Java Review 3

Rem Collier

Java Wrapper Classes

• In Java, the term wrapper class commonly refers to a set of Java classes that “objectify” the primitive Java types.– That is, for each primitive type, there is a corresponding Java

“Wrapper” class that represents that type.

– e.g. the wrapper for the int type is the Integer class.

• Wrapper Classes are based upon the well-known software engineering design pattern called the Wrapper pattern.– A design pattern is a template solution to a common problem. It

describes the problem and identifies the recommended solution(s) to that problem.

– Because design patterns deal with common problems, they are often quite abstract!

Wrapper Design Pattern

– Some code (the client) needs a class to use a certain interface (which it does not use).

• The problem:

– e.g. we want to store an int in a Vector, but Vectors do not accept primitive types.

• The Solution:– We create another class that wraps the underlying

class/type and provides an appropriate interface for the client.

– e.g. we create an Integer class that subclasses Object (as all classes do), allowing us to store wrapped int’s in Vectors.

Example that will not work

import java.util.Vector;

public class MyApp {

public static void main(String[] args) {

int myValue = 2;

Vector myVector = new Vector();

myVector.addElement(myValue);

for (int x=0; x < myVector.size(); x++) {

System.out.println(“Value: “ + myVector.get(x));

}

}

}

The compiler detectsan error here

Example that will work

import java.util.Vector;

public class MyApp {

public static void main(String[] args) {

int myValue = 2;

Vector myVector = new Vector();

myVector.addElement(new Integer(myValue));

for (int x=0; x < myVector.size(); x++) {

System.out.println(“Value: “ + myVector.get(x));

}

}

}

Here we wrap theint in the Integerclass

Java Wrapper Classes

Object

Boolean CharacterVoid Number String

Short DoubleInteger Long FloatByte

Some Useful Methods

• The Java Wrapper Classes include various useful methods:– Getting a value from a String

e.g. int value = Integer.parseInt(“33”);

sets value to 33.

– Converting a value to a String:

e.g. String str = Double.toString(33.34);

sets str to the String “33.34”.

– Getting a wrapper class instance for a value:

e.g. Integer obj = Integer.getInteger(“12”);

creates a new Integer object with value 12 and makes obj refer to that object.

Some Useful Methods

– Getting the maximum permitted int value

e.g. int value = Integer.MAX_VALUE;

sets value to 2147483647.

– Getting the minimum permitted int value

e.g. int value = Integer.MIN_VALUE;

sets value to -2147483647.

– Getting the value in an Integer object

e.g. int value = new Integer(545).intValue();

sets value to 545.

Reading a Double

import java.io.*;

class MyProgram {public static void main(String[] args) {

BufferedReader in = new BufferedReader(new InputStreamReader(System.in));

String line = null;

System.out.println("Input Something:" );try {

line = in.readLine();} catch (IOException ie) {

System.out.println("Exception caught: " + ie);}

try {double value = Double.parseDouble(line);System.out.println("Value: " + value);

} catch (NumberFormatException nfe) {System.out.println("You didn't enter a double number");

}}

}

Interfaces in Java

• In Object-Oriented terms, the interface of a class is the set of public methods and data members of that class.– These methods and data members describe how external code

(I.e. other classes) can interact with the class.

– Once a class is made publicly available for use, great care is required when changing that classes interface.

• E.g. consider how annoyed you would be if Sun changed the interface of the System class from System.out to System.cout…

• Java supports the definition of interfaces independent of implementation of that interface.– A Java interface is a named collection of method definitions

(without implementation). An interface can also include constant declarations.

– In Java, we define an interface using the interface keyword.

Example Interface

• Let us consider a general security service for web applications.– We don’t always know how we wish to validate users,

so we cannot implement a general user validation algorithm.

– So, we create an interface that describes what a security service should do, and leave it to the developer of a specific application to implement a particular security service.

interface SecurityService {

public boolean validateUser(String uid, String pwd);

public static final String ADMIN_USERNAME = “Admin”;

}

Interfaces in Java

• The definition of the methods described in a Java interface is the responsibility of any class that implements that interface.

• If a class implements an interface, then it must include definitions for all of the methods described in that interface.– If you do not include definitions for all the methods, the class will not

compile!

