Java/Class/Initialization block

Initialization block Demo

 
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();
  }
}

Initialization order

// : 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
  }
} ///:~

Java Instance Initialization

//: 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();
  }
} ///:~

Shared array

 
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();
  }
}

static Initialization block

 
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();
  }
}

To show that certain things really must be initialized

/*
 * 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");
  }
}