Now let's dive in...

Inheritance versus Interfaces

With inheritance, you design classes that have methods and attributes and then extend those methods and attributes in descendant classes adding functionality as the class hierarchy gets deeper and deeper. In a real sense, you design vertically top to bottom. Interfaces allow you to design horizontally througout your class tree. Interfaces define abstract methods and/or attributes in a psuedo abstract class that if used on a class must be implemented by the class.

Complete Example

More complete example...

//Declare class in Interface section.

TPerson = class(TObject)
private
  FName: String;
  FAge: Integer;
public
  constructor Create;
end;

//Implement class in Implementation section.

constructor TPerson.Create;
begin
  inherited;  //Call the parent Create method

  // Now set a default fruit name
  FName := 'unknown';
  FAge := 0;
end;

//Use class.

var
  Lisa: TPerson;
begin
  Lisa := TPerson.Create;

  //Defaults.
  ShowMessage(Lisa.FName + ' is ' + IntToStr(Lisa.FAge));

  //Use class.
  Lisa.FName := 'Lisa';
  Lisa.FAge := 34;
  ShowMessage(Lisa.FName + ' is ' + IntToStr(Lisa.FAge));
end;

Step by Step Example

The following is a step-by-step tutorial on creating your first Delphi class and does include an inheritance example. From this simple tutorial, you can then experiment with the various class inheritance features.

• Tutorial: A 10 Minute Your First Delphi Class Quick Start