Java/Class/Final

Blank final fields

 
// : c06:BlankFinal.java
// "Blank" final fields.
// From "Thinking in Java, 3rd ed." (c) Bruce Eckel 2002
// www.BruceEckel.ru. See copyright notice in CopyRight.txt.
class Poppet {
  private int i;
  Poppet(int ii) {
    i = ii;
  }
}
public class BlankFinal {
  private final int i = 0; // Initialized final
  private final int j; // Blank final
  private final Poppet p; // Blank final reference
  // Blank finals MUST be initialized in the constructor:
  public BlankFinal() {
    j = 1; // Initialize blank final
    p = new Poppet(1); // Initialize blank final reference
  }
  public BlankFinal(int x) {
    j = x; // Initialize blank final
    p = new Poppet(x); // Initialize blank final reference
  }
  public static void main(String[] args) {
    new BlankFinal();
    new BlankFinal(47);
  }
} ///:~

Experiment with final args to functions

 
/*
 * 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.
 */

import java.util.*;
/**
 * Experiment with "final" args to functions (new in 1.1)
 */
public class FinalArgs {
  public static void main(String argv[]) {
    new FinalArgs().run();
  }
  void run() {
    System.out.println("Hummm de dummm...");
    myFunc(Calendar.getInstance());
    System.out.println("Once upon a time...");
  }
  void myFunc(final Calendar d) {
    // d = null;  // this will not compile
    d.set(Calendar.YEAR, 1999); // this will compile, and changes the object
  }
}

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

 

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

Making an entire class final

 
// : c06:Jurassic.java
// Making an entire class final.
// From "Thinking in Java, 3rd ed." (c) Bruce Eckel 2002
// www.BruceEckel.ru. See copyright notice in CopyRight.txt.
class SmallBrain {
}
final class Dinosaur {
  int i = 7;
  int j = 1;
  SmallBrain x = new SmallBrain();
  void f() {
  }
}
//! class Further extends Dinosaur {}
// error: Cannot extend final class "Dinosaur"
public class Jurassic {
  public static void main(String[] args) {
    Dinosaur n = new Dinosaur();
    n.f();
    n.i = 40;
    n.j++;
  }
} ///:~

The effect of final on fields

 
// : c06:FinalData.java
// The effect of final on fields.
// From "Thinking in Java, 3rd ed." (c) Bruce Eckel 2002
// www.BruceEckel.ru. See copyright notice in CopyRight.txt.
import java.util.Random;
class Value {
  int i; // Package access
  public Value(int i) {
    this.i = i;
  }
}
public class FinalData {
  private static Random rand = new Random();
  private String id;
  public FinalData(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 Value v1 = new Value(11);
  private final Value v2 = new Value(22);
  private static final Value v3 = new Value(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) {
    FinalData fd1 = new FinalData("fd1");
    //! fd1.VAL_ONE++; // Error: can"t change value
    fd1.v2.i++; // Object isn"t constant!
    fd1.v1 = new Value(9); // OK -- not final
    for (int i = 0; i < fd1.a.length; i++)
      fd1.a[i]++; // Object isn"t constant!
    //! fd1.v2 = new Value(0); // Error: Can"t
    //! fd1.v3 = new Value(1); // change reference
    //! fd1.a = new int[3];
    System.out.println(fd1);
    System.out.println("Creating new FinalData");
    FinalData fd2 = new FinalData("fd2");
    System.out.println(fd1);
    System.out.println(fd2);
  }
} ///:~

Using final with method arguments

 
// : c06:FinalArguments.java
// Using final with method arguments.
// From "Thinking in Java, 3rd ed." (c) Bruce Eckel 2002
// www.BruceEckel.ru. See copyright notice in CopyRight.txt.
class Gizmo {
  public void spin() {
  }
}
public class FinalArguments {
  void with(final Gizmo g) {
    //! g = new Gizmo(); // Illegal -- g is final
  }
  void without(Gizmo g) {
    g = new Gizmo(); // OK -- g not final
    g.spin();
  }
  // void f(final int i) { i++; } // Can"t change
  // You can only read from a final primitive:
  int g(final int i) {
    return i + 1;
  }
  public static void main(String[] args) {
    FinalArguments bf = new FinalArguments();
    bf.without(null);
    bf.with(null);
  }
} ///:~