Java Tutorial/Class Definition/final
Содержание
- 1 Base class for demonstation of final methods
- 2 "Blank" final fields
- 3 Demonstrates how final variables are replaced at compilation time
- 4 Demonstration of final class members
- 5 Demonstration of final constants
- 6 Demonstration of final variables
- 7 final Variables
- 8 Java Final variable: Once created and initialized, its value can not be changed
- 9 Making an entire class final
- 10 The effect of final on fields
- 11 Using "final" with method arguments
- 12 You can override a private or private final method
Base class for demonstation of final methods
/*
* 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.
*
########## 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 ########## */
"Blank" final fields
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);
}
}
Demonstrates how final variables are replaced at compilation time
/*
* 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.
*
########## 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.
*
########## 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 ########## */
Demonstration of final class members
/*
* 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.
*
########## 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 ########## */
Demonstration of final constants
/*
* 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.
*
########## 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 ########## */
Demonstration of final variables
/*
* 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.
*
########## 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 ########## */
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.
final int numberOfMonths = 12;
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
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
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++;
}
}
The effect of final on fields
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);
}
}
fd1: i4 = 15, i5 = 5 Creating new FinalData fd1: i4 = 15, i5 = 5 fd2: i4 = 15, i5 = 5
Using "final" with method arguments
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);
}
}
You can override a private or private final method
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();
}
}
OverridingPrivate2.f() OverridingPrivate2.g()
