Device states

exercise No. 244

Q:

We consider an enum describing device states:

Device state enumeration Main method Result 
public enum Device {

 ON, OFF, STANDBY;

 private Device() {
  System.out.println("Hello!");
 }
}
 
public static void
  main(String[] args) {
  Device d = Device.STANDBY;
}
 
Hello!
Hello!
Hello!

Answer the following questions:

  • The Device() constructor is being declared private so how does it get invoked at all?

  • Why do we see "Hello!" three times on output though only Device.STANDBY shows up in the current example's main(...) method?

A:

ON, OFF and STANDBY are static references to Device instances being created inside our enum Device class. This happens the way being described in Figure 373, “Define a private Day constructor ”.

Thus our private constructor in question is being called from within enum Device class scope and therefore not being limited by access restrictions.

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