Why should we make the constructor private in class? As we always need the constructor to be public.
Some reasons where you may need private constructor:
The constructor can only be accessed from static factory method inside the class itself. Singleton can also belong to this category.
A utility class, that only contains static methods.
By providing a private constructor you prevent class instances from being created in any place other than this very class. There are several use cases for providing such constructor.
A. Your class instances are created in a static method. The static method is then declared as public.
class MyClass()
{
private:
MyClass() { }
public:
static MyClass * CreateInstance() { return new MyClass(); }
};
B. Your class is a singleton. This means, not more than one instance of your class exists in the program.
class MyClass()
{
private:
MyClass() { }
public:
MyClass & Instance()
{
static MyClass * aGlobalInst = new MyClass();
return *aGlobalInst;
}
};
C. (Only applies to the upcoming C++0x standard) You have several constructors. Some of them are declared public, others private. For reducing code size, public constructors 'call' private constructors which in turn do all the work. Your public constructors are thus called delegating constructors:
class MyClass
{
public:
MyClass() : MyClass(2010, 1, 1) { }
private:
MyClass(int theYear, int theMonth, int theDay) { /* do real work */ }
};
D. You want to limit object copying (for example, because of using a shared resource):
class MyClass
{
SharedResource * myResource;
private:
MyClass(const MyClass & theOriginal) { }
};
E. Your class is a utility class. That means, it only contains static members. In this case, no object instance must ever be created in the program.
To leave a "back door" that allows another friend class/function to construct an object in a way forbidden to the user. An example that comes to mind would be a container constructing an iterator (C++):
Iterator Container::begin() { return Iterator(this->beginPtr_); }
// Iterator(pointer_type p) constructor is private,
// and Container is a friend of Iterator.
Everyone is stuck on the Singleton thing, wow.
Other things:
Stop people from creating your class on the stack; make private constructors and only hand back pointers via a factory method.
Preventing creating copys of the class (private copy constructor)
This can be very useful for a constructor that contains common code; private constructors can be called by other constructors, using the 'this(...);' notation. By making the common initialization code in a private (or protected) constructor, you are also making explicitly clear that it is called only during construction, which is not so if it were simply a method:
public class Point {
public Point() {
this(0,0); // call common constructor
}
private Point(int x,int y) {
m_x = x; m_y = y;
}
};
There are some instances where you might not want to use a public constructor; for example if you want a singleton class.
If you are writing an assembly used by 3rd parties there could be a number of internal classes that you only want created by your assembly and not to be instantiated by users of your assembly.
This ensures that you (the class with private constructor) control how the contructor is called.
An example : A static factory method on the class could return objects as the factory method choses to allocate them (like a singleton factory for example).
We can also have private constructor,
to enfore the object's creation by a specific class
only(For security reasons).
One way to do it is through having a friend class.
C++ example:
class ClientClass;
class SecureClass
{
private:
SecureClass(); // Constructor is private.
friend class ClientClass; // All methods in
//ClientClass have access to private
// & protected methods of SecureClass.
};
class ClientClass
{
public:
ClientClass();
SecureClass* CreateSecureClass()
{
return (new SecureClass()); // we can access
// constructor of
// SecureClass as
// ClientClass is friend
// of SecureClass.
}
};
Note: Note: Only ClientClass (since it is friend of SecureClass)
can call SecureClass's Constructor.
You shouldn't make the constructor private. Period. Make it protected, so you can extend the class if you need to.
Edit: I'm standing by that, no matter how many downvotes you throw at this.
You're cutting off the potential for future development on the code. If other users or programmers are really determined to extend the class, then they'll just change the constructor to protected in source or bytecode. You will have accomplished nothing besides to make their life a little harder. Include a warning in your constructor's comments, and leave it at that.
If it's a utility class, the simpler, more correct, and more elegant solution is to mark the whole class "static final" to prevent extension. It doesn't do any good to just mark the constructor private; a really determined user may always use reflection to obtain the constructor.
Valid uses:
One good use of a protected
constructor is to force use of static
factory methods, which allow you to
limit instantiation or pool & reuse
expensive resources (DB connections,
native resources).
Singletons (usually not good practice, but sometimes necessary)
when you do not want users to create instances of this class or create class that inherits this class, like the java.lang.math, all the function in this package is static, all the functions can be called without creating an instance of math, so the constructor is announce as static.
If it's private, then you can't call it ==> you can't instantiate the class. Useful in some cases, like a singleton.
