Composition in Java is a design technique used to implement a "has-a" relationship between classes. It is achieved by using instance variables that refer to other objects. In composition, one class includes another class and is dependent on it so that it cannot functionally exist without the other class. For example, a Person class may have a Job class, and the Person object cannot exist without the Job object.
Composition is different from inheritance, which establishes an "is-a" relationship between classes. The benefits of using composition include code reusability, flexibility, and better testability of a class. By using composition, we can also change the member objects at runtime, dynamically changing the behavior of the program.
To implement composition, we use an instance variable that refers to another object. The contained object should not exist without the existence of its parent object. Composition is a restricted form of aggregation, where one class includes another class and is dependent on it.
An example of composition in Java is a University class that has a reference to a list of College classes. A University can have more than one College, and if a University is permanently closed, then all Colleges within that particular University will be closed because Colleges cannot exist without a University.
In summary, composition in Java is a design technique used to implement a "has-a" relationship between classes, achieved by using instance variables that refer to other objects. It offers code reusability, flexibility, and better testability of a class.
