Java中的静态初始化

Question

I know there are a lot of questions similar to this one , but i would like to get some detailed explanation about this issue.

Let say that i have this code -

 public class Finalexamples {

        public int num = 3;
        public static int num2;
        public static Finalexamples a;

        public Finalexamples(){
            Finalexamples.num2 = 4;
        }

        static{
            a = new Finalexamples();
             System.out.println(num2);
             Finalexamples.num2 = 5;
            }

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

}

1. When the JVM loads a .class file, does it first initialize the static variables with a default value, and afterward when the program running with the literal value?
2. In my example, the static block outputs "4" (fixed), if one can explain why isn't it 0?

Answer 1

Why 0 ? It should output 4 rather.

When the class is loaded, the static fields and statements are read and executed in the order of apparition.

So these are executed first :

public static int num2;
public static Finalexamples a;

Then this is invoked :

 static{
          a = new Finalexamples();
          System.out.println(num2);
          Finalexamples.num2 = 5;
      }

This : a = new Finalexamples(); executes : Finalexamples.num2 = 4;

So System.out.println(num2); displays 4

Answer 2

Ok, so first Java goes to your topmost static declaration in the class (defining num2 As an int) then it defines a as a Finalexamples object. It then goes to your static block. In the static block, it sees a = new Finalexamples(); so it then goes to create a via the constructor. (If it didn't do this step until the end of static initialization, a would be inaccessible until the end, since it wasn't constructed, which is not good!) So in our constructor for the variable a we set Finalexamples.num2 equal to 4. The constructor is done and Java returns to executing the static block, and prints out Finalexamples.num2 , which is now 4.

Answer 3

Know this flow first(this is for your program only),

1. Initialization of static variable.

    public static int num2; public static Finalexamples a; 

2. Execution of static block:

    static{ a = new Finalexamples(); System.out.println(num2); Finalexamples.num2 = 5; } 

3. As you are initializing the object reference 'a' of Finalexamples in static block, the constructor of Finalexamples class will be called.

    public Finalexamples(){ Finalexamples.num2 = 4; } 

4. The Finalexamples.main() method. Which has nothing.

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

Answering your questions.

A. Static variables are initialized only once , at the start of the execution .These variables will be initialized first, before the initialization of any instance variables. It is a variable which belongs to the class and not to object(instance). A single copy to be shared by all instances of the class

B. It's not '0' becuase you are initializing the object reference in static block with new object of Finalexamples. At this time the constructor will be called and it will set the vaule of num2 to 4 .

I hope this help!

Answer 4

hi your final output is 5 in java you two section first is Class Area and second is Heap area class area block contains three block method area,static area,constant pool.static block is run when class loading so your static variable is created in static block it is initialize at the run time of static block and for static variable multiple copy will not created in java so you can test your code like this

public class HelloWorld{

    public int num = 3;
    public static int num2;
    public static HelloWorld a;

    public HelloWorld(){
         System.out.println("Constructor Creation value of num2 "+num2);
         HelloWorld.num2 = 4;
         System.out.println("Variable Initialization in Constructor value of num2 "+num2);
    }

    static{

         System.out.println("Static bloack execution  value of num2 "+num2);
          a = new HelloWorld();
         HelloWorld.num2 = 5;
         System.out.println("Variable Initialization in Static bloack value of num2 "+num2);
        }

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