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Java - Access ModifiersJava - InheritanceJava - AggregationJava - PolymorphismJava - Method Overriding

Java - Association, Aggregation and Composition

📌 What You Will Learn
  • What class and object mean, with real-life examples
  • The four pillars: Encapsulation, Abstraction, Inheritance, Polymorphism
  • Constructors, this, super, static and final
  • Method overloading and method overriding (with a comparison table)
  • Abstract classes and interfaces (including the modern changes)
  • Association, Aggregation and Composition

Association, Aggregation and Composition is a core concept of the Java language. This lesson explains Association, Aggregation and Composition with complete, runnable code examples, clear step-by-step explanations, and common mistakes to avoid with exam-style MCQs at the end.

Association, Aggregation and Composition

ConceptMeaningExample
AssociationTwo objects are connected but live independently.A doctor treats a patient. Both exist separately.
AggregationA has-a relationship where the part can exist without the whole.A school has students. Students survive if the school closes.
CompositionA strong has-a relationship where the part cannot exist without the whole.A human has a heart. Remove the heart, the human is gone.
Example01
JCode Cell
1class Heart { }
2class Human {
3 Heart heart = new Heart(); // composition: human owns heart
4}
5class Student { }
6class School {
7 List<Student> students; // aggregation: school has students
8}
📝 Key Takeaways
  • Class = blueprint; Object = real thing made from it (memory in heap).
  • Four pillars: Encapsulation (hide data), Abstraction (show only essentials), Inheritance (reuse parent code), Polymorphism (one name, many forms).
  • Overloading = same class, different parameters, decided at compile time. Overriding = child redefines parent method, decided at run time.
  • Constructor initialises the object; name = class name, no return type.
  • this = current object; super = parent class; static = belongs to class; final = cannot change.
  • abstract class cannot be instantiated; interface is a contract a class implements.
  • Association (weak), Aggregation (has-a, part independent), Composition (strong has-a, part dies with whole).

🧠 Test Your Knowledge

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