Why Java Runs Static Blocks Before Constructors: A Deep Dive into Initialization Order

Simple class execution order

package practice;

public class Cbc {
    public static Cbc cbc = new Cbc();
    public static void main(String[] args) {
        System.out.println("Entering program entry!");
    }
    public Cbc() {
        System.out.println("I am Cbc constructor!");
    }
    static {
        System.out.println("I am Cbc static block!");
    }
}

I am Cbc constructor! I am Cbc static block! Entering program entry!

Execution order: static variable initialization, then static block, then the main method.

Multiple inheritance scenario

package practice;

public class Cbc extends Bbc {
    public static Cbc cbc = new Cbc();
    public static void main(String[] args) {
        System.out.println("Entering program entry!");
    }
    public Cbc() {
        System.out.println("I am Cbc constructor!");
    }
    static {
        System.out.println("I am Cbc static block!");
    }
}

class Bbc extends Abc {
    public static Bbc bbc = new Bbc();
    public Bbc() {
        System.out.println("I am Bbc constructor!");
    }
    static {
        System.out.println("I am Bbc static block!");
    }
}

class Abc {
    public static Abc abc = new Abc();
    public Abc() {
        System.out.println("I am Abc constructor!");
    }
    static {
        System.out.println("I am Abc static block!");
    }
}

I am Abc constructor! I am Abc static block! I am Abc constructor! I am Bbc constructor! I am Bbc static block! I am Abc constructor! I am Bbc constructor! I am Cbc constructor! I am Cbc static block! Entering program entry!

The output demonstrates that initialization proceeds from the top of the hierarchy (Abc) down through Bbc to Cbc. For each class, the static variable is created first, then its static block runs, and finally its constructors are invoked.

Modifying static fields in an inheritance chain

If a base class defines a static field, e.g., public static int num = 1; in Abc, a subclass can change its value in a static block. Setting num = 2 in Bbc 's static block makes the value 2 visible to Cbc and any other subclasses of Bbc, while code paths that do not involve Bbc continue to see the original value 1.

Test code demonstrating static field change

public class Cbc extends Bbc {
    public static void main(String[] args) {
        System.out.println("Entering program entry!");
        System.out.println("num value: " + num);
    }
}

class Bbc extends Abc {
    static {
        num = 2;
    }
}

class Abc {
    public static int num = 1;
}

Entering program entry! num value: 2

This confirms that the static block in Bbc overrides the inherited static field, affecting only execution paths that involve Bbc and its subclasses.