Lecture 6

Java I/O

Exceptions

Basic Swing

Java I/O classes

Flexible and somewhat slick, but a bit of a mess

Java classes for doing i/o

Includes file i/o, memory i/o, socket i/o, inter-process (pipes), etc.

All stored in package java.io Excellent example of OO design

– Very general and scaleable

Unfortunately, also obfuscates simple tasks. How to proceed

– Understand basic design

– Create some libraries to do common tasks

InputStream/OutputStream Start by studying the java.io.InputStream and java.

io.OutputStream API These are base class for performing all binary i/o Note that these classes are abstract each with a

single abstract method– abstract int read()– abstract void write(int)

Concrete subclasses must provide implementation of read/write that can get/put a single byte to/from the relevant source

Concrete subclasses of InputStream/OutputStream Since InputStream/OutputStream are

abstract, they cannot be used to create objects (of course, they can be used for typing).

A very common non-abstract subclass is FileOutputStream/FileInputStream.

These can be used in a simple way to do the most basic byte-based file io

Example with FileInputStream

/* class example DataInput1.java *//* assumes each char is one byte -- dangerousimport java.io.FileInputStream;public class DataInput1{ public static void main(String[] args) throws Exception{ String file = args[0]; int input; FileInputStream fin = new FileInputStream(file); while ( (input = fin.read()) != -1){ System.out.print((char) input); } }}

Example with FileOutputStream/* class example DataOutput1.java *//* assumes each char is a single byte */import java.io.FileOutputStream;public class DataOutput1{ public static void main(String[] args) throws Exception{ String file = args[0]; String output = "Hello World"; FileOutputStream fout = new FileOutputStream(file); char[] outputAsChars = output.toCharArray();

for (int i = 0; i < outputAsChars.length; ++i) fout.write(outputAsChars[i]); }}

Higher-level functionality FileInputStream and FileOuputStream allow you

to do pretty much any file i/o at a very low level. However, this is too low-level for Java. Java provides many more libraries to read/write

higher-level constructs:– characters– Strings– native datatypes– arrays – arbitrary objects (serialization)

Decorator Pattern

These capabilities are added using a design called the Decorator Pattern.

Purpose of Decorator

Best way to think of this is as follows:– There are two important issues when constructing an

i/o library• Where the i/o is going (file, etc).

• How the data is represented (String, native type, etc.)

– Rather than create a class for each combination, Decorator classes allow you to mix and match, augment functionality of base classes.

– This is a bit confusing but is very flexible.

– Decotators can also add other capabilities, such as peek ahead, push back, write line number, etc.

Java i/o decorators All Java i/o decorator classes inherit from

FilterInputStream and FilterOutputStream Look at the api for these classes and note a few

things:– They wrap instances of InputStream/OutputStream

respectively.– They inherit from InputStream/OutputStream

respectively This is an odd inheritence hierarchy but is

necessary to ensure that the FilterStreams support the same interface as the underlying class.

More on Filter Streams

Easiest way to think of the filter streams as wrapping an underlying class which they augment the functionality of.

Consider the respective constructors– FilterInputStream(InputStream in);– FilterOutputStream(OutputStream out);

In each case, the FilterStreams use an underlying presumably simpler inputstream and augment its functionality.

Some FilterStream examples to clarify this Perhaps most common FilterInputStream is

DataInputStream. Study the API and be sure you understand

the inheritance hierarchy DataInputStream stores an InputStream and

uses this to do higher-level i/o– readInt, readDouble, etc.

DataOutputStream is analogous

Example of DataInputStream

/* DataInputStream2 example in course examples */import java.io.DataOutputStream;import java.io.FileOutputStream;

public class DataOutput2{ public static void main(String[] args) throws Exception{ String file = args[0]; double[] data = {1.1,1.2,1.3,1.4,1.5}; DataOutputStream dout = new DataOutputStream (new FileOutputStream(file));

for (int i = 0; i < data.length; ++i){ dout.writeDouble(data[i]); } dout.close();}}

Example of DataInputStream/* DataOutput2 example in course examples */import java.io.DataInputStream;import java.io.FileInputStream;import java.io.EOFException;public class DataInput2{ public static void main(String[] args) throws Exception{ String file = args[0]; DataInputStream din = new DataInputStream(new FileInputStream(file)); double data; /* need an exception to know when end of file is hit */ try{ while (true){ data = din.readDouble(); System.out.println(data); } } catch (EOFException eofe){} din.close();}}

BufferedInputStream/BufferedOutputStream Another common set of decorator classes is

BufferedInputStream and BufferedOutputStream.

These augment the functionality of the underlying stream by providing system buffering for higher-performance i/o

They also add support for the mark method. Examples on next slide ...

