I don't understand why we use abstract method (abstract class) while we can use empty method of non-abstract class and then we override it. Does it sound fine? I am seeking to clarify this issue.
I give 2 examples
public abstract class MyClass {public abstract void foo();}
public MyChildClass extends MyClass {public void foo() {//..TODO}}
public class MyClass {public void foo(){//empty}}
public class MyChildClass extends MyClass {public void foo() {//..TODO}}
Which one is better?
I'll start by saying that you should try to use interfaces instead of abstract classes. Abstract classes couple the subclass to the implementation of the superclass. In a language like Java, the subclass can override any method even if the superclass did not intend to do so, and most people don't qualify their methods with "do not override" all the time.
At the lowest level, abstract methods give you two concrete protections at compile time:
They force you to override the method in a subclass
They disallow the creation of the abstract class
Before listing the use cases for abstract methods, I'll just say that "common functionality" is NOT a good reason for an abstract base class. If you need common functionality, just create a class that has the common methods, and let the various classes call these functions as they see fit.
So when would you use an abstract class? Here are some examples:
Template Method
In the template method pattern, you have all of your functionality, but there's just one internal aspect that's polymorphic, so you have subclasses that override that particular aspect.
For example, if you're implementing a cache, but the cache invalidation policy is polymorphic, you may have an abstract invalidate() method that is called internally by other methods, but it's up to subclasses to implement invalidate().
If there is a preferred default cache invalidation policy, then invalidate() could implement that default. But if that default is downright destructive in some cases, then it shouldn't be a default - it should be abstract, and the code that creates the cache should be forced to explicitly choose the invalidation policy.
This can also be achieved by passing an Invalidator class to the constructor (Strategy pattern), but if the invalidation logic needs to call methods of the cache, it's better to make those method protected and call them from a subclass (i.e. Template Method pattern).
Default implementation of other methods
In languages where interfaces cannot have default methods (e.g. Java 7), you can emulate it using abstract classes. All the interface methods will be abstract, but the default methods would be regular public methods.
Common Interface and Functionality
This is just a more generic version of the template method pattern. The difference is that the polymorphic methods are part of the API.
If your common functionality has a lot of overlap with the functionality you want to expose, and you don't want mountains of boilerplate code, you use an abstract class. For example:
interface File {
abstract Buffer read(int size);
abstract void write(Buffer buf);
abstract long getSize();
abstract void setSize();
// ... get/set creation time, get/set modification time, get
// file type etc.
abstract long getOwner();
abstract void setOwner(long owner);
}
abstract class AbstractFile extends File {
DataMap dataMap;
MetadataMap metaMap;
protected getDiskMap() { return dataMap; }
protected getMetaMap() { return metaMap; }
public Buffer read(int size) { /* loop here */ }
public void write(Buffer buf) { /* loop here */ }
public long getSize() { /* logic */ }
public void setSize() { /* logic */ }
// ... implementation of get/set creation time, get/set modification
// time, get file type etc.
}
abstract class HardDriveFile extends AbstractFile {
OwnershipMap ownerMap;
abstract long getOwner() { /* logic */ }
abstract void setOwner(long owner) { /* logic */ }
}
abstract class ThumbDriveFile extends AbstractFile {
// thumb drives have no ownership
abstract long getOwner() { return 0; }
abstract void setOwner(long owner) { /* no-op */ }
}
abstract class SomeOtherfile extends AbstractFile {
...
}
If we cut the middleman and have HardDriveFile and ThumbDriveFile (and possibly other types of files) implement File and spell out all the common methods, each calling a method of some common class, we would get mountains and mountains of boilerplate. So we inherit from an abstract base class, that has the abstract methods we want to specialize (e.g. based on the existence of an ownership map).
The naive thing to do would be to combine File and AbstractFile into a single class, which is where you'd get the abstract methods getOwner() and setOwner(), but it's better to hide abstract classes behind actual interfaces, to prevent the coupling between consumers of an API and the abstract class.
As far as I've understood in Dart is possible to use abstract classes to declare "interfaces" or "protocols" (if you come from objective-c).
Anyway I'm having trouble in finding a way to declare an optional method in the abstract class/interface.
If I declare a method in the abstract class A, and let the concrete class B implement A, I get a warning in the compiler.
I'd like to be able to declare a method as optional or at least to provide a default implementation without needing to "re-declare" it in a class that implements my interface.
abstract class A{
void abstractMethod();
}
class B implements A{
//not implementing abstract method here gives a warning
}
That's not how interfaces work. If your class states to implement an interface, then this is what it has to do.
You can split the interface
abstract class A {
void abstractMethod();
}
abstract class A1 extends A {
void optionalMethod();
}
class B implements A {
//not implementing abstract method here gives a warning
}
only when it states to implement A1 it has to implement optionalMethod.
Alternatively you can extend the abstract class
abstract class A{
void abstractMethod();
void optionalMethod(){};
}
class B extends A {
//not implementing abstract method here gives a warning
}
then only abstractMethod needs to be overridden because A doesn't provide an implementation.
Abstract methods defined in classes cannot be marked as optional. (At least not in the regular Dart language, I don't know of annotations that might support something like this.)
Any class that implements an interface must provide an implementation of all abstract methods, but, those method implementations may trivially throw an error to indicate that the method is not available.
Throw UnimplementedError if the implementing class is incomplete and the proper implementation is to be added later
Throw UnsupportedError if the implementing class does not intend to implement the method.
Note that UnimplementedError implements UnsupportedError.
Obviously you have to be judicious about what you choose to not implement. If it's in code that is not intended to be shared you can get away only implementing methods that you explicitly know are required. If it's in a library package intended to be shared with others you would need a good reason to not implement a method, and that reason should be well documented.
Example code:
abstract class A {
void abstractMethod();
}
class B implements A {
void abstractMethod() { throw new UnimplementedError(...); }
// or
void abstractMethod() { throw new UnsupportedError(...); }
}
See:
https://api.dartlang.org/stable/1.18.1/dart-core/UnimplementedError-class.html
https://api.dartlang.org/stable/1.18.1/dart-core/UnsupportedError-class.html
i have Abstract class
Public class Abstract baseClass
{
public abstract string GetString();
public abstract string GetString1();
}
public class DerivedClass : baseClass
{
public override string GetString()
{
return "test data";
}
public override string GetString1()
{
throw new NotImplementedException();
}
}
In above line of code, i have to implement both abstract method in derived class. But due to some reason i don't want to implement all methods, just one of them like GetString() only. How can it be done?
Thanks
If DerivedClass is going to offer common functionality to other classes, you can mark it as abstract, implement one of the methods here, and then inheritors will only have to implement the remaining method.
If you aren't going to support the other method in a given implementation, you still have to expose the method in your class, but similar to what you have here, you would typically throw a NotSupportedException. For void methods, you could simply return (do nothing).
Finally, if you want to separate out the things that have both methods and those that have only one, you can use interfaces.
public interface IBase
{
string GetString();
}
public interface IBasePlus : IBase
{
string GetStringPlus();
}
You can have one class that implements IBasePlus, but you can supply this to methods that take a parameter of type IBase, in which case you won't see the extra method.
Generally, if you don't implement all the abstract methods then your new class is also an abstract class. To get a concrete class, you need all the methods to be implemented. If you only want/need to implement a subset of the methods, consider using multiple interfaces (one interface with GetString and another with GetString1) rather than an abstract class. Then you can just implement the interfaces with the methods you want to use in the class.
Take the abstract keyword off the other method and provide a default implementation in the base class
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;
}