There's a discussion and some more examples here.
I saw a question from you addressing the same issue.
Simply if you don't want to allow the others to create instances, then keep the constuctor within a limited scope. The practical application (An example) is the singleton pattern.
Constructor is private for some purpose like when you need to implement singleton or limit the number of object of a class.
For instance in singleton implementation we have to make the constructor private
#include<iostream>
using namespace std;
class singletonClass
{
static int i;
static singletonClass* instance;
public:
static singletonClass* createInstance()
{
if(i==0)
{
instance =new singletonClass;
i=1;
}
return instance;
}
void test()
{
cout<<"successfully created instance";
}
};
int singletonClass::i=0;
singletonClass* singletonClass::instance=NULL;
int main()
{
singletonClass *temp=singletonClass::createInstance();//////return instance!!!
temp->test();
}
Again if you want to limit the object creation upto 10 then use the following
#include<iostream>
using namespace std;
class singletonClass
{
static int i;
static singletonClass* instance;
public:
static singletonClass* createInstance()
{
if(i<10)
{
instance =new singletonClass;
i++;
cout<<"created";
}
return instance;
}
};
int singletonClass::i=0;
singletonClass* singletonClass::instance=NULL;
int main()
{
singletonClass *temp=singletonClass::createInstance();//return an instance
singletonClass *temp1=singletonClass::createInstance();///return another instance
}
Thanks
You can have more than one constructor. C++ provides a default constructor and a default copy constructor if you don't provide one explicitly. Suppose you have a class that can only be constructed using some parameterized constructor. Maybe it initialized variables. If a user then uses this class without that constructor, they can cause no end of problems. A good general rule: If the default implementation is not valid, make both the default and copy constructor private and don't provide an implementation:
class C
{
public:
C(int x);
private:
C();
C(const C &);
};
Use the compiler to prevent users from using the object with the default constructors that are not valid.
Quoting from Effective Java, you can have a class with private constructor to have a utility class that defines constants (as static final fields).
(EDIT: As per the comment this is something which might be applicable only with Java, I'm unaware if this construct is applicable/needed in other OO languages (say C++))
An example as below:
public class Constants {
private Contants():
public static final int ADDRESS_UNIT = 32;
...
}
EDIT_1:
Again, below explanation is applicable in Java : (and referring from the book, Effective Java)
An instantiation of utility class like the one below ,though not harmful, doesn't serve
any purpose since they are not designed to be instantiated.
For example, say there is no private Constructor for class Constants.
A code chunk like below is valid but doesn't better convey intention of
the user of Constants class
unit = (this.length)/new Constants().ADDRESS_UNIT;
in contrast with code like
unit = (this.length)/Constants.ADDRESS_UNIT;
Also I think a private constructor conveys the intention of the designer of the Constants
(say) class better.
Java provides a default parameterless public constructor if no constructor
is provided, and if your intention is to prevent instantiation then a private constructor is
needed.
One cannot mark a top level class static and even a final class can be instantiated.
Utility classes could have private constructors. Users of the classes should not be able to instantiate these classes:
public final class UtilityClass {
private UtilityClass() {}
public static utilityMethod1() {
...
}
}
You may want to prevent a class to be instantiated freely. See the singleton design pattern as an example. In order to guarantee the uniqueness, you can't let anyone create an instance of it :-)
One of the important use is in SingleTon class
class Person
{
private Person()
{
//Its private, Hense cannot be Instantiated
}
public static Person GetInstance()
{
//return new instance of Person
// In here I will be able to access private constructor
}
};
Its also suitable, If your class has only static methods. i.e nobody needs to instantiate your class
It's really one obvious reason: you want to build an object, but it's not practical to do it (in term of interface) within the constructor.
The Factory example is quite obvious, let me demonstrate the Named Constructor idiom.
Say I have a class Complex which can represent a complex number.
class Complex { public: Complex(double,double); .... };
The question is: does the constructor expects the real and imaginary parts, or does it expects the norm and angle (polar coordinates) ?
I can change the interface to make it easier:
class Complex
{
public:
static Complex Regular(double, double = 0.0f);
static Complex Polar(double, double = 0.0f);
private:
Complex(double, double);
}; // class Complex
This is called the Named Constructor idiom: the class can only be built from scratch by explicitly stating which constructor we wish to use.
It's a special case of many construction methods. The Design Patterns provide a good number of ways to build object: Builder, Factory, Abstract Factory, ... and a private constructor will ensure that the user is properly constrained.
