In what order do static blocks and initialization

Question

I have two classes Parent and Child

public class Parent {    
    public Parent() {
        System.out.println("Parent Constructor");
    }    
    static {
        System.out.println("Parent static block");    
    }    
    {
        System.out.println("Parent initialisation  block");
    }
}

public class Child extends Parent {    
    {
        System.out.println("Child initialisation block");
    }
    static {
        System.out.println("Child static block");
    }

    public Child() {
        System.out.println("Child Constructor");
    }    
    public static void main(String[] args) {
        new Child();    
    }
}

The output of the above code will be

Parent static block
Child static block
Parent initialization  block
Parent Constructor
Child initialization block
Child Constructor

Why does Java execute the code in that order? What are the rules that determine the execution order?

Answer 1

I learn visually, so here's a visual representation of order, as a SSCCE:

public class Example {

  static {
    step(1);
  }

  public static int step_1 = step(2);
  public int step_6 = step(6);

  public Example() {
    step(8);
  }

  {
    step(7);
  }

  // Just for demonstration purposes:
  public static int step(int step) {
    System.out.println("Step " + step);
    return step;
  }
}

public class ExampleSubclass extends Example {

  {
    step(9);
  }

  public static int step_3 = step(3);
  public int step_10 = step(10);

  static {
    step(4);
  }

  public ExampleSubclass() {
    step(11);
  }

  public static void main(String[] args) {
    step(5);
    new ExampleSubclass();
    step(12);
  }
}

This prints:

Step 1
Step 2
Step 3
Step 4
Step 5
Step 6
Step 7
Step 8
Step 9
Step 10
Step 11
Step 12

Keep in mind that the order of the static parts matters; look back at the difference between the order of Example's static stuff and ExampleSubclass's.

Also note that the instance initialization block is always executed before the constructor, no matter the order. However, order does matter between an initialization block and a field initializer.

Answer 2

Static block in java is executed before main method. If we declare a Static block in java class it is executed when class loads. This is initialize with the static variables. It is mostly used in JDBC. Static block in java is executed every time when a class loads. This is also known as Static initialization block. Static block in java initializes when class load into memory , it means when JVM read the byte code. Initialization can be anything; it can be variable initialization or anything else which should be shared by all objects of that class. Static block is a normal block of code enclosed in braces { } and is preceded by static keyword.

so static block executed first.

Instance Initialization Blocks: Runs every time when the instance of the class is created.

so next Initialization block executed when instance of the class is created.

then Constructor executed

Answer 3

Just wanted to share my findings. I read in one of the answers on another thread that static blocks are executed first before static fields which is not correct. It depends on which comes first, static field or static block. Have a look at below code. It will try to put things in perspective.

1. JVM looks for a class which has public static void main(String args[]) so that it can load that class.
2. It then initialises static fields of this class(if they come before static blocks). These fields can call static methods of this class or another. If they call static method of this class then that method gets served. If they call static method of another class, then static fields or blocks of that class(depending on which comes first) gets initialised first, then this method call is served.
3. Then, it moves to static blocks.

4. It comes back to main method.

   class TestLab {
   static int method(String a) {
       System.out.println("in static method of TestLab" + " Coming from " + a);
       System.out.println("b is " + b);
       return 6;
   }
   
   static int a = method("Line 11");
   static int b = 7;
   
   TestLab() {
       System.out.println("Inside test lab constructor");
   }
   
   static {
       System.out.println("In static block of TestLab");
   }
   
   }
   
   public class Test1 {
   public static void main(String[] args) {
       System.out.println("inside main method of Test 1");
       int a = TestLab.method("Line 26");
   }
   
   // static Test ref=new Test();
   Test1() {
       System.out.println("Default Constructor of Test1");
   }
   
   {
       System.out.println("In instance block of Test1");
   }
   static int d = TestLab.method("Line 37");
   static int e = methodOfTest1();
   static {
       System.out.println("In Static Block of Test1");
   }
   
   static int methodOfTest1() {
       System.out.println("inside static method:mehtodOfTest1()");
       return 3;
   }
   }
   

Here is the output:

in static method of TestLab Coming from Line 11
b is 0
In static block of TestLab
in static method of TestLab Coming from Line 37
b is 7
inside static method:mehtodOfTest1()
In Static Block of Test1
inside main method of Test 1
in static method of TestLab Coming from Line 26
b is 7

Answer 4

1.Static init block executes at the time of class loading only ones. 2.Init block executes every time before creating object of the class.

link :- https://www.youtube.com/watch?v=6qG3JE0FbgA&t=2s

Answer 5

It would be very helpful to ckeck out the object construction process with a step by step debuger, having a view in which you can see how your object is goning through the phases. I found this very useful for clearing the perspective from a higher point of view. Eclipse can help you with this with it's debugger step into function.