Static initializer

Sometimes a Java class has code that needs to be run before anything happens. This is even before a constructor or field access.

A warning

Static initializers are confusing. It may be best to avoid them when possible. The order they are called is left undefined, but they are always called by the time they must be.

An example

In a static initializer, we can initialize static fields. We can even assign static final values. These initializers will be run before the class is used.

class Dog {
    static String color;

    static {
        // This is a static initializer.
        // ... It assigns a static String field.
        System.out.println("Static initializer");
        color = "red";
    }

    public void bark() {
        System.out.println("Woof");
        System.out.println("Color is " + color);
    }

    static {
        // Another static initializer.
        // ... It also assigns the String field.
        System.out.println("Static initializer 2");
        color = "brown";
    }
}

public class Program {
    public static void main(String[] args) {
        // Create a new instance and call its method.
        Dog dog = new Dog();
        dog.bark();
    }
}
Static initializer
Static initializer 2
Woof
Color is brown

Final static

A final variable is a constant. We cannot reassign it whenever we want to in a program. But in a static initializer, we can assign final values.

public class Program {
    final static int size;

    static {
        // Assign a final static in a static constructor.
        size = 100;
    }

    public static void main(String[] args) {
        System.out.println(size);
    }
}
100

A summary

With the term static, we indicate something exists only once in memory at a time. A static initializer is used to assign finals or statics when a class is first used.