Java/Class/Initialization block

Initialization block Demo

   <source lang="java">

class TryInitialization {

 static int[] values = new int[10]; 
 {
   System.out.println("Running initialization block.");
   for (int i = 0; i < values.length; i++)
     values[i] = (int) (100.0 * Math.random());
 }
 void listValues() {
   System.out.println(); 
   for (int i = 0; i < values.length; i++)
     System.out.print(" " + values[i]); 
   System.out.println(); 
 }
 public static void main(String[] args) {
   TryInitialization example = new TryInitialization();
   System.out.println("\nFirst object:");
   example.listValues();
   example = new TryInitialization();
   System.out.println("\nSecond object:");
   example.listValues();
 }

}

</source>
   
  
 
  

Initialization order

   <source lang="java">

// : c04:OrderOfInitialization.java // Demonstrates initialization order. // From "Thinking in Java, 3rd ed." (c) Bruce Eckel 2002 // www.BruceEckel.ru. See copyright notice in CopyRight.txt. // When the constructor is called to create a // Tag object, you"ll see a message: class Tag {

 Tag(int marker) {
   System.out.println("Tag(" + marker + ")");
 }

} class Card {

 Tag t1 = new Tag(1); // Before constructor
 Card() {
   // Indicate we"re in the constructor:
   System.out.println("Card()");
   t3 = new Tag(33); // Reinitialize t3
 }
 Tag t2 = new Tag(2); // After constructor
 void f() {
   System.out.println("f()");
 }
 Tag t3 = new Tag(3); // At end

} public class OrderOfInitialization {

 public static void main(String[] args) {
   Card t = new Card();
   t.f(); // Shows that construction is done
 }

} ///:~

      </source>
   
  
 
  

Java Instance Initialization

   <source lang="java">

//: c04:Mugs.java // Java "Instance Initialization." // From "Thinking in Java, 3rd ed." (c) Bruce Eckel 2002 // www.BruceEckel.ru. See copyright notice in CopyRight.txt. class Mug {

 Mug(int marker) {
   System.out.println("Mug(" + marker + ")");
 }
 void f(int marker) {
   System.out.println("f(" + marker + ")");
 }

} public class Mugs {

 Mug c1;
 Mug c2;
 {
   c1 = new Mug(1);
   c2 = new Mug(2);
   System.out.println("c1 & c2 initialized");
 }
 Mugs() {
   System.out.println("Mugs()");
 }
 public static void main(String[] args) {
   System.out.println("Inside main()");
   Mugs x = new Mugs();
 }

} ///:~

      </source>
   
  
 
  

Shared array

   <source lang="java">

class TryInitialization {

 static int[] values = new int[10]; 
 {
   System.out.println("Running initialization block.");
   for (int i = 0; i < values.length; i++)
     values[i] = (int) (100.0 * Math.random());
 }
 void listValues() {
   for (int i = 0; i < values.length; i++)
     System.out.print(" " + values[i]); 
 }
 public static void main(String[] args) {
   TryInitialization example = new TryInitialization();
   example.listValues();
   TryInitialization nextexample = new TryInitialization();
   nextexample.listValues();
   example.listValues();
 }

}

</source>
   
  
 
  

static Initialization block

   <source lang="java">

class TryInitialization {

 static int[] values = new int[10]; 
 static {
   System.out.println("Running initialization block.");
   for (int i = 0; i < values.length; i++)
     values[i] = (int) (100.0 * Math.random());
 }
 void listValues() {
   for (int i = 0; i < values.length; i++)
     System.out.print(" " + values[i]); 
 }
 public static void main(String[] args) {
   TryInitialization example = new TryInitialization();
   example.listValues();
   example = new TryInitialization();
   example.listValues();
 }

}

</source>
   
  
 
  

To show that certain things really must be initialized

   <source lang="java">

/*

* Copyright (c) Ian F. Darwin, http://www.darwinsys.ru/, 1996-2002.
* All rights reserved. Software written by Ian F. Darwin and others.
* $Id: LICENSE,v 1.8 2004/02/09 03:33:38 ian Exp $
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
*    notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
*    notice, this list of conditions and the following disclaimer in the
*    documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS""
* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
* TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
* PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS
* BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
* POSSIBILITY OF SUCH DAMAGE.
* 
* Java, the Duke mascot, and all variants of Sun"s Java "steaming coffee
* cup" logo are trademarks of Sun Microsystems. Sun"s, and James Gosling"s,
* pioneering role in inventing and promulgating (and standardizing) the Java 
* language and environment is gratefully acknowledged.
* 
* The pioneering role of Dennis Ritchie and Bjarne Stroustrup, of AT&T, for
* inventing predecessor languages C and C++ is also gratefully acknowledged.
*/

/* To show that certain things really must be initialized */ public class InitializersDemo {

 public Object o;
 public int i;
 public static void main(String[] av) {
   Object mo;
   int mi;
   InitializersDemo t = new InitializersDemo();
   if (t.o == null)
     System.out.println("o is null");
   if (t.i == 0)
     System.out.println("i is zero");
   if (mo == null)    // EXPECT COMPILE ERROR
     System.out.println("mo is null");
   if (mi == 0)    // EXPECT COMPILE ERROR
     System.out.println("mi is zero");
 }

}

      </source>