ch01_classesobjects

1. Classes and objects

• some class basics is part of OCA, see:
  • OCA, chapter 1 Classes, static classes, inner classes
  • OCA, chapter 10 Abstract
  • OCE, chapter 11 Interfaces
• POJO vs JavaBean: www.geeksforgeeks.org/pojo-vs-java-beans/
• JavaBean vs EJB: stackoverflow.com/questions/2460048/difference-between-java-bean-and-enterprise-java-beans/

Inheritance

• provide storing of common attributes and behavior in parent class
• child class can (depends on access privilege of attribute or method):
  • use attributes and methods from parent
  • overshadow attribute (with same name) from parent
  • expand or override methods from parent class
• it is not possible to assign instance of parent class into variable of type the child class
  • child class (used as variable reference type) may contains attributes or methods which are not defined in parent class (used as variable object type)
  • similar situation is with class that implements some interfaces

Constructors

• see OCA, chapter 5 Constructors
• child constructor always call (implicitly or explicitly) parent constructor first
  • explicit use super() or super(param) in case of parameterized parent constructor

Methods

• override method in child class:
  • can access to first parent method implementation using super.method();
  • can't add other checked exception to throws clause
• throws clause of override method in child class can:
  • add unchecked exception
  • use same exception as parent method
  • use subclass of exception used in parent method
  • hide checked exception (omit throws clause)

Immutable class

• see official Oracle guideline: docs.oracle.com/javase/tutorial/essential/concurrency/imstrat
• example from Java API is e.g. java.lang.String

Rules of immutability

• make all fields final and private
• don't provide "setter" methods
• don't allow subclasses to override methods (declare the class as final or make the constructor private and construct instances in factory methods)
• if the instance fields include references to mutable objects, don't allow those objects to be changed:
  • don't provide methods that modify the mutable objects.
  • don't share references to the mutable objects
    • if necessary, create copies and return references to the copies)
  • never store references to external, mutable objects passed to the constructor
    • if necessary, create copies, and store references to the copies)

Instanceof

• determines if object is instance of class or interface
• if the reference type of checked object is unreachable with checked class, the compilation error is caused
  • it's not relevant if reference type is interface (compilation error is not caused in this case)