Inheritance and Polymorphisan

Inheritance

  • What:
    • student和faculty都是person。总结出一个person的super class
    • 把属于person的共同代码写在parent class里，包括instance和method。
    • 把属于student和faculty自己的代码写在subclass里，包括instance和method。
  • Why:
    • 维护代码方便
    • 把不同的class放在同一个data structure里，方便操作
      • Person[] p = new Person[3];
      • p[0] = new Person();
      • p[1] = new Student();
      • p[2] = new Faculty();
  • Note:
    • Person p = new Student()--is allowed because student is a person. But compiler will see p as a person, only during runtime, p will be regarded as student.

Polymorphism

  • What:
    • Superclass reference to subclass object: Person s = new Student(“Cara”, 1)
    • Subclass can call their own version of methods (overriding)
  • Why:
    • Need different version of one methods
  • Notes:
    • When compile: compiler only know reference type (for example: person), be caution for compile errors (for example: use methods only in student not in person, compiler only looks for person class) (sometimes need casting to subclass)
    • When run: will execute the real type

Overloading vs. Overriding

  • Same class has same method name with different parameters
  • Subclass has same method name with same parameter

Visibility

  • Public: can access from any class
  • Protected: can access from same class/same package/any subclass
  • Package: can access from same class/same package
  • Private: can access from same class
  • Rule of thumb: make member variables private, methods can be private or public (for example: method for others to access member value is normally public, while helper method does not want others to use is private)
  • Protected or package rarely used

Construction

  • Construction happens from subclass to parent class
  • No constructor: java gives a default one with no arguments
  • First line should say either this or super, otherwise, java will insert super() for you
  • How to initialize parent instance in subclass? call super(arguments)

Abstract classes vs. interface (this part not clear, need rewrite)

  1. Abstract, define instance and methods
  2. public abstract class Person{
  3. public abstract void Monthlypaycheck (); }
  4. Interface, define method only public interface Comparable{ public abstract int compareto(); }

  5. why?

    • Force subclasses to have this method
    • Person class do not define actual implementation