Java-Inheritance and Interface

• sub-class inherit public and protected variables and methods from super-class
  • exp. car(super)-suv(sub)
  • although it still contain private function and variables, user can not use it in sub class
    • compiler error
  • the sub-class can add new variable and methods or override exist method from super-class
    • overwrite a method, and compiler will choose function base on object type
      • this is called gynamic method lookup
        • allow us to treat object of different class in a uniform way
      • this  feaure is called polymorphism
        • make program extensible
    • variable can not been override
    • use super.accessor()  / super.mutator()  to get/change private variable value
    • use super() to call constructor of super class(use in sub-constructor, must be first line)
  • sub class data can convert to super-class data
    • if A is super class, B is sub class of A
    • A a = new B() create a B type variable
    • A a = (A) b convert a B type variable to A type
• abstract class
  • a class with abstract methods can work as super class to many class
  • basically contain common ideas of all sub-class
  • if extend abstrat class, must implement all abstract methods
  • you can not instantiate an abstract calss
    • because abstract class do not have all property the real object does
    • means you can not create a object with abstruct class
• you can prevent others over write method/variable using final
• tostring method
  • return a string contain all info of current object
  • all class have such method because all class extend object class
  • can inherit by sub class and use super.toString()+… to overwrite(save time)
• the == is different to .equals()methods
  • all class have such method because all class extend object class
• instanceof operator
  • check if object is an data of given type
    • (obj instanceof class)
  • return a boolean value
  • do not use type test to seperate case of super and sub class
    • write a method in super class, overwrite in sub class with different task
• use getClass method to get and compare object`s class

 

 

• Interface
  • a list of methods and their signatures
    • these methods just have name, parameter and return type, but not decleared
    • atomatic public
    • do not contain instance variable/static method
      • instance variable: variable decleared outside class
      • static method: method do not apply to object
  • a class implements a Interface must have all methods contained by that Interface
    • all implement method must be public
  • <T> a placeholder for method is some interface to make sure these method can imply to any  data type
  • exp.
    • measureable interface
      • getMeasure() method
    • comparable interface
      • compareTo() method
  • can have variable which is public static final
    • not changable but can be access by any class implement to it