Java/Class/Static

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Explicit static initialization with the static clause

   <source lang="java">

// : c04:ExplicitStatic.java // Explicit static initialization with the "static" clause. // From "Thinking in Java, 3rd ed." (c) Bruce Eckel 2002 // www.BruceEckel.ru. See copyright notice in CopyRight.txt. class Cup {

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

} class Cups {

 static Cup c1;
 static Cup c2;
 static {
   c1 = new Cup(1);
   c2 = new Cup(2);
 }
 Cups() {
   System.out.println("Cups()");
 }

} public class ExplicitStatic {

 public static void main(String[] args) {
   System.out.println("Inside main()");
   Cups.c1.f(99); // (1)
 }
 // static Cups x = new Cups(); // (2)
 // static Cups y = new Cups(); // (2)

} ///:~


 </source>
   
  
 
  



Java static member variable example

   <source lang="java">

public class Main {

 public static void main(String[] args) {
   Counter object1 = new Counter();
   System.out.println(object1.getNumberOfObjects());
   Counter object2 = new Counter();
   System.out.println(object2.getNumberOfObjects());
 }

} class Counter {

 static int counter = 0;
 public Counter() {
   counter++;
 }
 public int getNumberOfObjects() {
   return counter;
 }

}

 </source>
   
  
 
  



Java static method

   <source lang="java">

public class Main {

 public static void main(String[] args) {
   int result = add(1, 2);
   System.out.println("(1+2) is : " + result);
 }
 public static int add(int first, int second) {
   return first + second;
 }

} //(1+2) is : 3

 </source>
   
  
 
  



Show that you can"t have static variables in a method

   <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.
*/

/**

* Show that you can"t have static variables in a method, unlike C/C++
*/

public class NoLocalStatics {

 public static void main(String[] argv) {
   NoLocalStatics t = new NoLocalStatics();
   t.process();
 }
 void process() {
   static int a = 42;      // EXPECT COMPILE ERROR
   System.out.println("Process: " + a);
 }

}


 </source>
   
  
 
  



Show that you do inherit static fields

   <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.
*/

// Show that you do inherit static fields. /** This is the class that we extend. */ class AnOtherClass {

 AnOtherClass() {
   System.out.println("In AnOtherClass::<init>");
 }
 static int foo = 24;
 static int bar() {
   return 42;
 }

} /** This is the subclass */ public class InheritStatics extends AnOtherClass {

 public static void main(String[] c) {
   new InheritStatics().run();
 }
 public void run() {
   System.out.println("In InheritStatics::run");
   System.out.println("Foo is " + foo);
   System.out.println("Bar is " + bar());
 }

}


 </source>
   
  
 
  



Static field, constructor and exception

   <source lang="java">

public class Main {

 static Bar bar;
 static {
   try {
     bar = new Bar();
   } catch (Exception e) {
     e.printStackTrace();
   }
 }
 public static void main(String[] argv) {
   System.out.println(bar);
 }

} class Bar {

 public Bar() throws Exception {
 }
 public String toString() {
   return "Bar";
 }

}

 </source>
   
  
 
  



Static Init Demo

   <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.
*/

/**

* Show static initializers. These are run when the class is loaded.
* If you run the main program here, you will see the static
* initializer before the "About to load class", since the class
* is loaded before main gets run. If you copy the guts of main into
* another class, the messages will come out in the "expected" order.
*
* @author Ian F. Darwin, http://www.darwinsys.ru/
* @version $Id: StaticIniDemo.java,v 1.5 2004/02/09 03:33:54 ian Exp $
*/

public class StaticIniDemo {

 /** A static initializer - called when class is loaded. */
 static {
   System.out.println("In static initializer");
 }
 /** A constructor -- called when object is instantiated. */
 public StaticIniDemo() {
   System.out.println("In Constructor");
 }
 public static void main(String[] a) {
   try {
     System.err.println("About to load class");
     Class c = Class.forName("StaticIniDemo");
     System.err.println("About to construct instance");
     Object sd = c.newInstance();
     System.err.println("Object is " + sd);
   } catch (Exception e) {
     e.printStackTrace();
   }
 }

}



 </source>
   
  
 
  



Using Static Variables

   <source lang="java">

/* In this example, the Box class contains a static variable, numBoxes, which is incremented each time a Box object is created. The main() method of the TestStaticVar class creates two Box objects, then prints out the value of the static variable.

  • /

class Box {

 double width;
 public static int numBoxes = 0; // static variable is declared and initialized
       
 public Box() {
   width = 5.0;
   numBoxes++; // numBoxes is incremented to count number of objects.
 }

}

public class TestStaticVar {

 public static void main (String args[]) {
   Box box1 = new Box();
   Box box2 = new Box();
       
   System.out.println("Number of objects = " + Box.numBoxes);
 } 

}


 </source>