Cross Ref > OOP Details

General Info: Abstract Class / Abstract Member

An abstract class member is a member that is specified in a class but not implemented. Classes that inherit from the class will have to implement the abstract member. Abstract members are a technique for ensuring a common interface with descendant classes. An abstract class is a class you cannot instantiate. A pure abstract class is a class with only abstract members.

Languages Focus

Abstraction is supported at various levels with each language. A language could enforce abstraction at the class level (either enforcing a no-instantiation rule or a only abstract members rule), and with class members (member methods and/or properties).

Java:   abstract Java supports marking a full class as abstract as well as class members. A subclass must either implement the abstract members or you must declare the subclass abstract (which delays the implementation to it's subclass). Syntax Example: public abstract class Dog {   abstract void Bark(); }  Java Abstraction (abstract)

C#:   abstract, override C# supports abstract class members and abstract classes using the abstract modifier. An abstract class is a class with one or more abstract members and you cannot instantiate an abstract class. However, you can have additional implemented methods and properties. An abstract member is either a method (implicitly virtual), property, indexer, or event in an abstract class. You can add abstract members ONLY to abstract classes using the abstract keyword. Then you override it in a descendant class with Override. Syntax Example: abstract public class Cyborg : System.Object { abstract public void Speak(string pMessage); } public class Series600 : Cyborg { public override void Speak(string pMessage) { MessageBox.Show(pMessage); } }  C# Abstraction (abstract, override)

A. In Dephi, class helpers allow you to extend a class without using inheritance. With a class helper, you do not have to create and use a new class descending from a class but instead you enhance the class directly and continue using it as you always have (even just with the DCU).

B. In general terms, developers sometimes use the term to refer to any class that helps out another class.

Java:  "Class Helpers" Not Supported However, developers sometimes use the term "class helper" to refer to code that helps out a class. Not truly the meaning we are using here, but you should be aware of the term's general usage.	C#:  "Class Helpers" Not Supported However, developers sometimes use the term "class helper" to refer to code that helps out a class. Not truly the meaning we are using here, but you should be aware of the term's general usage.

A.k.a. Class Contract and Design by Contracts.

A contract with a method that must be true upon calling (pre) or exiting (post). A pre-condition contract must be true when the method is called. A post-condition contract must be true when exiting. If either are not true, an error is raised. For example, you can use code contracts to check for the validity of input parameters, and results

An invariant is also a code contract which validates the state of the object required by the method.

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C#:  "Code Contracts" Not Supported Although not currently supported, there are plans for the next version of VS.Net and .Net using Requires and Ensures keywords. Look for code contracts in VS.Net 2010 and .Net 4. Read About It: Introduction to Code Contracts If you code with VS.Net 2008 Pro or above and wish to implement contracts now, checkout the following download: Managed Contract Tools   Syntax Example: //Future syntax may look something like: string SetAge(int pAge) { Contract.Requires(pAge>0); } string GetAge() { Contract.Ensures(Contract.Result() != null); }

General Info: Class Constructor

Constructors are called when you instantiate an object from a class. This is where you can initialize variables and put code you wish executed each time the class is created. When you initially set the member fields and properties of an object, you are initializing the state of the object. The state of an object is the values of all it's member fields and properties at a given time.

Languages Focus

What is the syntax? Can you overload constructors? Is a special method name reserved for constructors?

Java:  "Constructors" Use class name A method with the same name as the class. Syntax Example:   public class Cyborg{ //Constructors have the same name as the class. public Cyborg() { } }

C#:  "Constructors" Use class name In C#, a constructor is called whenever a class or struct is created. A constructor is a method with the same name as the class with no return value and you can overload the constructor. If you do not create a constructor, C# will create an implicit constructor that initializes all member fields to their default values. Constructors can execute at two different times. Static constructors are executed by the CLR before any objects are instantiated. Regular constructors are executed when you create an object. Syntax Example: public class Cyborg { public string CyborgName; //Constructor. public Cyborg(string pName) { CyborgName = pName; } }  C# Constructors (Use class name)

General Info: Class Destructor

A special class method called when an object instance of a class is destroyed. With some languages they are called when the object instance goes out of scope, with some languages you specifically have to call the destructor in code to destroy the object, and others use a garbage collector to dispose of object instances at specific times.

Desctructors are commonly used to free the object instance but with languages that have a garbage collector object instances are disposed of when appropriate. Either way, destructors or their equivalent are commonly used to free up resources allocated in the class constructor.

Languages Focus

Are object instances freed with a garbage collector? Or, do you have to destroy object instances.

Java:  "finalize" finalize() Java has a garbage collection algorythm that runs as a background task so it has no destructors. You can use the finalize() method to close additonal resources such as file handles. Syntax Example: protected void finalize() throws Throwable { try { close(); // close open files } finally { super.finalize(); } }

C#:  "Finalizer" ~ClassName In C# you cannot explicitly destroy a managed object. Instead, the .Net Framework's garbage collector (GC) takes care of destroying all objects. The GC destroys the objects only when necessary. Some situations of necessity are when memory is exhausted or you explicitly call the System.GC.Collect() method. In general, you never need to call System.GC.Collect(). In .Net, a finalizer is used to free non-managed objects such as a file or network resource. In C#, a finalizer is a method with the same name as the class but preceded with a tilde (as in ~ClassName). The finalizer method implicity creates an Object.Finalize method (you cannot directly call nor override the Object.Finalize method). Because you don't know when the garbage collector will call your finalizer, Microsoft recommends you implement the IDisposable interface for non-managed resources and call it's Dispose() method at the appropriate time. Syntax Example: class Cyborg { public: //Destructor for class Cyborg. ~Cyborg(); { //Free non-managed resources here. } };  C# Finalizer (~ClassName)

An element of coding where you define a common set of properties and methods for use with the design of two or more classes.

