No enclosing instance of (class here) is accessible

Question

I have a class called GreenhouseControls that has a bunch of

classes built into it such as:

public class ThermostatNight extends Event {
    public ThermostatNight(long delayTime) {
      super(delayTime);
    }
    public void action() {
      // hardware control code here.
      thermostat = "Night";
    }
    public String toString() {
      return "Thermostat on night setting";
    }
  }

I pull values from a text file to get event names such as "ThermostatNight" and time values such as "2000". To instantiate a new object with those values I built a EventFactory that accepts the values as arguments.

This is the class I have built to create new event objects from text file values:

    public class EventFactory{

        public static Event createSpecificEvent(String eventName, long delayTime) {

            Event event = null;

            switch(eventName) {
            case "ThermostatNight": 
                event = new ThermostatNight(delayTime); // Compiler error
                break;
            case "ThermostatDay": 
                event = new ThermostatDay(delayTime);
                break;
            case "LightOn": 
                event = new LightOn(delayTime);
                break;
            case "LightOff": 
                event = new LightOff(delayTime);
                break;
                ...
        }
}

Everything was working well when I ran the program until I pulled the EventFactory class out of GreenhouseControls and into it's own independent class. Now I am getting a compile time error that says:

"No enclosing instance of type GreenhouseControls is accessible. Must qualify the allocation with an enclosing instance of type GreenhouseControls (e.g. x.new A() where x is an instance of GreenhouseControls)."

See in-line comment in EventFactory class to see where the error occurs at "new ThermostatNight(delayTime)"

Answer 1

Your classes are currently inner classes, which need to be constructed in the context of instances of the containing class. Your options are:

• Specify an instance of the outer class when constructing an instance of the inner class
• Declare your nested classes as static classes, at which point they won't be inner classes any more
• Move the nested classes out of the containing class, making them top level classes

Personally I'd go for the last option if possible - nested classes can be useful at times, but you should only use them when there's a real benefit. There are various restrictions which can be quite subtle and which are best avoided if possible. Do you have any compelling reason to make these nested classes?

Answer 2

The error means that a non-static inner class ThermostatNight is being instantiated from a static method. You need to make the class static (the most likely solution given your code), make the method non-static, or provide an instance explicitly.

public static /*<<==Add this*/ class ThermostatNight extends Event {
    public ThermostatNight(long delayTime) {
      super(delayTime);
    }
    public void action() {
      // hardware control code here.
      thermostat = "Night";
    }
    public String toString() {
      return "Thermostat on night setting";
    }
}

To provide an instance explicitly, use this example:

public static Event createSpecificEvent(GreenhouseControls ctrl, String eventName, long delayTime) {
    Event event = null;
    switch(eventName) {
    case "ThermostatNight": 
        event = ctrl.new ThermostatNight(delayTime);
    ...
}