When are computed the functions inside an Enum's variable?

user_5899172 0
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Consider the following code :

public enum MyOwnEnum    {

    JANVIER("janvier", "3101"),
    FEVRIER("février", new DateCalculator().getLeapYear()),
    MARS("mars", "3103"),
    AVRIL("avril", "3004");

    // Two variables, the protected constructor, getters

}

How would the code behave ? Would the method be computed directly at compile time and fixed, or would it be computed each time someone calls FEVRIER through MyOwnEnum.FEVRIER or MyOwnEnum.valueOf("FEVRIER") ? Or fixed, but at runtime ?

runtime compilation enums java
2 Answers

The enum constants are created once at runtime, or more specifically, at class-loading time.

Consider the following code:

public enum TestEnum {
    ONE("One at " + System.nanoTime()),
    TWO("Two at " + System.nanoTime());

    String value;

    TestEnum(String value) {
        System.out.println(value);
        this.value = value;
    }
}

public class Test {
    public static void main(String[] args) throws Exception {
        System.out.println("Loading");

        Class.forName("TestEnum"); // Load class

        System.out.println("Evaluating");

        System.out.println(TestEnum.ONE.value); // Evaluate value one
        System.out.println(TestEnum.TWO.value); // Evaluate value two
    }
}

This generates the following output:

Loading
One at 31207575500045
Two at 31207575625697
Evaluating
One at 31207575500045
Two at 31207575625697

Note that the values don't change on subsequent invocations.

user_35589600

If you look at the byte code generated by your ENUM you can see that the above code is similar as below code:

class MyOwnEnum{
    public static MyOwnEnum JANVIER = new MyOwnEnum("janvier", "3101");
    public static MyOwnEnum FEVRIER = new MyOwnEnum(("février", new DateCalculator().getLeapYear()));
    public static MyOwnEnum MARS = new MyOwnEnum("mars", "3103");
}

That means your code will create the Object of DateCalculator() and call getLeapYear() when it will init the static variable FEVRIER and maintain all properties of static variable:

  1. A static variable which belongs to the class and not to object(instance)
  2. Static variables are initialized only once , at the start of the execution.
  3. Static variables will be initialized first, before the initialization of any instance variables
  4. A single copy to be shared by all instances of the class
  5. A static variable can be accessed directly by the class name and doesn’t need any object.

As FEVRIER is a static variable public static MyOwnEnum FEVRIER = new MyOwnEnum(("février", new DateCalculator().getLeapYear())); this lines of code will execute only once per class loader loaded the class.

Here is the byte code for reference:

21: ldc           #26                 // String FEVRIER
      23: iconst_1
      24: ldc           #27                 // String fΘvrier
      26: new           #29                 // class com/java8/demo/DateCalculator
      29: dup
      30: invokespecial #31                 // Method com/java8/demo/DateCalculator."<init>":()V
      33: invokevirtual #33                 // Method com/java8/demo/DateCalculator.getLeapYear:()Ljava/lang/String;
      36: invokespecial #20                 // Method "<init>":(Ljava/lang/String;ILjava/lang/String;Ljava/lang/String;)V
      39: putstatic     #37                 // Field FEVRIER:Lcom/java8/demo/MyOwnEnum;
      42: new           #1                  // class com/java8/demo/MyOwnEnum
      45: dup
      46: ldc           #39                 // String MARS
      48: iconst_2
      49: ldc           #40                 // String mars
      51: ldc           #42                 // String 3103
      53: invokespecial #20                 // Method "<init>":(Ljava/lang/String;ILjava/lang/String;Ljava/lang/String;)V
      56: putstatic     #44                 // Field MARS:Lcom/java8/demo/MyOwnEnum;
      59: new           #1                  // class com/java8/demo/MyOwnEnum
user_50394980
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