Why does a "customised Enum constructor" return the info of all the other Enum type?

user_13048460 0
Viewed 38
enum TrafficSignal {

    //this will call enum constructor with one String argument
    RED("wait"), 
    GREEN("go"), 
    ORANGE("slow down");

    private String action;

    public String getAction() {
        return this.action;
    }

    // enum constructor - can not be public or protected
    TrafficSignal(String action){
        this.action = action;
        System.out.println(this.action);
    }
}


public class EnumConstructorExample{

    public static void main(String args[]) {

        // Only one Enum object initialized/instaniated
       TrafficSignal c1 = TrafficSignal.GREEN;
    }
}

Output:

wait
go
slow down

I am just wondering why the output would give the information of all the other Enum types despite the fact I only initialized one enum object (TrafficSignal.GREEN).

enums java
3 Answers

Enum values are constants (public static final fields) on the enum class. When the enum class itself is initialized, it goes through the normal class initialization process, which includes initializing all fields (including value instances) and running static initializer blocks. You would get the same behavior using Class.forName without actually referring to any of the values.

user_11898850

All the enum’s objects instantiated automatically when class loads

You said:

despite the fact I only initialized one enum object (TrafficSignal.GREEN).

Incorrect. You did not instantiate an enum object.

You accessed an already-existing object of the enum class. That object was instantiated when the enum’s class was loaded.

Indeed, all of the enum’s objects were instantiated when the class loaded. As part of instantiating one object for each of the constant names, its constructor is automatically called. All this happened when the enum class was loaded. So all this happened before your code was executed. The class used in your code must first be loaded before your code can execute. So, three named constants mean the constructor is called three times, each time printing the action member field value before you gained access to the object referenced by the constant named GREEN.

user_6427060

enum can have constructor and it is executed separately for each enum constant at the time of enum class loading.

user_93534080
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