How Do Static and Instance Blocks Work in Java Enums?

How do the execution orders of static and instance blocks differ in Java Enums compared to standard classes?

enum CoffeeSize {

    BIG(8), LARGE(10), HUGE(12), OVERWHELMING();
    private int ounces;

    static {
        System.out.println("static block ");
    }
    {
        System.out.println("instance block");
    }

    private CoffeeSize(int ounces) {
        this.ounces = ounces;
        System.out.println(ounces);
    }
    private CoffeeSize() {
        this.ounces = 20;
        System.out.println(ounces);
    }

    public int getOunces() {
        return ounces;
    }
} // Enum Definition

In Java, both static and instance initializer blocks play a critical role in class initialization. However, in the context of enums, the execution sequence differs from regular classes, which can lead to confusion.

enum CoffeeSize {

    BIG(8), LARGE(10), HUGE(12), OVERWHELMING();
    private int ounces;

    static {
        System.out.println("static block ");
    }
    {
        System.out.println("instance block");
    }

    private CoffeeSize(int ounces) {
        this.ounces = ounces;
        System.out.println(ounces);
    }
    private CoffeeSize() {
        this.ounces = 20;
        System.out.println(ounces);
    }

    public int getOunces() {
        return ounces;
    }
} 

// Output: 
// instance block
// 8
// instance block
// 10
// instance block
// 12
// instance block
// 20
// static block

Causes

• In standard Java classes, the static block runs once when the class is loaded, followed by instance blocks that run every time a new instance is created.
• For enums, the static block is invoked only after all the constants (instances) are created and initialized, which is why it appears to run last.

Solutions

• To clarify the behavior, remember that in an enum, each value is treated as a singleton instance, which means the instance blocks run during the initialization of each instance.
• By analyzing the order of execution, you can predict the flow of control and the resulting output.

Mistake: Confusing the order of execution of instance and static blocks in enums compared to regular classes.

Solution: Remember: in enums, instance blocks execute for every defined constant before the static block.

Mistake: Assuming that static blocks run immediately during the class loading process like in conventional classes.

Solution: Understand that static blocks in enums are executed after all constants have been initialized.