Dependency
Change in structure or behaviour of a class affects the other related class, then there is a dependency between those two classes. It need not be the same vice-versa. When one class contains the other class it this happens.
Composition
Composition is a special case of aggregation. In a more specific manner, a restricted aggregation is called composition. When an object contains the other object, if the contained object cannot exist without the existence of container object, then it is called composition.
Concrete examples in Java from here and here
Dependency
class Employee {
private Address address;
// constructor
public Employee( Address newAddress ) {
this.address = newAddress;
}
public Address getAddress() {
return this.address;
}
public void setAddress( Address newAddress ) {
this.address = newAddress;
}
}
Composition
final class Car {
private final Engine engine;
Car(EngineSpecs specs) {
engine = new Engine(specs);
}
void move() {
engine.work();
}
}
The difference can be seen in the two constructors:
Dependency: The Address object comes from outside, it's allocated somewhere else. This means that the Address and Employee objects exists separately, and only depend on each other.
Composition: Here you see that a new Engine is created inside Car. The Engine object is part of the Car. This means that a Car is composed of an Engine.