Today, I will briefly mention about the class initialization. When we execute any java program (e.g.: java InitTest), Java Virtual Machine loads and link the class, execute initializers, and invokes the main class. Initialization process has some rules. Before we go into the details, spend a few minutes to figure out the output.

class SuperClass {
    static { System.out.print("0 "); }
}

public class InitTest extends SuperClass{
   public InitTest(){
      s1 = sM1("1");
   }
   static String s1 = sM1("a");
   String s3 = sM1("2");{
      s1 = sM1("3");
   }
   static{
      s1 = sM1("b");
   }
   static String s2 = sM1("c");
   String s4 = sM1("4");
   public static void main(String args[]){
       InitTest it = new InitTest(); 
   }
   private static String sM1(String s){
      System.out.println(s);  return s;
   }
}

The order of initialization process are as follows:

1. Superclasses are initialized before subclasses
2. Static variable declarations and initializers are initialized in textual order
3. Instance variable declarations and initializers are initialized in textual order
4. Constructors

After InitTest is loaded, it must be initialized before main can be invoked. Whether you created an instance of the class or not, first rule 1 and rule 2 runs. If the class is referred to without a new call, only rules 1 and 2 apply. Rule 3 and rule 4 relate to instances and constructors. They have to wait until there is code to instantiate the object.

Let’s execute the code step by step:

Rule 1: Superclasses are initialized before subclasses.  SuperClass is initialized first.

static { System.out.print("0 "); }
output: 0

Rule 2: Static variable declarations and initializers are initialized in textual order. Notice that the order of appearance in the class is important.

 static String s1 = sM1("a");
 static{
   s1 = sM1("b");
 }
 static String s2 = sM1("c");
 Output: 0 a b c

Rule 3: Instance variable declarations and initializers are initialized in textual order. Again, the order is important.

 String s3 = sM1("2");
 { s1 = sM1("3"); }
 String s4 = sM1("4");
 Output: 0 a b c 2 3 4

Rule 4: Constructors. After main method is invoked, a new class instance is explicitly created with new InitTest().

 public InitTest(){
   s1 = sM1("1");
 }
 Output:  0 a b c 2 3 4 1

Thanks for reading!

For more information, see Java Language Specification.