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Class initialization

  • Although we tend to think of initialization in terms of assigning values to variables, initialization is so much more. 
  • For example, initialization might involve opening a file and reading its contents into a memory buffer, registering a database driver, preparing a memory buffer to hold an image's contents, acquiring the resources necessary for playing a video, and so on.
  • Java supports initialization via language features collectively known as initializers.
  • Class initialization is different from Object initialization and it happens before Object initialization.


 

  • Class Initialization
    • A program contains no of classes, before the execution of java program the class loader loads the public class having the PSVM.
    • Then the byte code verifier verifies the class.
    • Then class initializes.
    • The class fields are set to default values.
    • For Example:


       

      • static boolean b;//flase
      • static byte by;//0
      • static char c;//
      • static double d;//0.0
      • static float f;//0.0
      • static int i;//0
      • static long l;//0
      • static short s;//0
      • static String st;//null


         

  • We can also explicitly assign values to the class fields.
  • Those values are assigned to the fields just after the class is loaded and before any user defined method including main executes.
  • This initialization is done using <clinit>method of the JVM. It uses machine level instructions to assign those values.
  • Although a subsequently declared class field can refer to a previously declared class field, the reverse is not true: You cannot declare a class field initializer that refers to a class field declared later in source code. In other words, Java does not permit forward references with class field initializers, as the following code fragment demonstrates:
    • static int second = 1 + first;
    • static int first = 3;
  • In the cases when you need to read the contents of the file before the main method executes then we need to use the static block.
    • static
    •    {
    •       System.out.println ("Acquiring filenames");
    •       filenames = new File (".").list ();
    •       System.out.println ("Filenames acquired");
    •    }
  • Class block initializers are useful. For example, Sun's JDBC (Java Database Connectivity) API uses class block initializers to simplify database driver registration. Consider the following code fragment:
    • Class.forName ("sun.jdbc.odbc.JdbcOdbcDriver");
  • Any variable declared in the block is having the scope only upto that block.
  • JVM allows us to declare any constant field without any explicit initializer, but u have to initialize it before its first use.
    • class ClassInitializationDemo5
    • {
    •    final static double PI;
    •    static
    •    {
    •       PI = 3.14159;
    •       int i;
    •       for (i = 0; i < 5; i++)
    •            System.out.println (i);
    •    }
    •    static int j = i;//this vl cause problem as the I is local to that block
    •    public static void main (String [] args)
    •    {
    •       System.out.println ("PI = " + PI);//this vl show 3.14 as the initialization is done in static block
    •    }
    • }
  • When a class hierarchy is involved, the compiler creates a separate<clinit> method for each class in that hierarchy. At runtime, the JVM loads all hierarchy classes and calls their <clinit> methods in a top-to-bottom order. 
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