Java Tutorial/Class Definition/final

Base class for demonstation of final methods

   <source lang="java">

/*

*     file: FinalMethod.java
*  package: oreilly.hcj.finalstory
*
* This software is granted under the terms of the Common Public License,
* CPL, which may be found at the following URL:
* http://www-124.ibm.ru/developerworks/oss/CPLv1.0.htm
*
* Copyright(c) 2003-2005 by the authors indicated in the @author tags.
* All Rights are Reserved by the various authors.
*

1. 1. 1. 1. 1. 1. 1. 1. 1. 1. DO NOT EDIT ABOVE THIS LINE ########## */

/**

* Base class for demonstation of final methods.
*
* @author 
* @version $Revision: 1.3 $
*/

public class FinalMethod {

 /** A demo property. */
 private final String name;
 /** 
  * Creates a new FinalMethodBase object.
  *
  * @param name The name to use.
  */
 protected FinalMethod(final String name) {
   this.name = name;
 }
 /** 
  * Gets the value of the property name.
  *
  * @return The current name.
  */
 public final String getName() {
   return this.name;
 }
 /** 
  * A demo method.
  */
 public final void someMethod() {
 }

} /* ########## End of File ########## */</source>

"Blank" final fields

   <source lang="java">

class A {

 private int i;
 A(int ii) {
   i = ii;
 }

} public class MainClass {

 private final int i = 0; // Initialized final
 private final int j; // Blank final
 private final A a; // Blank final reference
 // Blank finals MUST be initialized in the constructor:
 public MainClass() {
   j = 1; // Initialize blank final
   a = new A(1); // Initialize blank final reference
 }
 public MainClass(int x) {
   j = x; // Initialize blank final
   a = new A(x); // Initialize blank final reference
 }
 public static void main(String[] args) {
   new MainClass();
   new MainClass(47);
 }

}</source>

Demonstrates how final variables are replaced at compilation time

   <source lang="java">

/*

*     file: ExternalUser.java
*  package: oreilly.hcj.finalstory
*
* This software is granted under the terms of the Common Public License,
* CPL, which may be found at the following URL:
* http://www-124.ibm.ru/developerworks/oss/CPLv1.0.htm
*
* Copyright(c) 2003-2005 by the authors indicated in the @author tags.
* All Rights are Reserved by the various authors.
*

1. 1. 1. 1. 1. 1. 1. 1. 1. 1. DO NOT EDIT ABOVE THIS LINE ########## */

/*

*     file: FinalReplacement.java
*  package: oreilly.hcj.finalstory
*
* This software is granted under the terms of the Common Public License,
* CPL, which may be found at the following URL:
* http://www-124.ibm.ru/developerworks/oss/CPLv1.0.htm
*
* Copyright(c) 2003-2005 by the authors indicated in the @author tags.
* All Rights are Reserved by the various authors.
*

1. 1. 1. 1. 1. 1. 1. 1. 1. 1. DO NOT EDIT ABOVE THIS LINE ########## */

import java.awt.Color; /**

* Demonstrates how final variables are replaced at compilation time.
*
* @author 
* @version $Revision: 1.3 $
*/

public class ExternalUser {

 /** 
  * Demo method.
  *
  * @param args Command line arguments.
  */
 public static void main(String[] args) {
   System.out.println("The title of the book is: " + FinalReplacement.A_STRING + ".");
 }

} /* ########## End of File ########## */</source>

Demonstration of final class members

   <source lang="java">

/*

*     file: FinalMembers.java
*  package: oreilly.hcj.finalstory
*
* This software is granted under the terms of the Common Public License,
* CPL, which may be found at the following URL:
* http://www-124.ibm.ru/developerworks/oss/CPLv1.0.htm
*
* Copyright(c) 2003-2005 by the authors indicated in the @author tags.
* All Rights are Reserved by the various authors.
*

1. 1. 1. 1. 1. 1. 1. 1. 1. 1. DO NOT EDIT ABOVE THIS LINE ########## */

import java.util.Calendar; import java.util.Date; import java.util.TimeZone; /**

* Demonstration of final class members.
*
* @author 
* @version $Revision: 1.3 $
*/