In addition to the better-known uses…
To implement the Method Object pattern, which I’d summarize as:
“Private constructor, public static method”
“Object for implementation, function for interface”
If you want to implement a function using an object, and the object is not useful outside of doing a one-off computation (by a method call), then you have a Throwaway Object. You can encapsulate the object creation and method call in a static method, preventing this common anti-pattern:
z = new A(x,y).call();
…replacing it with a (namespaced) function call:
z = A.f(x,y);
The caller never needs to know or care that you’re using an object internally, yielding a cleaner interface, and preventing garbage from the object hanging around or incorrect use of the object.
For example, if you want to break up a computation across methods foo, bar, and zork, for example to share state without having to pass many values in and out of functions, you could implement it as follows:
class A {
public static Z f(x, y) {
A a = new A(x, y);
a.foo();
a.bar();
return a.zork();
}
private A(X x, Y y) { /* ... */ };
}
This Method Object pattern is given in Smalltalk Best Practice Patterns, Kent Beck, pages 34–37, where it is the last step of a refactoring pattern, ending:
Replace the original method with one that creates an instance of the new class, constructed with the parameters and receiver of the original method, and invokes “compute”.
This differs significantly from the other examples here: the class is instantiable (unlike a utility class), but the instances are private (unlike factory methods, including singletons etc.), and can live on the stack, since they never escape.
This pattern is very useful in bottoms-up OOP, where objects are used to simplify low-level implementation, but are not necessarily exposed externally, and contrasts with the top-down OOP that is often presented and begins with high-level interfaces.
Sometimes is useful if you want to control how and when (and how many) instances of an object are created.
Among others, used in patterns:
Singleton pattern
Builder pattern
On use of private constructors could also be to increase readability/maintainability in the face of domain-driven design.
From "Microsoft .NET - Architecing Applications for the Enterprise, 2nd Edition":
var request = new OrderRequest(1234);
Quote, "There are two problems here. First, when looking at the code, one can hardly guess what’s going
on. An instance of OrderRequest is being created, but why and using which data? What’s 1234? This
leads to the second problem: you are violating the ubiquitous language of the bounded context. The
language probably says something like this: a customer can issue an order request and is allowed to
specify a purchase ID. If that’s the case, here’s a better way to get a new OrderRequest instance:"
var request = OrderRequest.CreateForCustomer(1234);
where
private OrderRequest() { ... }
public OrderRequest CreateForCustomer (int customerId)
{
var request = new OrderRequest();
...
return request;
}
I'm not advocating this for every single class, but for the above DDD scenario I think it makes perfect sense to prevent a direct creation of a new object.
If you create a private constructor you need to create the object inside the class
enter code here#include<iostream>
//factory method
using namespace std;
class Test
{
private:
Test(){
cout<<"Object created"<<endl;
}
public:
static Test* m1(){
Test *t = new Test();
return t;
}
void m2(){
cout<<"m2-Test"<<endl;
}
};
int main(){
Test *t = Test::m1();
t->m2();
return 0;
}
Related
I'm learning OOP and trying to write a simple program that will execute some method every time when a specific varible will change.
I have two classes:
public class SomeClass {
private OtherClass object;
public OtherClass getObject() {
return this.object;
}
public void setObject(OtherClass object) {
objectChanged();
this.object = object;
}
private void objectChanged() {
System.out.println("Object has changed");
}
}
public class OtherClass {
private int value = 5;
public int getValue() {
return this.value;
}
public void setValue(int value) {
this.value = value;
}
}
The variable objectChanged should be called every time when variable "object" is changed. My first naive idea was to put the method call inside of set function. But what if you change the object after you set it? Like this:
SomeClass someObject = new SomeClass();
OtherClass otherObject = new OtherClass();
someObject.setObject(otherObject); //"Object has changed"
otherObject.setValue(10); //nothing happens yet
I need someObject to realize that object stored inside of it changed its value to 10, but how do i do it? Is it even possible in OOP?
It looks to be reasonable, but one should consider many things. This is why there is no automatic way to do it in general. It is not part of the OOP paradigm as such. If this would be some automatic behavior, it would cause huge overhead as it is not often needed to observe changes this way. But you can, of course, implement your way depending on your concrete requirements.
There are at least two approaches.
In MVVM (like WPF) there is an INotifyPropertyChanged interface (let's call it a pattern) you can use to trigger a notification yourself, mutch like you did with SomeClass. However when you are nesting objects, you need to wire up that mechanism.to cascade: you should do the same with OtherClass and also connect the actual instances to bubble up changes.
