{"id":665,"date":"2016-03-04T10:58:48","date_gmt":"2016-03-04T10:58:48","guid":{"rendered":"http:\/\/abhiandroid.com\/java\/?page_id=665"},"modified":"2016-03-04T10:58:48","modified_gmt":"2016-03-04T10:58:48","slug":"composition","status":"publish","type":"page","link":"https:\/\/abhiandroid.com\/java\/composition","title":{"rendered":"Composition In Java With Example"},"content":{"rendered":"

We have discussed the concept of inheritance which is a powerful mechanism of reusing code, minimizing data redundancy, and improving the organization of object oriented system. Inheritance is suitable only when classes are in a relationship in which child class is a<\/strong> parent class. For example: A Car<\/strong> is a Vehicle,<\/strong> so the class Car<\/strong> has all the features or properties of class Vehicle <\/strong>and in addition to its own features. However, we cannot always have is a <\/strong>relationship between objects of different classes. Let us say with example: A car is not a kind of engine. To represent such a relationship, we have an alternative to inheritance known as composition. It is applied when classes are in a relationship in which child class has<\/strong> a <\/strong>parent class.<\/p>\n

Unlike Inheritance in which a subclass extends the functionality of a superclass, in composition, a class reuses the functionality by creating a reference to the object of the class it wants to reuse. For example: A car has a engine, a window has a button, a zoo has a tiger.<\/p>\n

Composition is a special case of aggregation. In other words, a restricted aggregation is called composition. When an object contains the other object and the contained object cannot exist without the other object, then it is called composition.<\/p>\n

Program Example of Composition:<\/strong><\/h4>\n

Let us consider the following program that demonstrates the concept of composition.<\/p>\n

Step 1: <\/strong><\/span>First we create a class Bike<\/strong> in which we declare and define data members and methods:<\/p>\n

class\u00a0Bike\r\n{\r\n\r\n  \/\/\u00a0declaring data members and methods\r\n\r\n  private\u00a0String\u00a0color;\r\n\r\n  private\u00a0int\u00a0wheels;\r\n\r\n  public\u00a0void\u00a0bikeFeatures()\r\n\r\n  {\r\n\r\n    System.out.println(\"Bike\u00a0Color=\u00a0\"+color\u00a0+\u00a0\"\u00a0wheels=\u00a0\"\u00a0+\u00a0wheels);\r\n\r\n  }\r\n\r\n  public\u00a0void\u00a0setColor(String\u00a0color)\r\n\r\n  {\r\n\r\n    this.color\u00a0=\u00a0color;\r\n\r\n  }\r\n\r\n  public\u00a0void\u00a0setwheels(int\u00a0wheels)\r\n\r\n  {\r\n\r\n    this.wheels\u00a0=\u00a0wheels;\r\n\r\n  }\r\n\r\n}\r\n<\/pre>\n
Step 2: <\/strong><\/span>Second we create a class Honda <\/strong>which extends the above class Bike<\/strong>. Here Honda<\/strong> class uses HondaEngine<\/strong> class object start()<\/strong> method via composition. Now we can say that Honda<\/strong> class HAS-A HondaEngine<\/strong>:<\/p>\n
class\u00a0Honda\u00a0extends\u00a0Bike\r\n\r\n{\r\n\r\n  \/\/inherits all properties of bike class\r\n\r\n  public\u00a0void\u00a0setStart()\r\n\r\n  {\r\n\r\n    HondaEngine\u00a0e\u00a0=\u00a0new\u00a0HondaEngine();\r\n\r\n    e.start();\r\n\r\n  }\r\n\r\n}\r\n<\/pre>\n
Step 3: <\/strong><\/span>Third we create a class HondaEngine <\/strong>through which we uses this class object in above class Honda:<\/strong><\/p>\n
class\u00a0HondaEngine\r\n\r\n{\r\n\r\n  public\u00a0void\u00a0start()\r\n\r\n  {\r\n\r\n    System.out.println(\"Engine has been\u00a0started.\");\r\n\r\n  }\r\n\r\n  public\u00a0void\u00a0stop()\r\n\r\n  {\r\n\r\n    System.out.println(\"Engine\u00a0has been stopped.\");\r\n\r\n  }\r\n\r\n}\r\n<\/pre>\n
Step 4:<\/strong><\/span> Fourth we create a class CompositionDemo<\/strong> in which we make an object of Honda<\/strong> class and initialized it:<\/p>\n
class\u00a0CompositionDemo\r\n\r\n{\r\n\r\n  public\u00a0static\u00a0void\u00a0main(String[]\u00a0args)\r\n\r\n  {\r\n\r\n    Honda\u00a0h\u00a0=\u00a0new\u00a0Honda();\r\n\r\n    h.setColor(\"Black\");\r\n\r\n    h.setwheels(2);\r\n\r\n    h.bikeFeatures();\r\n\r\n    h.setStart();\r\n\r\n  }\r\n\r\n}\r\n<\/pre>\n
Output:<\/strong><\/p>\n
Bike color= Black \u00a0\u00a0wheels= 2\r\nEngine has been started\r\n<\/pre>\n
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Importance of Composition:<\/strong><\/h4>\n