Association vs Aggregation vs Composition in Java

Association, aggregation, and composition describe three types of relationships between objects in object-oriented design — they differ in how strongly one object's lifecycle is tied to another's. There is no special Java syntax for any of them; the distinction lives entirely in who creates the object and who holds the reference.

Composition — owns and creates:

• Composition is the tightest binding: the owner creates the part using new directly in its constructor — Car does this.engine = new Engine(), meaning the car's construction directly causes the engine's creation
• The part cannot exist independently — when Car is garbage-collected with no other references to its engine, the engine is collected too; this maps to CASCADE DELETE in a relational database
• A real giveaway: if you can't create the part without creating the owner first, it's composition

Aggregation — uses but doesn't own:

• Aggregation is a looser relationship: the part is constructed elsewhere and passed in — Department(List professors) receives professors that were created independently
• Professors continue to exist after the department is dissolved — they were never owned by the department, just referenced; the department going away doesn't trigger their destruction
• Spring @Autowired is always aggregation — the beans are created and managed by the container, not by the class that receives them

Association — interacts without ownership:

• Association is the loosest: two objects interact but neither creates nor manages the other's lifecycle; the relationship exists only for the duration of an interaction
• This shows up as a method parameter or a temporary local reference — a Teacher who grades a Student doesn't create the student and doesn't hold a permanent field reference

I recommend the lifecycle test when uncertain: if you delete the parent and the child should disappear with it, that is composition. If the child continues to exist independently, that is aggregation.