See: https://rehansaeed.com/tag/design-patterns/
An other option is the Observable pattern. Each time the object changes state, you emit an instance - the current instance. However, you should care to emit unmutable objects. At least by using an interface that makes it read-only. But you still need to wire up the object tree to react to the changes of nested objects.
If your platform supports reflection, and you create a proper toolset, you could make this wiring up quite simple. But again: this is not strictly related to the paradigm.
I read many posts about the "Interface" and "Abstract Class"
Basically, we use "Abstract Class" when we talking about the characteristic of the Object.
And we use "Interface" when we taling about what the object capable can do.
But it still confuse so I make up an example for myself to practice.
so now I thinking of a Object 'Cargo;
public abstract class cargo {
protected int id;
public abstract int getWidth(int width);
public abstract int setWidth(int width);
public abstract int setHeight(int h);
public abstract int getHeight(int h);
public abstract int setDepth(int d);
public abstract int getDepth(int d);
public abstract int volume(int w,int h,int d);
public int getId(){
return this.id;
}
public abstract int setId();
public abstract void setBrand();
public abstract void getBrand( );
.....so on , still have a lot of characteristic of a cargo
}
//in the other class
public class usaCargo extends cargo{
....
private
}
So here is few Question about my design.
1.So in the real programming project world, are we actually doing like above? for me i think it's ok design, we meet the basic characteristic of cargo.
if we setup "private id" , then we actually can't use "id" this variable in any subclass because it's private, so is that mean every variable we defined in abstract class must be either public/ protected?
can someone give some suitable example so my cargo can implement some interface?
public interface registration{
public void lastWarrantyCheck();
}
But seems not suitable here...
we dont usually define variable inside interface, do we ??
I try to gain more sense on OOP . Forgive my long questions.
You would define variables in the Abstract class so that methods defined in the abstract class have variables to use. The scope of those variables depend on how you want concrete classes to access those variables:
private should be used when you want to force a concrete class to go through a getter or setter defined in the abstract class.
protected should be used when you want to give the concrete class direct access to the variable.
public should be used when you want the variable to be accessible by any class.
A reasonable interface that a Cargo object might implement could be Shippable as in how to move the cargo from a source to a destination. Some cargo may be shipped via freight train, some might be shippable by airplane, etc. It is up to the concrete class to implement Shippable and define just how that type of cargo would be shipped.
public interface Shippable {
public void ship();
}
Lastly a variable defined in an interface must be public static and final meaning it would be a constant variable.
Hope this clears it up for you!
Abstract classes can contain implementation, so they can have private variables and methods. Interfaces on the other hand cannot.
You can find some examples on how to implement interfaces here. However, I included how you would implement your registration example below.
public class Cargo implements Registration{
public void lastWarrantyCheck(){
System.out.println("Last warranty check");
}
}
Interface variables are possible, but they should only include constant declarations (variable declarations that are declared to be both static and final). More information about this can be found here.
Variables in an abstract class may be declared as protected, and they will only be available within it and any extending classes. Private variables are never accessible inside extending classes.
Interfaces provide a list of functions that are required by the classes that implement them. For example, you might use an interface hasWarranty to define all the functions that an object would need to handle warranty-related activities.
public interface hasWarranty {
public void lastWarrantyCheck();
public void checkWarranty();
}
Then, any objects that need to perform warranty-related activities should implement that interface:
// Disclaimer: been away from Java for a long time, so please interpret as pseudo-code.
// Will compile
public class Car implements hasWarranty {
public void lastWarrantyCheck() {
... need to have this exact function or program won't compile ...
}
public void checkWarranty() {
... need to have this exact function or program won't compile ...
}
}
// Missing one of the required functions defined in hasWarranty
public class Bus implements hasWarranty {
public void lastWarrantyCheck() {
... need to have this exact function or program won't compile ...
}
}
Only constants, really, as variables declared in an interface are immutable and are shared by all objects that implement that interface. They are implicitly "static final".
I have an abstract base class and an implementation class like:
public abstract class Base
{
public Base getInstance( Class<? extends Base> clazz )
{
//expected to return a singleton instance of clazz's class
}
public abstract absMeth();
}
public A extends Base
{
//expected to be a singleton
}
In this example I can make A to be a singleton and even write getInstance in Base to return a singleton object of A for every call, doing this way:
public abstract class Base
{
public Base getInstance( Class<? extends Base> clazz )
{
try
{
return clazz.getDeclaredMethod("getInstance").invoke(null,null);
}
}
public abstract void absMeth();
}
public A extends Base
{
private static A inst;
private A(){}
public static A getInstance( )
{
if( inst!= null)
inst = new A();
return inst;
}
public void absMeth(){
//...
