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Aggregation in Java

Theory

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Aggregation in Java

Aggregation is HAS-A: one object uses another. Student has an Address. Model it with a field: Address address; inside Student. Not extends.

Address can still exist if Student goes away — weaker ownership. Inheritance is IS-A. Aggregation is HAS-A. Composition is a stronger “part-of” idea (engine inside car).

Using extends when the real relationship is HAS-A is the usual design mistake. Interview line: Student HAS-A Address — field reference, not extends.

Let's take this on the board with one tiny Main class — no extra files. Aggregation in Java — output: Asha - Pune. Address can exist without Student — that is aggregation (HAS-A), not IS-A. Start from main, go line by line, and stop at each print. That output is the proof for Aggregation.

Exam tip

Say HAS-A with Student/Address. Contrast with extends.

Example

class Address {
  String city;
  Address(String city) { this.city = city; }
}
class Student {
  String name;
  Address address; // HAS-A
  Student(String name, Address address) {
    this.name = name;
    this.address = address;
  }
}
public class Main {
  public static void main(String[] args) {
    Address a = new Address("Pune");
    Student s = new Student("Asha", a);
    System.out.println(s.name + " - " + s.address.city);
  }
}

Aggregation in Java — output: Asha - Pune. Address can exist without Student — that is aggregation (HAS-A), not IS-A.

Short notes

DefAggregation = HAS-A. One object uses another as a field.
RuleStudent has Address. Not Student extends Address.
DiffIS-A = inheritance. HAS-A = aggregation. Composition = stronger part-of.
RememberAddress can live even if Student is gone.

class Address { String city; Address(String city) { this.city = city; } } class Student { String name; Address address; // HAS-A Student(String name, Address address) { this.name = name; this.address = address; } } public class Main { public static void main(String[] args) { Address a = new Address("Pune"); Student s = new Student("Asha", a); System.out.println(s.name + " - " + s.address.city); } }

Aggregation in Java

Example

Short notes

Questions

Aggregation in Java

Example

Short notes

Questions