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In software engineering, the Twin pattern is a software design pattern that allows developers to model multiple inheritance in programming languages that do not support multiple inheritance. This pattern avoids many of the problems with multiple inheritance. [1]
Instead of having a single class which is derived from two super-classes, have two separate sub-classes each derived from one of the two super-classes. These two sub-classes are closely coupled, so, both can be viewed as a Twin object having two ends. [1]
The twin pattern can be used:
There will be two or more parent classes which are used to be inherited. There will be sub-classes each of which is derived from one of the super-classes. The sub-classes are mutually linked via fields, and each sub-class may override the methods inherited from the super-class. New methods and fields are usually declared in one sub-class. [1]
The following diagram shows the typical structure of multiple inheritance:
The following diagram shows the Twin pattern structure after replacing the previous multiple inheritance structure:
Each child class is responsible for the protocol inherited from its parent. It handles the messages from this protocol and forwards other messages to its partner class. [1]
Clients of the twin pattern reference one of the twin objects directly and the other via its twin field. [1]
Clients that rely on the protocols of parent classes communicate with objects of the respective child class. [1]
The following code is a sketched implementation of a computer game board with moving balls.
Class for the game board:
publicclassGameboardextendsCanvas{publicintwidth,height;publicGameItemfirstItem;…}Code sketch for GameItem class:
publicabstractclassGameItem{Gameboardboard;intposX,posY;GameItemnext;publicabstractvoiddraw();publicabstractvoidclick(MouseEvente);publicabstractbooleanintersects(GameItemother);publicabstractvoidcollideWith(GameItemother);publicvoidcheck(){GameItemx;for(x=board.firstItem;x!=null;x=x.next)if(intersects(x))collideWith(x);}publicstaticBallItemnewBall(intposX,intposY,intradius){//method of GameBoardBallItemballItem=newBallItem(posX,posY,radius);BallThreadballThread=newBallThread();ballItem.twin=ballThread;ballThread.twin=ballItem;returnballItem;}}Code sketch for the BallItem class:
publicclassBallItemextendsGameItem{BallThreadtwin;intradius;intdx,dy;booleansuspended;publicvoiddraw(){board.getGraphics().drawOval(posX-radius,posY-radius,2*radius,2*radius);}publicvoidmove(){posX+=dx;posY+=dy;}publicvoidclick(){if(suspended)twin.resume();elsetwin.suspend();suspended=!suspended;}publicbooleanintersects(GameItemother){if(otherinstanceofWall)returnposX-radius<=other.posX&&other.posX<=posX+radius||posY-radius<=other.posY&&other.posY<=posY+radius;elsereturnfalse;}publicvoidcollideWith(GameItemother){Wallwall=(Wall)other;if(wall.isVertical)dx=-dx;elsedy=-dy;}}Code sketch for BallThread class:
publicclassBallThreadextendsThread{BallItemtwin;publicvoidrun(){while(true){twin.draw();/*erase*/twin.move();twin.draw();}}}The following issues should be considered:
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