}
}
But my concern is how do I ensure that if somebody writes another class class B extends Base it should also be a singleton and it necessarily implements a static method called getInstance?
In other words I need to enforce this as a specification for all classes extending with the Base class.
You cannot trust classes that extend you to create a single instance of themselves1: even if you could somehow ensure that they all implement getInstance, there is no way to tell that inside that method they check inst before constructing a new instance of themselves.
Stay in control of the process: create a Map<Class,Base>, and instantiate the class passed in through reflection2. Now your code can decide whether to create an instance or not, without relying on the getInstance of a subclass.
1 A popular saying goes, "If you want a job done right, do it yourself."
2 Here is a link describing a solution based on setAccessible(true)
Singleton is a design pattern, not a language feature. It is pretty much impossible to somehow enforce it on the inheritance tree through syntax.
It certainly is possible to require all subclasses to implement a method by declaring it abstract but there is no way to control implementation details. Singleton is all about implementation details.
But why is this a concern at all? Do not make your app dependant on internal details of someone else's code. It is Bad Design™ and having this issue is a sure sign of it. Code against a well-defined interface and avoid relying on internal details.
I have looked around and could not find any similar question.
Here is the paragraph I got from Wikipedia:
Polymorphism is not the same as method overloading or method overriding. Polymorphism is only concerned with the application of specific implementations to an interface or a more generic base class. Method overloading refers to methods that have the same name but different signatures inside the same class. Method overriding is where a subclass replaces the implementation of one or more of its parent's methods. Neither method overloading nor method overriding are by themselves implementations of polymorphism.
Could anyone here explain it more clearly, especially the part "Polymorphism is not the same as method overriding"? I am confused now. Thanks in advance.
Polymorphism (very simply said) is a possibility to use a derived class where a base class is expected:
class Base {
}
class Derived extends Base {
}
Base v = new Derived(); // OK
Method overriding, on the other hand, is as Wiki says a way to change the method behavior in a derived class:
class Shape {
void draw() { /* Nothing here, could be abstract*/ }
}
class Square extends Shape {
#Override
void draw() { /* Draw the square here */ }
}
Overloading is unrelated to inheritance, it allows defining more functions with the same name that differ only in the arguments they take.
You can have polymorphism in a language that does not allow method overriding (or even inheritance). e.g. by having several different objects implement the same interface. Polymorphism just means that you can have different concrete implementations of the same abstract interface. Some languages discourage or disallow inheritance but allow this kind of polymorphism in the spirit of programming with abstractions.
You could also theoretically have method overriding without polymorphism in a language that doesn't allow virtual method dispatch. The effect would be that you could create a new class with overridden methods, but you wouldn't be able to use it in place of the parent class. I'm not aware of any mainstream language that does this.
Polymorphism is not about methods being overridden; it is about the objects determining the implementation of a particular process. An easy example - but by no means the only example - is with inheritance:
A Novel is a type of Book. It has most of the same methods, and everything you can do to a Book can also be done to a Novel. Therefore, any method that accepts a Book as an argument can also deal with a Novel as an argument. (Example would include .read(), .write(), .burn()). This is, per se, not referring to the fact that a Novel can overwrite a Book method. Instead, it is referring to a feature of abstraction. If a professor assigns a Book to be read, he/she doesn't care how you read it - just that you do. Similarly, a calling program doesn't care how an object of type Book is read, just that it is. If the object is a Novel, it will be read as a Novel. If it is not a novel but is still a book, it will be read as a Book.
Book:
private void read(){
#Read the book.
}
Novel:
private void read(){
#Read a book, and complain about how long it is, because it's a novel!
}
Overloading methods is just referring to having two methods with the same name but a different number of arguments. Example:
writeNovel(int numPages, String name)
writeNovel(String name)
Overloading is having, in the same class, many methods with the same name, but differents parameters.
Overriding is having, in an inherited class, the same method+parameters of a base class. Thus, depending on the class of the object, either the base method, or the inherited method will be called.
Polymorphism is the fact that, an instance of an inherited class can replace an instance of a base class, when given as a parameters.
