.isEmpty(key)) { return null; } return properties.getProperty(key.trim()); } private  PropertyUtils(); static String getString(String key); static String getString(String key, String defaultVal); static int getInt(String key); static int getInt(String key, int defaultValue); static Boolean getBoolean(String key); static long getLong(String key); static long getLong(String key, long defaultVal); static double getDouble(String key); static double getDouble(String key, double defaultVal); static String[] getArray(String key, String splitStr); static T getEnum(String key, Class<T> type,
                                         T defaultValue);  }### Answer:
@Test public void testGetString() { String result = PropertyUtils.getString("alert.type"); logger.info(result); assertEquals(result, "EMAIL"); result = PropertyUtils.getString("mail.server.host"); assertEquals(result, "xxx.xxx.test"); result = PropertyUtils.getString("abc"); assertNull(result); result = PropertyUtils.getString(null); assertNull(result); }