• A powerful feature of Java interfaces is that they can be implemented in more than one class!– For example, our SecurityService interface could be implemented as:

• A DatabaseSecurityService, which looks validates the username and password against a database table, or

• A SimpleSecurityService, which validates the username and password against hardcoded values for the username and password.

Interface Example

interface SecurityService {

public boolean validateUser(String uid, String pwd);

public static final String ADMIN_USERNAME = “Admin”;

}

class SimpleSecurityService implements SecurityService {

public boolean validateUser(String uid, String pwd) {

if (uid.equals(ADMIN_USERNAME) &&

pwd.equals(“god”)) {

return true;

}

return false;

}

}

Using Interfaces + Implementations

• Java interfaces can be used as types.– For example:

SecurityService service;

• Variables or attributes whose type is an interface can be used to reference any class the implements that interface.– For example:

SecurityService service = new SimpleSecurityService();

• When an object is cast as an interface, only those methods declared in the interface may be accessed.– For example:

boolean valid = service.validateUser(“rem”, “hi”);

A simple login program

import java.io.*;

class MyProgram {

public static String readUserInput() {

BufferedReader in = new BufferedReader(

new InputStreamReader(System.in));

try {

return in.readLine();

} catch (IOException ie) {

System.out.println("Exception caught: " + ie);

}

return null;

}

A simple login program

public static void main(String[] args) {

System.out.print(“Username:”);

String username = readUserInput();

System.out.print(“Password:”);

String password = readUserInput();

SecurityService service = new SimpleSecurityService();

if (service.validateUser(username, password)) {

System.out.println(“Valid User”);

} else {

System.out.println(“Invalid User”);

}

}

}

Some Comments…

• Another way in which we could have coded this example is through inheritance and abstract classes.

class SecurityService {

public abstract boolean validateUser(String uid, String pwd);

public static final String ADMIN_USERNAME = “Admin”;

}

class SimpleSecurityService extends SecurityService {

public boolean validateUser(String uid, String pwd) {

if (uid.equals(ADMIN_USERNAME) && pwd.equals(“god”)) {

return true;

}

return false;

}

So, what is the need for interfaces?

• It is not always possible for us to use inheritance to deliver the required interface.– Remember: Java supports only single inheritance. That is,

a class has exactly one parent class.

• Whilst a class can have only one parent class, it can implement many interfaces.– A class includes both private and public methods and

attributes.

– Java interfaces only define what methods must be implemented, not how to implement them!

– So, we can write classes that subclass some parent class and implement one or more interfaces!

– But when is this useful?

Another Example

• Lets consider an alarm clock service.– This service notifies objects after a certain amount of

time has passed.

– It is implemented through two components:• An AlarmClock class that implements the service, and• A Sleeper interface that classes wishing to use the service

must implement.

– Whenever an object wishes to sleep, it contacts an alarm clock instance requesting that it be allowed to sleep for a specified time.

– Once that specified time has passed, the alarm clock tells the sleeping object to “wake up”.

The AlarmClock class

import java.util.ArrayList;

class AlarmClock {

public static void letMeSleepFor(Sleeper sleeper, long time) {

new AlarmThread(sleeper, time).start();

}

}

The AlarmThread class

class AlarmThread extends Thread {

private Sleeper sleeper;

private long time;

public AlarmThread(Sleeper sleeper, long time) {

this.sleeper = sleeper;

this.time = time;

}

public void run() {

try {

sleep(time);

} catch (Exception e) {

System.out.println(“Failed to sleep”);

}

sleeper.wakeUp();

}

}

The Sleeper interface

interface Sleeper {

public static final long ONE_MINUTE = 60000;

public static final long ONE_SECOND = 1000;

public void wakeUp();

}

public class TestSleeper implements Sleeper {

private int iterations;

public TestSleeper() {

iterations = 0;

}

public void wakeUp() {

Iterations++;

System.out.println("Woken up for the " + iterations + " time.");

AlarmClock.letMeSleepFor(this, ONE_SECOND);

}

}

The Applet POC

• So, what happens when we want an Applet to use this service?

class ClockApplet extends Applet implements Sleeper {

// . . .

public void wakeUp() {

repaint();

AlarmClock.letMeSleepFor(ONE_MINUTE);

}

// . . .

}

• Obviously, we could not use inheritance here…