Aggregation in Java: Definition and Examples

Aggregation implies ownership, not just association

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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)  {
this.name = name;
}
public String getName()
{
return name;
}
}
public class Student {
private Subject[] studyAreas = new Subject[10];
//the rest of the Student class