Lecture Summary 6

Inheritance

• Inheritance allows a software developer to derive a new class from an existing one, for the purpose of reuse, enhancement, adaptation, etc.
• superclass (a.k.a. base class)
• subclass (a.k.a. derived class, extended class)
• Inheritance models the is-a relationship.
• If class E is an extended class of class B, then any object of E can act-as an object of B.
• Examples: Book.java, Dictionary.java, Words.java
• Protected visibility
• Super constructor
• Examples: Book2.java, Dictionary2.java, Words2.java
• Overriding
• Super reference
• Examples: Book3.java, Dictionary3a.java, Dictionary3b.java, Books.java
• Single vs. multiple inheritance
• A class may have only one superclass.
• Overloading vs. overriding
• Overloading

 More than one methods have the same name but different  signatures
• Overriding

 Replacing the implementation of a methods in the superclass with one of your own.
• You can only override a method with the same signature.
• You can only override instance methods.
• The Object class
• The root of Java class hierarchy
• Defines some common methods:
• public String toString()
• public boolean equals(Object other)
• If a class does not explicitly declare a superclass, its superclass is Object.
• Abstract class
• An abstract class is a class with partial implementation.
• It implements behaviors that are common to all subclasses, but defers to the subclasses to implement others (abstract methods).
• An abstract class cannot be instantiated
• The subclass of an abstract class must override the abstract methods of the parent, or it too will be considered abstract
• Interface
• Interfaces are classes with no implementation.
• Interfaces represent pure design.
• Abstract classes represent mixed design and implementation.
• An interface consists of only abstract methods and constants, i.e., static and final.
• All methods and constants are public.
• No static methods.
• An interface cannot be instantiated
• Inheritance relation also applies to interfaces
• superinterface and subinterface
• Multiple inheritance allowed
• An interface may extend multiple interfaces
• A class my implement multiple interfaces
• Type conversion
• Implicit conversion
• Numeric variables: Any numeric types can be converted to another numeric type with larger range, e.g.

char ==> int, int ==> long, int ==> float, float ==> double.
• Object reference: An object reference of class C can be converted to a reference of a superclass of C.
• Explicit conversion (cast)
• Numeric variables: Any numeric types can be explicitly cast to any other numeric type. May lose bits, precision.
• Object reference: Cast an object reference of a class to a reference of any other class is:
• syntactically allowed; but
• runtime checked.
• Cast object references:

class Student { ... }
class Undergraduate extends Student { ... }
class Graduate extends Student { ... }

Student student1, student2;
student1 = new Undergraduate(); // ok
student2 = new Graduate();      // ok

Graduate student3;
student3 = student2; // compilation error

student3 = (Graduate) student2; // explicit cast, ok

student3 = (Graduate) student1; // compilation ok, run-time error

• A polymorphic reference is one which can refer to different types of objects.
• Example: Books2.java