• The data (attributes), methods (behaviors), and all functionality related to concept the class represents is coded inside the class.
• Data hiding enforces this - data is private, meaning it can only be accessed through methods written by the class designer.
• The methods define the interface between the program using the class and the object's data.
• Rules about the class behavior and how the data relates (often called the invariant of the class) cannot be bypassed by the calling function.
  
• In Java, data hiding is done with access control; instance variables should be declared as private.
  

• A class is composed of attributes, which can be primitives, but are often of other classes.
• The relationship between these classes is called composition.
• Note: sometimes this is called containment, but be careful, that term has a specific and different meaning in many contexts, including Java, where it usually means a Container Class.

• strong: "hasa", attribute, instance variable, fully encapsulated in the class, life of attribute = life of the object, life of the association = life of the object
  
• weak: not really an attribute; the association is temporary. It could be passed as an argument, or have reference, be a reference in a container (like an array); the object has its own life beyond the use of the reference.
  

• A new class can be created from an existing class.
• The new class inherits the attributes and behaviors (instance variables and methods) for the existing class.
• Classes related by inheritance are called parent/child classes or base/derived classes or super-/sub-classes.
• A set of classes related by inheritance is called a class hierarchy.
  

1. have a hierarchy
2. have a parent reference that refers to an object of the child
3. child class overrides a method in the parent

When the method is called with the parent reference, it figures out which exact method to call (uses dynamic or late binding).