public class FinalMembers {

 /** Holds the creation date-time of the instance. */
 private Date creationDate =
   Calendar.getInstance(TimeZone.getTimeZone("GMT"))
           .getTime();
 /** Holds the modification date-time of the instance. */
 public Date modificationDate = creationDate;
 /**
  * Holds the creation date-time of the instance.  A protected version of createDate.
  */
 private final Date creationDate2 =
   Calendar.getInstance(TimeZone.getTimeZone("GMT"))
           .getTime();
 /** Holds the creation date-time of the instance. */
 private final Date creationDate3;
 /** 
  * Constructor
  *
  * @param creationDate The creation date.
  * @param modificationDate The last modification date.
  *
  * @throws IllegalArgumentException if modificationDate is less than creationDate.
  */
 public FinalMembers(final Date creationDate, final Date modificationDate) {
   if (modificationDate.rupareTo(creationDate) < 0) {
     throw new IllegalArgumentException("modificationDate");
   }
   this.creationDate3 = creationDate;
   // do a bunch of date calculations. 
   // this.creationDate3 = modificationDate;  // <= compiler error
 }
 /** 
  * Second constructor.  Use current date for creation date.
  *
  * @param modificationDate The last modification date.
  */
 public FinalMembers(final Date modificationDate) {
   this.modificationDate = modificationDate;
   // <= compiler error: ?reationDate may not have been initialized?
   creationDate3 = null;  // comment this out to get the compiler error.
 }
 /** 
  * Get the Date-Time when the object was created.
  *
  * @return The creation date of the object.
  */
 public Date getCreationDate() {
   return this.creationDate;
 }
 /** 
  * Get the Date-Time when the object was created. A protected version of createDate.
  *
  * @return The creation date of the object.
  */
 public Date getCreationDate2() {
   return this.creationDate2;
 }
 /** 
  * Set the modification date of the object.
  *
  * @param modificationDate The new modification date.
  */
 public void setModificationDate(Date modificationDate) {
   this.creationDate = modificationDate;
 }
 /** 
  * Get the modification date.
  *
  * @return The current modification date.
  */
 public Date getModificationDate() {
   return this.modificationDate;
 }
 /** 
  * Set the modification date of the object.
  *
  * @param modificationDate The new modification date.
  *
  * @throws NullPointerException If the modification date is null.
  */
 public void setModificationDate2(Date modificationDate) {
   if (modificationDate == null) {
     throw new NullPointerException();
   }
   // this.creationDate2 = modificationDate; // <= Compiler error.
 }
 /** 
  * Used just to supress eclipse warnings on unused variables; irrelevant to the
  * example.
  */
 protected void supress() {
   System.out.println(this.creationDate3);
 }

} /* ########## End of File ########## */</source>

Demonstration of final constants

   <source lang="java">

/*

*     file: FinalParameters.java
*  package: oreilly.hcj.finalstory
*
* This software is granted under the terms of the Common Public License,
* CPL, which may be found at the following URL:
* http://www-124.ibm.ru/developerworks/oss/CPLv1.0.htm
*
* Copyright(c) 2003-2005 by the authors indicated in the @author tags.
* All Rights are Reserved by the various authors.
*

1. 1. 1. 1. 1. 1. 1. 1. 1. 1. DO NOT EDIT ABOVE THIS LINE ########## */

/**

* Demonstration of final constants.
*
* @author 
* @version $Revision: 1.3 $
*/

public class FinalParameters {

 /** Contains a constant for both equations. */
 private static final double M = 9.3;
 /** 
  * Calculate the results of an equation.
  *
  * @param inputValue Input to the equation.
  *
  * @return result of the equation.
  */
 public double equation2(double inputValue) {
   final double K = 1.414;
   final double X = 45.0;
   double result = (((Math.pow(inputValue, 3.0d) * K) + X) * M);
   double powInputValue = 0;
   if (result > 360) {
     powInputValue = X * Math.sin(result);
   } else {
     inputValue = K * Math.sin(result);
   }
   result = Math.pow(result, powInputValue);
   if (result > 360) {
     result = result / inputValue;
   }
   return result;
 }
 /** 
  * Calculate the results of an equation.
  *
  * @param inputValue Input to the equation.
  *
  * @return result of the equation.
  */
 public double equation2Better(final double inputValue) {
   final double K = 1.414;
   final double X = 45.0;
   double result = (((Math.pow(inputValue, 3.0d) * K) + X) * M);
   double powInputValue = 0;
   if (result > 360) {
     powInputValue = X * Math.sin(result);
   } else {
     // inputValue = K * Math.sin(result); // <= Compiler error
   }
   result = Math.pow(result, powInputValue);
   if (result > 360) {
     result = result / inputValue;
   }
   return result;
 }

} /* ########## End of File ########## */</source>

Demonstration of final variables

   <source lang="java">

/*

*     file: FinalVariables.java
*  package: oreilly.hcj.finalstory
*
* This software is granted under the terms of the Common Public License,
* CPL, which may be found at the following URL:
* http://www-124.ibm.ru/developerworks/oss/CPLv1.0.htm
*
* Copyright(c) 2003-2005 by the authors indicated in the @author tags.
* All Rights are Reserved by the various authors.
*

