Why are instance initialization blocks executed before constructors

Question

I know that static blocks are initialized at the time the class is loaded and since the class is loaded only once in a program,they are initialized only once.

IIB (Instance initialization blocks) are initialized every time an instance of the class is made, and the same for constructors: they are executed during object creation.

I don't understand why in the below program IIB is executed in prior to Constructors. Code-

public class Hello {

    public static void main(String args[]) {
        C obj = new C();
    }
}

class A {

    static {
        System.out.println("Inside static block of A");
    }

    {
        System.out.println("Inside IIB of A");
    }

    A() {
        System.out.println("Inside NO-ARGS constructor of A");
    }
}

class B extends A {

    static {
        System.out.println("Inside static block of B");
    }

    {
        System.out.println("Inside IIB of B");
    }

    B() {
        System.out.println("Inside NO-ARGS constructor of B");
    }
}

class C extends B {

    static {
        System.out.println("Inside static block of C");
    }

    {
        System.out.println("Inside IIB of C");
    }

    C() {
        System.out.println("Inside NO-ARGS constructor of C");
    }
}

Why IIB is executed first compared to constructors?

Actually constructors are called first, but your sysout is executed after the initialization block, which is inserted as the first statement (after super()) by the compiler. docs.oracle.com/javase/tutorial/java/javaOO/initial.html — Pavel Horal
– Pavel Horal, Commented Jun 28, 2015 at 14:52

Pavel Horal · Accepted Answer · 2015-06-28 15:12:53Z

Java compiler injects initializer blocks at the beginning of your constructors (after calling a superconstructor). To give you a better understanding I've compiled the following class

public class Foo extends SuperFoo {
    private String foo1 = "hello";
    private String foo2;
    private String foo3;
    {
        foo2 = "world";
    }
    public Foo() {
        foo3 = "!!!";
    }
}

and run it through a javap decompiler:

Compiled from "Foo.java"
public class Foo extends SuperFoo {
  private java.lang.String foo1;
  private java.lang.String foo2;
  private java.lang.String foo3;
  public Foo();
    Code:
       0: aload_0
       1: invokespecial #12                 // Method SuperFoo."<init>":()V
       4: aload_0
       5: ldc           #14                 // String hello
       7: putfield      #16                 // Field foo1:Ljava/lang/String;
      10: aload_0
      11: ldc           #18                 // String world
      13: putfield      #20                 // Field foo2:Ljava/lang/String;
      16: aload_0
      17: ldc           #22                 // String !!!
      19: putfield      #24                 // Field foo3:Ljava/lang/String;
      22: return
}

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