OO relationships between classes
0 likes•69 views
This document discusses different types of relationships between classes in object-oriented programming, including inheritance, composition/aggregation, association, using helper classes, and instantiation with generics. Inheritance allows a class to reuse features of another superclass, establishing an "is-a" relationship. Composition/aggregation indicate "has-a" relationships between classes. Association represents collaboration between classes. Helper classes are used as parameters or variables but do not represent associations. Generics allow defining classes that can work with different data types.
OO relationships between classes
- 1. OO Relationships between Classes Sujit Kumar Zenolocity LLC
- 2. Overview • • • • • Inheritance Composition and Aggregation Association Using (Helper Classes) Instantiation (Generics)
- 3. Inheritance • Allows a class to reuse features & functionalities of another class. • Class it inherits from is also called Base Class or Super Class or Parent class. • The inheriting class is called Sub Class or Derived Class or Child class. • Sub Class can add more attributes & methods. • Establishes an “IS-A” relationship from sub class object to super class object.
- 4. Inheritance Hierarchies • Examples of Inheritance Hierarchy: Shape Hierarchy, Living Being Hierarchy. • Specialization – go down the hierarchy. • Generalization – go up the hierarchy.
- 5. Example: LivingBeing Class Hierarchy
- 6. Java Keywords related to Inheritance • extends => inherits from Example: public class Plant extends LivingBeing { … … } • super(…) => invoke the base class constructor, must be the first statement in the sub class constructor. • super.someMethod() => scope resolution, helps to invoke a method with the same name from the base class.
- 7. Properties of Inheritance • Every sub class object “is an” object of the base class as well. • Examples: Triangle is a Shape, Plant is a LivingBeing • Inheritance is NOT symmetric. “A is a B” does not imply “B is-an A”. • Inheritance is transitive. If “A is a B” & “B is a C” => A “is a “ C.
- 8. Casting w.r.t Inherited Classes • Example Class B inherits from Class A. • A a = new B(); • The above is valid as every B is an A, upcasting is implicit. • B b = (B) a; // downcasting has to be explicit, potential ClassCastException. • B b = new A(); // Invalid as every B is not an A
- 9. Composition & Aggregation • Indicates “has a” relationship between classes. • Examples: Car has an engine, LaptopBag has a Laptop. • Aggregation => component can exist independently of aggregate object. Example: Laptop can exist independently of LaptopBag. • Composition => component cannot exist independently of composite object. Example: Engine cannot exist independently of Car.
- 10. Association • Classes that collaborate with each other within the business domain. • Can be uni-directional or bi-directional. • Example: Customer and Teller at a Retail Bank. The Teller serves many Customers. The Customer can be serviced by many Tellers. • Cardinality: 1-to-1, 1-to-Many, Many-to-Many. • Many-to-Many may be broken into two 1-toMany relationships using a junction (link) class.
- 11. Using • A class uses a helper class as a parameter to a method or as a local variable of a method. • Not same as association or aggregation. • Example: In fluid dynamics, a method of a class which calculates the flow in 2 dimensions can use a ComplexNumber class as a parameter or local variable.
- 12. Instantiation (Generics) • Not an object level relationship, but a class-toclass relationship. • Share the definition of a class across different data types. • Example: A Queue can hold objects of type HumanBeing or Animal or Integer. Behavior of Queue (FIFO) is same irrespective of whether it is holding Animals or HumanBeings or Integers.
- 13. UML Notation for OO Relationships • See the UML Notation Summary pdf file