OOP - Association

When writing a program, variables that hold value types are created on the stack.

Variables that hold object types, create the object on the heap, and a reference to the objects is stored in the variable on the stack.

When we create an instance of the class, and call the constructor...

ClassName ObjectName = new ClassName();

...the constructor first creates the object on the heap, then sets the initial values of any fields in the object. If successful, it returns the reference to the object to the variable.

Classes, however, do not exist in our programs independently.

Association defines a relationship between different objects that allows one object to perform an action on another’s behalf. This is called delegation.

One object ‘has a’ relationship with another object.
These relationships are implemented through references to other objects.

We already know that an object can have fields that store simple values (our atomic data types).

Objects can also have fields that hold references to other objects. You should never hold copies of data from other objects, only references to them.