Both interfaces and abstract classes are types of abstraction. With interfaces, like abstract classes, you cannot provide any implementation. However, unlike abstract classes, interfaces are not based on inheritance. You can apply an Interface to any class in your class tree. In a real sense, interfaces are a technique for designing horizontally in a class hierarchy (as opposed to inheritance where you design vertically). Using interfaces in your class design allows your system to evolve without breaking existing code.

Java:  "Interfaces" Yes  Java Interfaces (Yes)

C#:  "Interfaces" interface Classes and structs can inherit from interfaces in a manner similar to how classes can inherit a base class or struct, but a class or struct can inherit more than one interface and it inherits only the method names and signatures, because the interface itself contains no implementations. class MyClass: IMyInterface {   public object Clone() { return null; } // IMyInterface implemented here... } Syntax Example: interface IMyInterface {   bool IsValid(); }  C# Interfaces (interface)

General Info: Method Overriding

Where you define or implement a virtual method in a parent class and then replace it in a descendant class.

When you decide to declare a method as virtual, you are giving permission to derived classes to extend and override the method with their own implementation. You can have the extended method call the parent method's code too.

In most OO languages you can also choose to hide a parent method. When you introduce a new implementation of the same named method with the same signature without overriding, you are hiding the parent method.

[Not specified yet. Coming...]

C#:   virtual, override In C#, you specify a virtual method with the virtual keyword in a parent class and extend (or replace) it in a descendant class using the override keyword. Use the base keyword in the descendant method to execute the code in the parent method, i.e. base.SomeMethod(). Syntax Example: class Robot { public virtual void Speak() { } } class Cyborg:Robot { public override void Speak() { } }  C# Overriding (virtual, override)

A partial class, or partial type, is a class that can be split into two or more source code files and/or two or more locations within the same source file. Each partial class is known as a class part or just a part. Logically, partial classes do not make any difference to the compiler. The compiler puts the class together at compile time and treats the final class or type as a single entity exactly the same as if all the source code was in a single location.

Languages Focus

For languages that have implemented partial classes, you need to know usage details and restrictions. Can you split a class into two or more files? Can you split a class within a source code file into two or more locations? What are the details of inheritance? Does it apply to interfaces as well?

Java:  "Partial Classes" Not Supported	C#:  "Partial Classes" partial C# uses the keyword partial to specify a partial class. All parts must be in the same namespace. Syntax Example: class partial Cyborg: System.Object { }  C# Partial Classes (partial)

A coding technique where the same named function, operator, or object behaves differently depending on outside input or influences. Usually implemented as parameter overloading where the same named function is overloaded with other versions that are called either with a different type or number of parameters. Polymorphism is a general coding technique and other specific implementations are common such as inheritance, operator overloading, and interfaces.

Languages Focus

Many languages support built-in polymorphism such as a "+" operator that can add both integers and decimals. The following documents the ability to implement developer defined polymorphism.

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C#:  C# supports the following types of polymorphism: Parametric Polymorphism (Method Overloading) Inheritance-Based Polymorphism Interface-Based Polymorphism (Interfaces) Ad-Hoc Polymorphism (Operator Overloading) More Info / Comment

Languages Focus

How do you prevent another class from inheriting and/or prevent a class from overriding a member.

Java:  "final class" Final In Java, there is the concept of a final class. More Info / Comment

C#:   sealed With C#, use the sealed keyword to prevent a class from being inherited from and to prevent a method from being overridden. A method marked sealed must override an ancestor method. If you mark a class sealed, all members are implicitly not overridable so the sealed keyword on members is not legal. Syntax Example: public class Machine : System.Object { public virtual void Speak(String pSentence) { MessageBox.Show(pSentence); } } public class Robot : Machine { public sealed override void Speak(String pSentence) { MessageBox.Show(pSentence); } } public sealed class Cyborg : Robot { }

General Info: Static Class / Static Member

A static member is a member you can have access to without instantiating the class into an object. For example, you can read and write static properties and call static methods without ever creating the class. Static members are also called class members (class methods, class properties, etc.) since they belong to the class and not to a specific object. A static class is a class that contains only static members. In the UML, these classes are described as utility classes.

Languages Focus

Languages that support static members usually at least support static member fields (the data). Some languages also support static methods, properties, etc. in which case the class member is held in memory at one location and shared with all objects. Finally, some languages support static classes which usually means the compiler will make sure a static class contains only static members.

Java:  "Static Members" static When calling a static method from within the same class, you don't need to specify the class name. Syntax Example: class MyUtils { //Static method. public static void MyStaticMethod() { } }  Java Static Members (static)

C#:  "Static Members" static C# supports both static members and static classes using the static keyword. You can add a static method, field, property, or event to an existing class. Also, you can designate a class as static and the compiler will ensure all members in that class are static. You can add a constructor to a static class to initialize values. The CLR automatically loads static classes with the program or namespace. Syntax Example: //Static Class Example public static class MyStaticClass { //Static Method Example public static void MyStaticMethod() { // static method code } }  C# Static Members (static)