Download dynamic binding, Interfaces

CS 106
Introduction to Computer Science I
04 / 16 / 2014
Instructor: Michael Eckmann
Today’s Topics
• Comments and/or Questions?
• Dynamic binding
• Interfaces
– Comparable interface
– Polymorphism via interfaces
Michael Eckmann - Skidmore
College - CS 106 - Spring 2014
Binding
• Binding of a variable to a type usually occurs at compile-time. This
means that when Java compiles the source code, it can figure out the exact
type (class) associated with each variable name in the code at the time the
code is compiled.
• In the case of polymorphism, this binding of a variable to a type can only
be done at runtime. Why?
• This is called late binding or dynamic binding. Java needs to determine
the ACTUAL type of the object being referred to by the reference.
Pet p;
// some code here to assign an object to p
p.speak();
Recap
• Inheritance (super/sub, parent/child) use extends
keyword.
• Abstract vs. Instantiable
• Overriding methods
• Polymorphism & Dynamic method binding
– e.g. when have an array of Pets, each reference is
a polymorphic reference either a Dog,
TalkingDog or Cat and can call speak() directly
on the Pet reference and the Java runtime system
is able to call the correct speak() method based on
the actual type of the object that the Pet reference
refers to. This choosing the correct speak()
method to call at runtime is dynamic method
binding.
Overloaded methods
• Overloaded methods are those that have the same name but
different numbers or types of parameters.
• It has nothing to do with the super / subclass (parent / child)
relationships that we've been talking about.
• Does anyone remember when we used Overloaded methods?
Interfaces
• An interface is a collection of constants and abstract methods.
– An abstract method is one that is defined by a header
(signature) but no body. An abstract method, when defined
in an interface, does not provide an implementation.
• An interface can never be instantiated. What does that mean
again?
Interfaces
• The purpose of interfaces are to formally define the ways in
which we can interact with a class (that implements the
interface).
• A class is said to implement an interface if it does so with the
correct syntax and provides definitions for each of the abstract
methods in the interface.
• A class can implement multiple interfaces, but may only
extend one class directly.
Interfaces
• We can create our own interfaces (and we may do this later).
• We can also use interfaces in the Java API.
Interfaces
• There are many interfaces in the Java API available for us to
use.
• One of these is the interface Comparable
• It contains one abstract method:
int compareTo( objectToBeCompared )
It is meant to return a negative integer, zero, or a positive integer
as this object is less than, equal to, or greater than the
objectToBeCompared. Negative means <, Positive means >
and 0 means equal.
It is up to a class that implements the Comparable interface to
determine what it means for its objects to be <, > or = to each
other. That is specified in the implementation of the method.
Interfaces
• Example use of Comparable with this method:
int compareTo( objectToBeCompared )
• Let's edit our Card class to implement Comparable.
• Will Card need a compareTo method?
Interfaces
• Example use of Comparable with this method:
int compareTo( objectToBeCompared )
• Let's edit our Card class to implement Comparable.
• Will Card need a compareTo method?
– Yes, but we already wrote it to those exact
specifications
Interfaces
• Let's revisit the bubbleSort method that sorted ints and make it
sort Comparables.
• Why?
Interfaces
• Let's revisit the bubbleSort method that sorted an array of ints
and make it sort an array of Comparables.
Because it will be more flexible --- it will be able to sort an array
of objects of any class that implements the Comparable
interface
• It can sort an array of Cards
• It can sort an array of Employees
– Assuming there is an Employee class and it
implements the Comparable interface
• It can sort an array of String (b/c String
implements Comparable)
• It can sort an array of any class
– As long as that class implements the
Comparable interface
Interfaces
• We know String has a compareTo method.
• It implements the Comparable interface. Let's look at String in
the Java API.