Aggregation in Java Definition and Examples

Aggregation in Java  is a relationship between two classes that is best described as a has-a and whole/part relationship. It is a more specialized version of the association relationship. The aggregate class contains a reference to another class and is said to have ownership of that class. Each class referenced is considered to be part-of the aggregate class. Ownership occurs because there can be no cyclic references in an aggregation relationship. If Class A contains a reference to Class B and Class B contains a reference to Class A then no clear ownership can be determined and the relationship is simply one of association. For example, if you imagine that a Student class that stores information about individual students at a school. Now assume a Subject class that holds the details about a particular subject (e.g., history, geography). If the Student class is defined to contain a Subject object then it can be said that the Student object has-a Subject object. The Subject object also makes up part-of the Student object —  after all, there is no student without a subject to study. The Student object, therefore, owns the Subject object. Examples Define an aggregation relationship between Student class and the Subject class  as follows:   public class Subject {private String name;public void setName(String name)  Ã‚  {this.name name;}public String getName(){return name;}}public class Student {private Subject[] studyAreas new Subject[10];//the rest of the Student class}