Java Packages and Libraries

M Taimoor Khantaimoorkhan@ciit-attock.edu.pk

Content

• What is a class library?• What is a package?• Java class libraries, what do they

provide?• The root class : java.lang.object

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The Java class library• Thousands of classes• Tens of thousands of methods• Many useful classes that make life much easier• A Java programmer must be able to work with the

librarieso http://java.sun.com/javase/reference/api.jsp

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Working with the library

You should:• know some important classes by name• know how to find out about other classes

o http://java.sun.com/javase/6/docs/api/

Remember:• We only need to know the interface, not the

implementation

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The public face of the class

The exit conditionString input = reader.getInput();

if(input.startsWith("bye")) { finished = true;}

• Where does ‘startsWith’ come from?• What is it? What does it do?• How can we find out?

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Reading class documentation

• Documentation of the Java libraries in HTML format;

• Readable in a web browser• Class API: Application Programmers’ Interface• Interface description for all library classes

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Interface vs implementation

The documentation includes• the name of the class;

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Interface vs implementation

The documentation includes• a general description of the class;

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Interface vs implementation

The documentation includes• a list of constructors

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Interface vs implementation

The documentation includes• a list of methods

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Interface vs implementation

The documentation includes• return values and parameters for constructors

and methods

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Interface vs implementation

The documentation does not include

• private fields (most fields are private)• private methods• the implementation (source code) for each

method

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Using library classes• Classes from the library must be imported using

an import statement (except classes from java.lang).

• They can then be used like classes from the current project.

• http://java.sun.com/docs/books/tutorial/java/package/usepkgs.html

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“The Java compiler automatically imports” •java.lang package•the package of the current file source code

Packages and import• Classes are organised in packages.

• Single classes may be imported:

import java.util.ArrayList;

• Whole packages can be imported:

import java.util.*;

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Some of the Packages listed in the API

Packages are NOT hierarchicalUsing “import java.awt.*;” does not import java.awt.color.*;

Using Random• The library class Random can be used to generate random numbers

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import java.util.Random;...Random randomGenerator = new Random();...int index1 = randomGenerator.nextInt();int index2 = randomGenerator.nextInt(100);

Generating random responses

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public Responder(){ randomGenerator = new Random(); responses = new ArrayList<String>(); fillResponses();}

public String generateResponse(){ int index = randomGenerator.nextInt(responses.size()); return responses.get(index);}

public void fillResponses(){ ...}

Java Packages• An overview can be found at this link• http://java.sun.com/javase/6/docs/api/overview-summary.ht

ml

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Java Packages• Inside each package you will find

o InterfacesoClasseso Enumerationso Exceptions

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java.lang• “Provides classes that are fundamental to the design of the

Java programming language.”• Boolean, Byte, Double, Integer, Long, String • Math • System • Object

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java.util• “Contains the collections framework…date and

time facilities,… string tokenizer.. random-number generator..”

• ArrayList<E>, LinkedList<E>, Stack<E> • Calendar, Date • Formatter• Scanner• StringTokenizer• Random • Iterator<E>

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java.io• “Provides for system input and output through

data streams, serialization and the file system.”• Console • File • FileReader • BufferedInputStream • StreamTokenizer

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GUI libraries• java.awt

o java.awt.Event

• javax.imageio • javax.swing

o http://java.sun.com/docs/books/tutorial/uiswing/

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All classes are Objects

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Packages

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Reading & Exercises• P136 – p147• Collections

o http://java.sun.com/docs/books/tutorial/collections/index.html

• Swingo http://java.sun.com/docs/books/tutorial/ui/index.html

• Classes and Objectso http://java.sun.com/docs/books/tutorial/java/javaOO/inde

x.html

• Genericso http://java.sun.com/docs/books/tutorial/java/generics/ind

ex.htm

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References• http://java.sun.com/javase/reference/api.jsp• http://java.sun.com/docs/books/tutorial/java/package/mana

gingfiles.html• http://en.wikipedia.org/wiki/Classpath_(Java)• http://condor.depaul.edu/~glancast/224class/docs/lecJun16.

html

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