When does an initialization block run?

[Richard Hayward]

Does an initialization block run before or after statements in a class defining and maybe initializing instance variables?

This code compiles & runs, printing "1". Line 3, where x is defined, must have run before the init block, otherwise I'd expect an error at line 2 when the init block attempts to access a variable x that doesn't exist. So, given that the init block must have executed after line 3, how come the program doesn't print out "2"?

class MyClass {
  {x=2;}          //instance init block
  int x =1;  
}

public class InitializationTest {
  public static void main(String[] args) {
    System.out.println(new MyClass().x);
  }
}

[Hauke Ingmar Schmidt]

They run in textual order.

class MyClass {
	int x = 1;
	{
		x = 2;
	} // instance init block
}

Now you get 2.

Not very intuitive, indeed. int x = 1; will de facto be split int int x; and x = 1 and executed at separate times; the declaration when needed (before the initializer), the assignment in textual order.

[Richard Hayward]

Thank you Hauke.

Hauke Ingmar Schmidt wrote:They run in textual order.

Not very intuitive, indeed. int x = 1; will de facto be split int int x; and x = 1 and executed at separate times; the declaration when needed (before the initializer), the assignment in textual order.

I thought it must be doing something weird like that.

So my code

class MyClass {
  {x=2;}       
  int x =1;  
}

is somehow reorganized to this

class MyClass {
  int x;
  {
    x=2;
    x=1;
  }       
}

All such assignments happening before any constructor runs.

[Gajendra Kangokar]

@Hauke Ingmar Schmidt

"They run in textual order" what does this mean?

[Richard Hayward]

Gajendra Kangokar wrote:@Hauke Ingmar Schmidt

"They run in textual order" what does this mean?

The order in which the statements are encountered in the source code, except that, in lines such as

int x =1;

variable declarations are separated from the assignment part of the statement, and executed first.

Seems weird.

[Henry Wong]

Richard Hayward wrote:
The order in which the statements are encountered in the source code, except that, in lines such as

int x =1;

variable declarations are separated from the assignment part of the statement, and executed first.

Seems weird.

Since this is in reference to runtime, I think it is better to think about when memory is allocated (and zeroed) for instance variables, and when they are initialized. The JLS regards, initializations of instance variables, and instance initializer blocks, running in "textual order"... but obviously, you can't allocate the memory for the instance variable fields at that point -- it needs to be allocated much earlier than that.

Henry


BTW, it is not everyday that the original poster, who ask the question, is the one who follows up to help out others with the same issue (a month later). Nice. You earned a cow.