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).

Corel Paradox:   Not Supported	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)

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.

Corel Paradox:  "Class Helpers" Not Supported	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.

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.

Corel Paradox:  "Code Contracts" Not Supported

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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?

Corel Paradox:  "Constructors" Not Supported	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() { } }

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.

Corel Paradox:   Not Supported	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(); } }

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.

Corel Paradox:  "Interfaces" Not Supported	Java:  "Interfaces" Yes  Java Interfaces (Yes)

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.

Corel Paradox:   Not Supported

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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?

Corel Paradox:  "Partial Classes" Not Supported	Java:  "Partial Classes" Not Supported

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.

Corel Paradox:   Not Supported Built-in: In ObjectPAL, polymorphism is the capability of the built-in objects to act differently depending on the context in which they are being used. For example, Table.Open and TCursor.Open. The Open method works differently depending on the object type used. Custom: No. However, you can have the same named method or procedure so long as they are in different libraries. This is important if you use libraries in a class-like OOP way and wish to implement some form of interfaces-like polymorphism (i.e. libMember.GetName and libVendor.GetName). More Info / Comment  Introduction to OO for the Paradox Community

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Languages Focus

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

Corel Paradox:   Not Supported	Java:  "final class" Final In Java, there is the concept of a final class. More Info / Comment

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.

Corel Paradox:  "Static Members" Not Supported	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)