CODEWITHSUNDEEP
javastatic-blockvariable-initialization
Stefan Jankovic
Stefan Jankovic

Reputation: 63

Declaring static initializer block before declaring variable explanation

How does this code work fine and prints 9?

public class Dog{

    static {
        age=9;
    }

    static int age=7;

}

And this code doesn't compile(illegal forward reference)? Notice I changed age in static block.

public class Dog{

    static {
        age++;
    }

    static int age=7;

}

Another question is how do both of them even work? From my prior Java knowledge I knew a rule that:

you can't access variable before declaring them

. So how does static block know what variable age actually is?

Upvotes: 0

Views: 67

Answers (1)

lucid
lucid

Reputation: 2900

public class Dog{

   static {
      age=9;
   }

   static int age=7;
}

Static blocks and static variable initialisations are executed in the order in which they appear in source file. (java documentation point 9

Next, execute either the class variable initializers and static initializers of the class, or the field initializers of the interface, in textual order, as though they were a single block.

In Above case, you are doing an assignment before declaring a variable which is permitted by java in certain cases. Forward References During Field Initialization

The use is on the left-hand side of an assignment;

public class Dog {
   static {
      age++;
   }
   static int age=7;
}

In this case, you are reading it before declaring it which is not permitted. That's why you are getting an illegal forward reference exception.

j = 200; // ok - assignment
j = j + 1; // error - right hand side reads before declaration

Upvotes: 2

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