Inheritance in Java Programming

Inheritance

• Access Attributes (from Horton's Beginning Java 2, pp. 213, 214, 238)
  
  
• Animals Inheritance examples:
  • version 1 (handout (doc), or directory): Basic inheritance using extends, no-args constructor in superclass needed, overloaded constructors. Using classes Animal.java, Dog.java, Driver.java
  • version 2 (handout (doc) or directory): Chaining constructors using this, invoking superclass constructor using super, for Animal2.java, Dog2.java, Driver2.java
    • Overriding superclass toString method: access specifier (i.e. public) for the overriding class may not be more restrictive than that of the superclass.
    • Cannot string more than one super together. (i.e. super.super.toString() is invalid.)
  • version 3 ( handout (doc) or directory): Polymorphism, where the type of an object is determined at runtime and changes the behavior of the program. 3 Different animals all have the sound method implemented.
    Animal3.java, Cat.java, Dog3.java, Dolphin.java, Poodle.java, Dolphin.java, and Driver3.java
    For polymorphism to work (e.g. objects of type Animal all call the correct class corresponding to the actual type of each one) the following must be true:
    • The method call (e.g. sound() ) must be through an object of the base class (e.g. Animal).
    • The method in the derived class must have the same argument signature and return type as that in the base class.
    • The method in the derived class must have an access specifier no more restrictive than that of the base class
  • version #4: abstract classes.
    Animal4.java, Cat4.java, Dog4.java, Driver4.java
    • A class with one or more abstract methods is itself an abstract class, which cannot be instantiated. It is useful for organizing subclasses.
    • An abstract class method declaration ends with a semi-colon, e.g. method sound() in class Animal4.
      
      
• Another Inheritance Example: Subclassing (doc file, from Liskov), and the corrresponding class handout for the Account class (doc file)
  
  
• All user-defined classes themselves are actually subclasses of super-class Object. Class Object has some public methods which we commonly override:
  • toString() - This gives us a representation of an object as a String
  • equals() - This compares the calling object with the object sent as a parameter, returning true if they are actually the same object, not just containing the same values.
  • getClass() - This returns an object of type Class that identifies the class of the current object.
    • We could use this, for instance, if we want to print only elements of class Dog.
    • This also illustrates using qualified names to access methods outside the current package.
  • hashCode() - This calculates an integer hash code value for an object and is used for storing objects in hash tables.