code: public static boolean getBoolean(String key, boolean def) {
          String value = get(key);
          if (value == null) {
              return def;
          }
  
          value = value.trim().toLowerCase();
          if (value.isEmpty()) {
              return def;
          }
  
          if ("true".equals(value) || "yes".equals(value) || "1".equals(value)) {
              return true;
          }
  
          if ("false".equals(value) || "no".equals(value) || "0".equals(value)) {
              return false;
          }
  
          logger.warn(
                  "Unable to parse the boolean system property '{}':{} - using the default value: {}",
                  key, value, def
          );
  
          return def;
      }
docs: Returns the value of the Java system property with the specified
  {@code key}, while falling back to the specified default value if
  the property access fails.
  
  @return the property value.
  {@code def} if there's no such property or if an access to the
  specified property is not allowed.