what is thisit doesn' quite look like our earlier class diagrams that' because it isn' class diagramthis is an object diagramalso called an instance diagram it describes the system at specific state in timeand is describing specific instances of objectsnot the interaction between classes rememberboth players are members of the same classso the class diagram looks little differentplayer chess set make move the diagram shows that exactly two players can interact with one chess set it also indicates that any one player can be playing with only one chess set at time howeverwe're discussing compositionnot umlso let' think about what the chess set is composed of we don' care what the player is composed of at this time we can assume that the player has heart and brainamong other organsbut these are irrelevant to our model indeedthere is nothing stopping said player from being deep blue itselfwhich has neither heart nor brain the chess setthenis composed of board and pieces the board further comprises positions you could argue that pieces are not part of the chess set because you could replace the pieces in chess set with different set of pieces while this is unlikely or impossible in computerized version of chessit introduces us to aggregation aggregation is almost exactly like composition the difference is that aggregate objects can exist independently it would be impossible for position to be associated with different chess boardso we say the board is composed of positions but the pieceswhich might exist independently of the chess setare said to be in an aggregate relationship with that set another way to differentiate between aggregation and composition is to think about the lifespan of the object if the composite (outsideobject controls when the related (insideobjects are created and destroyedcomposition is most suitable if the related object is created independently of the composite objector can outlast that objectan aggregate relationship makes more sense alsokeep in mind that composition is aggregationaggregation is simply more general form of composition any composite relationship is also an aggregate relationshipbut not vice versa