Java – Aggregation versus Composition


I've had a hard time understanding the difference between composition and aggregation in UML. Can someone please offer me a good compare and contrast between them? I'd also love to learn to recognize the difference between them in code and/or to see a short software/code example.

Edit: Part of the reason why I ask is because of a reverse documentation activity that we're doing at work. We have written the code, but we need to go back and create class diagrams for the code. We'd just like to capture the associations properly.

Best Solution

As a rule of thumb: enter image description here

class Person {
    private Heart heart;
    private List<Hand> hands;

class City {
    private List<Tree> trees;
    private List<Car> cars

In composition (Person, Heart, Hand), "sub objects" (Heart, Hand) will be destroyed as soon as Person is destroyed.

In aggregation (City, Tree, Car) "sub objects" (Tree, Car) will NOT be destroyed when City is destroyed.

The bottom line is, composition stresses on mutual existence, and in aggregation, this property is NOT required.