Java/Class/Static

Explicit static initialization with the static clause

 
// : 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)
} ///:~

Java static member variable example

 

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;
  }
}

Java static method

 
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

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

 
/*
 * 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);
  }
}

Show that you do inherit static fields

 
/*
 * 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());
  }
}

Static field, constructor and exception

 
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";
  }
}

Static Init Demo

 
/*
 * 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();
    }
  }
}

Using Static Variables

 
/*
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);
  } 
}