Java Inheritance

• Inheritance is the process by which a new class is created from another class

  1. The new class is called a derived class, or child class
  2. The original class is called the base class (super class) or parent class

• A derived class automatically has all the instance variables and methods that the base class has, and it can have additional methods and/or instance variables as well

• keyword: extends

  public clase HourlyEmployee extends Employee{
    private double wageRate;
    private double hour;
  
    public HourlyEmployee(){
      super();
      wageRate = 0;
      hour = 0;
    }
  }
  

• super(): A derived class uses a constructor from the base class to initialize all the data inherited from the base class

  1. A call to the base class constructor can never use the name of the base class, but uses the keyword super instead
  2. A call to super must always be the first action taken in a constructor definition

• Although a derived class inherits methods from the base class, it can change or override an inherited method if necessary

• final keyword:

  1. If the modifier final is placed before the definition of a method, then that method may not be redefined in a derived class
  2. If the modifier final is placed before the definition of a class, then that class may not be used as a base class to derive other classes

• Overiding vs Overloading

  • Overiding: 修改base class中某個method的內容，但parameter數量不變
  • Overloading: 某method支援多種數量的parameter

• implements keyword: Generally, the implements keyword is used with classes to inherit the properties of an interface. Interfaces can never be extended by a class.

• The Java programming language supports multiple inheritance of type, which is the ability of a class to implement more than one interface.