Chapter 14

Abstract Classes and Interfaces

Abstract Classes

• An abstract class extracts common features and functionality of a family of objects

• An abstract class provides a uniform interface for dealing with a family of objects

• Abstract classes are defined with the abstract keyword

Using abstract classes

• Concrete instances of abstract classes cannot be instantiated with the new operator

• However, variables of abstract class types can be declared and used to hold objects that extend (inherit from) the abstract class

• Abstract classes can have constructors that are called by subtypes using the super keyword (even though they cannot be created)

Abstract Methods

• Methods can be defined using the abstract keyword

• If a class contains abstract methods, the class must be declared abstract as well

• Abstract methods do not have a body• Abstract methods are implemented by subclasses• If a subclass does not implement all abstract

methods in the superclass, it must be declared abstract

What is the purpose?

• Abstract methods define a common interface that all subclasses will implement

• The methods can be called on a supertype variable using polymorphism without regard to what actual subtype is stored in the variable.

• Allows generic treatment of a family of subtypes

Other points of interest

• An abstract class does not have to define any abstract methods

• A subclass can be defined abstract even if the superclass is concrete (e.g. java.lang.Object)

• A subclass can override a superclass method and define it to be abstract

Interfaces

• Interfaces are like abstract classes—but• Interfaces are declared with the interface

keyword• Interfaces can only define abstract methods• Interfaces can only declare constants

• Interfaces are compiled into separate byte-code files just like classes

How to use interfaces

• A class can extend only one class by inheritance (using the extends keyword)

• A class can implement an unlimited number of interfaces (using the implements keyword)

• An interface is “contract” of functionality• Classes can be designed to use and provide

specific functionality via an defined interface (the Application Programming Interface)

Java and Multiple Inheritance

• Java does not support inheriting from multiple classes (linear inheritance model)

• This avoids many problems encountered in C++

• Java can simulate multiple inheritance by having a class implement multiple interfaces that provide the functionality of multiple “class abstractions”

The Comparable interface

• public interface Comparable {public int compareTo(Object o); }

• Classes implementing this interface can be compared to each other to obtain an ordering relationship

• public class MyClass extends Object implements Comparable { public int compareTo(Object o) { // code to compare objects } }

An informative example

A Caveat

• compareTo() returns zero if two objects have the same ordering relationship

• Generally, this would imply that the overriden equals() method inherited from java.lang.Object would also return true for the two objects

• Often the implementations will be congruent, but not always

ActionListener

• ActionListener is an interface for handling GUI events such as button clicks

• A class that implements ActionListener overrides the actionPerformed() method to provide custom processing to respond to GUI events

• A GUI element “adds” an object implementing ActionListener and then sends events (e.g. button clicks) to the object for processing through the actionPerformed() method

How does ActionListener work?

• Explain how ActionListener works under the covers if students do not appear too restless or bored

• Is event handling in Java kind of like a “come from” statement (opposite of goto)?

• ActionListeners are often implemented as anonymous inner classes--but that is a topic for next semester!

The Cloneable Interface

• Cloneable is an empty interface (no methods or constants defined)

• Empty interfaces are called marker interfaces and signal that an object implements certain characteristics

• In this case, Cloneable indicates that the class overrides the clone() method defined for java.lang.Object

clone()

• The definition of clone() is:protected native Object clone() throws CloneNotSupportedException;

• Cloning relies on the underlying system to create a copy of an object—native code

• clone() is a protected method, but it can be made public when overriden

• clone() can throw an exception that you should handle

Shallow Copy

• The overriden method clone() can be implemented by calling super.clone()

• Implementing it this way does a shallow copy• A shallow copy will copy the contents of each

field of the object to the new object• The value of primitive types are copied• The references to objects are copied• References will be equal even though the

cloned objects are not equal

Deep Copy

• A deep copy creates duplicates of the objects referenced by fields rather than simply copying the references

• Deep copy is implemented by first calling super.clone() and then adding code to duplicate objects

• Deep copy is important if the two objects modify internal objects and should not be using the same objects

Multithreading

• Multithreading means running two different paths of execution simultaneously

• Java has two ways to do multithreading: the good way and the other way

• The good way is to implement the Runnable interface

• The other way is to extend the Thread class (if you need to override its methods)

Runnable

• The Runnable interface has just one method:void run(); // no parameters

• The run() method is where thread execution starts, which is effectively the main method for the thread

• A class that implements Runnable can create a Thread with itself as the parameter and run itself in a thread

An example of multithreading

Setting up and calling threads

Results of running multiple threads

Interface vs. Abstract Class

Interface• All variables must be

public static final

• Constructors are not allowed

• All methods must bepublic abstract

Abstract Class• No restrictions on variables

• Constructors are allowed and can be invoked by subclasses

• No restrictions on methods

When to use

• An abstract class represents common functionality extracted from a family of related objects

• An interface represents common functionality that can be implemented by unrelated objects

• What are some examples typifying this division of usage?

Primitive Data Types as Objects

• All primitive data types can be represented as objects

• Doing this would make the language perfectly uniform

• It would also make the language very slow• Wrapper Classes are the objects that “wrap”

primitive data types• Wrapper classes allow generic programming

Wrapper Classes

• Boolean• Character• Byte• Short• Integer• Long• Float• Double

Number

• The numeric wrapper classes extend the abstract Number class

• Number has methods to convert a value to various primitive types

• Each wrapper class overrides the toString(), equals() and compareTo() methods

• Each wrapper class knows the MAX_VALUE and MIN_VALUE for the types it wraps

Conversion methods

• shortValue()• intValue()• longValue()• floatValue()• doubleValue()

• valueOf(String strValue) // returns an object initialized with the value specified in strValue

Parsing methods

• public static xyz parseXyz(String s)• public static xyz parseXyz(String s, int radix)• Static methods called on wrapper class

Integer.parseInt(“536”, 16);• Returns the primitive type (xyz) represented

by the string• Can choose a radix value of 2, 8, 10, or 16 (the

default is 10)*

BigInteger and BigDecimal

• Java provides two classes for handling very large or very precise numbers

• BigInteger and BigDecimal can represent numbers of any size or precision*

• Both classes implement add(), subtract(), multiply(), divide(), and remainder()

• Both implement compareTo()• Be careful with BigDecimal.divide()*

Example using big numbers

CS4a Lab 1 reduxCalculating Pi using BigDecimal

• The irrational number pi can be calculated by the following formula:

• Pi = )• Calculate and store this value in a BigDecimal

variable • Expand the formula to 20 digits of precision• Display the results either on the console or in

a dialog box

Problems to consider

• Will this require doing arithmetic with more digits of precision that just 20?

• What kinds of cumulative errors have we discussed in class that need to be avoided?

• How many digits of calculation precision will be required to accurately define 20 digits of pi?

• How many terms need to be expanded?

Details

• Remember to have a cover sheet and include appropriate comments and printouts of the results.

• Assignment due: 4:30 p.m. November 1, 2012