E.g. :
class Shape {
public void draw() {
//code here
}
public void draw(int size) {
//this is overloading
}
}
class Square inherits Shape {
public void draw() {
//some other code : this is overriding
}
public void draw(color c) {
//this is overloading too
}
}
class Work {
public myMethod(Shape s) {
//using polymophism, you can give to this method
//a Shape, but also a Square, because Square inherits Shape.
}
}
See it ?
Polymorphing is the fact that, the same object, can be used as an instance of its own class, its base class, or even as an interface.
Polymorphism refers to the fact that an instance of a type can be treated just like any instance of any of its supertypes. Polymorphism means 'many forms'.
Say you had a type named Dog. You then have a type named Spaniel which inherits from Dog. An instance of Spaniel can be used wherever a Dog is used - it can be treated just like any other Dog instance. This is polymorphism.
Method overriding is what a subclass may do to methods in a base class. Dog may contain a Bark method. Spaniel can override that method to provide a more specific implementation. Overriding methods does not affect polymorphism - the fact that you've overriden a Dog method in Spaniel does not enable you to or prevent you from treating a Spaniel like a dog.
Method overloading is simply the act of giving different methods which take different parameters the same name.
I hope that helps.
Frankly:
Polymorphism is using many types which have specific things in common in one implementation which only needs the common things, where as method overloading is using one implementation for each type.
When you override a method, you change its implementation. Polymorphism will use your implementation, or a base implementation, depending on your language (does it support virtual methods?) and depending on the class instance you've created.
Overloading a method is something else, it means using the same method with a different amount of parameters.
The combination of this (overriding), plus the possibility to use base classes or interfaces and still call an overriden method somewhere up the chain, is called polymorphism.
Example:
interface IVehicle
{
void Drive();
}
class Car : IVehicle
{
public Drive() { /* drive a car */ }
}
class MotorBike : IVehicle
{
public Drive() { /* drive a motorbike */ }
}
class Program
{
public int Main()
{
var myCar = new Car();
var myMotorBike = new MotorBike();
this.DriveAVehicle(myCar); // drive myCar
this.DriveAVehicle(myMotorBike); // drive a motobike
this.DriveAVhehicle(); // drive a default car
}
// drive any vehicle that implements IVehicle
// this is polymorphism in action
public DriveAVehicle(IVehicle vehicle)
{
vehicle.Drive();
}
// overload, creates a default car and drives it
// another part of OO, not directly related to polymorphism
public DriveAVehicle()
{
// typically, overloads just perform shortcuts to the method
// with the real implemenation, making it easier for users of the class
this.DriveAVehicle(new Car());
}
}
class XFactory {
private XFactory() {}
static void getX() {
if(...)
return new A(new XFactory());
else
return new B(new XFactory());
}
}
class A {
private A() {}
public A(XFactory xf) {}
}
class B {
private B() {}
public A(XFactory xf) {}
}
By this way I can ensure only Factory can create instances of it's belonging Classes.
Is this right approach or there is any other alternative/good approach?
The common approach (in C++) is to make the "belonging classes" constructors private, and have them declare the factory class as friend.
I would make classes A and B friends of XFactory, and keep all their constructors private. Therefore, only XFactory has access to their constructors.
That is, in C++. In Java or C#, I don't see any clean way of enforcing that at compile-time. Your example is far from fool-proof and even a bit confusing, since as long as one has an instance of XFactory, he can pass it to the constructor of A or B and instantiate them directly like that.
If you were up for hacks and could not make your constructors private, you could:
Make your factory a global singleton and to create an object:
Create a random key
Add that key to a private list in the factory object of keys in use
Pass the key to the constructor
Have the constructor retrieve the global factory object and call it to validate the key.
If they key validation fails, scuttle your program (call exit, die, ... whatever is appropriate). Or possibly email a stack tract to an admin. This is the kind of thing that should be caught quickly.
(Do I get hack points?)
Jacob
In Java you can make the constructors private and provide the factory in the form of a public nested class, since nested classes have access to the private members of the class in which they are declared.
public class ExampleClass {
private ExampleClass() {
}
public class NestedFactory {
public ExampleClass createExample() {
return new ExampleClass();
}
}
Anyone who wanted to could create an instance of ExampleClass.NestedFactory and through it instantiate ExampleClasses.
I haven't been able to figure out a way to do this that lets you then inherit from ExampleClass since the Java compiler demands that you specify a constructor for the superclass... so that's a disadvantage.