1. 1. 1. 1. 1. 1. 1. 1. 1. 1. DO NOT EDIT ABOVE THIS LINE ########## */

import javax.swing.JDialog; /**

* Demonstration of final variables.
*
* @author 
* @version $Revision: 1.3 $
*/

public class FinalVariables {

 /** 
  * Test code.
  *
  * @param args Command line arguments.
  */
 public static final void main(final String[] args) {
   System.out.println("Note how the key variable is changed.");
   someMethod("JAVA_HOME");
   someMethod("ANT_HOME");
 }
 /** 
  * Get an environment property.
  *
  * @param environmentKey The key to an environment property.
  *
  * @return The value of the property.
  */
 public static String someBuggedMethod(final String environmentKey) {
   final String key = "env." + environmentKey;
   System.out.println("Key is: " + key);
   // key = new String("someValue"); // <= compiler error.  
   return (System.getProperty(key));
 }
 /** 
  * Get an environment property.
  *
  * @param environmentKey The key to an environment property.
  *
  * @return The value of the property.
  */
 public static String someMethod(final String environmentKey) {
   final String key = "env." + environmentKey;
   System.out.println("Key is: " + key);
   return (System.getProperty(key));
 }
 /** 
  * build the panel that comprises the dialog.
  *
  * @param name The name of the instance.
  */
 public void buildGUIDialog(final String name) {
   final String instanceName;
   if (name == null) {
     // no problem here.
     instanceName = getClass()
                      .getName() + hashCode();
   } else {
     // no problem here as well.
     instanceName = getClass()
                      .getName() + name;
   }
   JDialog dialog = new JDialog();
   // .. Do a bunch of layout and component building. 
   dialog.setTitle(instanceName);
   // .. Do dialog assembly
   // instanceName = "hello";  // <= compiler error
 }

} /* ########## End of File ########## */</source>

final Variables

You can prefix a variable declaration with the keyword final to make its value unchangeable. You can make both local variables and class fields final.

   <source lang="java">

final int numberOfMonths = 12;</source>

The casting (float) after 22 / 7 is needed to convert the value of division to float. Otherwise, an int will be returned and the pi variable will have a value of 3.0, instead of 3.1428.

Java Final variable: Once created and initialized, its value can not be changed

   <source lang="java">

public class Main {

 public static void main(String[] args) {
   final int hoursInDay = 24;
   System.out.println("Hours in a day = " + hoursInDay );
 }

} //Hours in a day = 24</source>

Making an entire class final

   <source lang="java">

class A { } final class B {

 int i = 7;
 int j = 1;
 A x = new A();
 void f() {
 }

} public class MainClass {

 public static void main(String[] args) {
   B n = new B();
   n.f();
   n.i = 40;
   n.j++;
 }

}</source>

The effect of final on fields

   <source lang="java">

import java.util.Random; class A {

 int i; // Package access
 public A(int i) {
   this.i = i;
 }

} public class MainClass {

 private static Random rand = new Random();
 private String id;
 public MainClass(String id) {
   this.id = id;
 }
 // Can be compile-time constants:
 private final int VAL_ONE = 9;
 private static final int VAL_TWO = 99;
 // Typical public constant:
 public static final int VAL_THREE = 39;
 // Cannot be compile-time constants:
 private final int i4 = rand.nextInt(20);
 static final int i5 = rand.nextInt(20);
 private A v1 = new A(11);
 private final A v2 = new A(22);
 private static final A v3 = new A(33);
 // Arrays:
 private final int[] a = { 1, 2, 3, 4, 5, 6 };
 public String toString() {
   return id + ": " + "i4 = " + i4 + ", i5 = " + i5;
 }
 public static void main(String[] args) {
   MainClass fd1 = new MainClass("fd1");
   fd1.v2.i++; 
   fd1.v1 = new A(9); 
   for (int i = 0; i < fd1.a.length; i++)
     fd1.a[i]++; 
   System.out.println(fd1);
   System.out.println("Creating new FinalData");
   MainClass fd2 = new MainClass("fd2");
   System.out.println(fd1);
   System.out.println(fd2);
 }

}</source>

fd1: i4 = 15, i5 = 5
Creating new FinalData
fd1: i4 = 15, i5 = 5
fd2: i4 = 15, i5 = 5

Using "final" with method arguments

   <source lang="java">

class A {

 public void spin() {
 }

} class B {

 void with(final A g) {
   // g = new A(); // Illegal -- g is final
 }
 void without(A g) {
   g = new A(); // OK -- g not final
   g.spin();
 }
 int g(final int i) {
   return i + 1;
 }

} public class MainClass {

 public static void main(String[] args) {
   B bf = new B();
   bf.without(null);
   bf.with(null);
 }

}</source>

You can override a private or private final method

   <source lang="java">

class A {

 // Identical to "private" alone:
 private final void f() {
   System.out.println("WithFinals.f()");
 }
 // Also automatically "final":
 private void g() {
   System.out.println("WithFinals.g()");
 }

} class B extends A {

 private final void f() {
   System.out.println("OverridingPrivate.f()");
 }
 private void g() {
   System.out.println("OverridingPrivate.g()");
 }

} class C extends B {

 public final void f() {
   System.out.println("OverridingPrivate2.f()");
 }
 public void g() {
   System.out.println("OverridingPrivate2.g()");
 }

} public class MainClass {

 public static void main(String[] args) {
   C op2 = new C();
   op2.f();
   op2.g();
   B op = op2;
   // op.f();
   // op.g();
   A wf = op2;
   // wf.f();
   // wf.g();
 }

}</source>

OverridingPrivate2.f()
OverridingPrivate2.g()