Will Java Final variables have default values?

Question

I have a program like this:

class Test {      final int x;      {         printX();     }      Test() {         System.out.println("const called");     }      void printX() {         System.out.println("Here x is " + x);     }      public static void main(String[] args) {         Test t = new Test();     }  } 

If I try to execute it, i am getting compiler error as : variable x might not have been initialized based on java default values i should get the below output right??

"Here x is 0". 

Will final variables have dafault values?

if I change my code like this,

class Test {      final int x;      {         printX();         x = 7;         printX();     }      Test() {         System.out.println("const called");     }      void printX() {         System.out.println("Here x is " + x);     }      public static void main(String[] args) {         Test t = new Test();     }  } 

I am getting output as:

Here x is 0                                                                                       Here x is 7                                                                                      const called 

Can anyone please explain this behavior..

Answer 1

http://docs.oracle.com/javase/tutorial/java/javaOO/initial.html, chapter "Initializing Instance Members":

The Java compiler copies initializer blocks into every constructor.

That is to say:

{     printX(); }  Test() {     System.out.println("const called"); } 

behaves exactly like:

Test() {     printX();     System.out.println("const called"); } 

As you can thus see, once an instance has been created, the final field has not been definitely assigned, while (from http://docs.oracle.com/javase/specs/jls/se7/html/jls-8.html#jls-8.3.1.2):

A blank final instance variable must be definitely assigned at the end of every constructor of the class in which it is declared; otherwise a compile-time error occurs.

While it does not seem to be stated explitely in the docs (at least I have not been able to find it), a final field must temporary take its default value before the end of the constructor, so that it has a predictable value if you read it before its assignment.

Default values: http://docs.oracle.com/javase/specs/jls/se7/html/jls-4.html#jls-4.12.5

On your second snippet, x is initialized on instance creation, so the compiler does not complain:

Test() {     printX();     x = 7;     printX();     System.out.println("const called"); } 

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Also note that the following approach doesn't work. Using default value of final variable is only allowed through a method.

Test() {     System.out.println("Here x is " + x); // Compile time error : variable 'x' might not be initialized     x = 7;     System.out.println("Here x is " + x);     System.out.println("const called"); }