BufferedInputStream Exampleimport java.io.*; /public class DataInput3{ public static void main(String[] args) throws Exception{ String file = args[0]; DataInputStream din = new DataInputStream (new BufferedInputStream (new FileInputStream(file))); double data; /* need an exception to know when end of file is hit */ try{ while (true){ data = din.readDouble(); System.out.println(data); } } catch (EOFException eofe){} din.close();}}

BufferedOutputStream exampleimport java.io.BufferedOutputStream;import java.io.DataOutputStream;import java.io.FileOutputStream;

public class DataOutput3{

public static void main(String[] args) throws Exception{ String file = args[0]; double[] data = {1.1,1.2,1.3,1.4,1.5}; DataOutputStream dout = new DataOutputStream (new BufferedOutputStream (new FileOutputStream(file)));

for (int i = 0; i < data.length; ++i){ dout.writeDouble(data[i]); } dout.close();}}

Other output streams

FileOutputStream is probably the most common. However, note that we could replace

FileOutputStream with other Outputstream in these examples.

In this case, the same decorated or undecorated data will be sent to some other device.

Good example of this is thread communicatoin, memory i/o, and socket i/o (using Socket class).

Character-based i/o

Reader and Writer classes

Reader/Writer

Java maintains a second class hierarchy for performing higher-level character-based i/o.

The two base classes in this case are– java.io.Reader – java.io.Writer

Study the API for these classes. Very similar to InputStream/OutputStream Here I’ll show how to do some common i/o

tasks as examples

Atypical FileWriter Example/* example Writer1.java in course examples *//* using a simple FileWriter for String-based i/o */import java.io.FileWriter;public class Writer1{ public static void main(String[] args) throws Exception{ String file = args[0]; String output = "Hello World!"; FileWriter fw = new FileWriter(file); fw.write(output); fw.close(); }}

Reading lines from stdinimport java.io.BufferedReader;import java.io.InputStreamReader;public class Reader1{ public static void main(String[] args) throws Exception{ /* convert System.in, which is an InputStream, to a Reader by wrapping in InputStreamReader, then wrap everything in BufferedReader */ String input; BufferedReader bin = new BufferedReader (new InputStreamReader (System.in)); while ( (input = bin.readLine()) != null){ System.out.println("you typed " + input); }}}

Reading by line from file

import java.io.BufferedReader; /*Reader2.java */import java.io.InputStreamReader;import java.io.FileInputStream;public class Reader2{ public static void main(String[] args) throws Exception{ /* convert a FileInputStream, which is an InputStream, to a Reader by wrapping in InputStreamReader, then wrap everything in BufferedReader and call the readLine method to get a line at a time */ String input; String file = args[0]; BufferedReader bin = new BufferedReader (new InputStreamReader (new FileInputStream(file))); while ( (input = bin.readLine()) != null){ System.out.println(input); } }}

Exercise

Study the jdk API for GZIPOutputStream and GZIPInputStream. Write a program that reads and writes gzip files.

Serialization

Objects can be written to streams also. This process is known as serialization.

This is a huge convenience compared with having to marshal and unmarshal iv’s.

But the issue is even deeper – how are methods represented, objects that contain objects as iv’s, etc.

Java takes care of all of this with a very nice serialization interface.

Serialization classes Relevant classes

– java.io.ObjectInputStream – java.io.ObjectOutputStream

Note that these required an underlying Input/OutputStream to do their work.

For a class to be serializable, it also must implement the Serializable interface (no methods).

Finally, a class-scope variable can be declared as transient, meaning that it is ignored during serialization.

Serialization Example/* simple example of Serialization -- writing an object directly to an OutputStream without having to marshal and unmarshal */import java.io.*;public class Serialization{ public static void main(String[] args) throws Exception{ String flag = args[0]; String file = args[1]; Currency c = new Currency("US Dollar", "USD“, 10, 5); Currency d; if (flag.equals("-w")){ ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream(new File(file))); out.writeObject(c); } else if (flag.equals("-r")){ ObjectInputStream in = new ObjectInputStream(new FileInputStream(new File(file))); System.out.println("Reading serialized object"); d = (Currency) in.readObject(); }}}

Related Topics

java.io.File class– Very nice. Many methods for portably

manipulating files java.io.Socket class

– Provides Input/OutputStreams for communication across ports of different computers

PrintWriter class (e.g. println method) Writing zip files, jar files, etc.

Suggested Readings

Eckel’s detailed section on i/o Patterns in Java, A Catalog of Reusable

Design Patterns Illustratred with UML, Mark Grand, Wiley Press.

Design Patterns, Elements of Reusable Object-Oriented Software, Gamma et al.

Java Exceptions

Intro to Exceptions

What are exceptions?– Events that occur during the execution of a

program that interrupt the normal flow of control.

One technique for handling Exceptions is to use return statements in method calls.

This is fine, but java provides a much more general and flexible formalism that forces programmers to consider exceptional cases.

Exception Class hierarchy

Object

Exception Error

Throwable

NullPointerException

RuntimeExceptionmany

IndexOutOfBounds

• must handle• may handle• too serious to catch

Exception Handling Basics

Three parts to Exception handling1. claiming exception

2. throwing exception

3. catching exception

A method has the option of throwing one or more exceptions when specified conditions occur. This exception must be claimed by the method. Another method calling this method must either catch or rethrow the exception. (unless it is a RuntimeException)

Claiming Exceptions

Method declaration must specify every exception that the method potentially throws

MethodDeclaration throws Exception1, Exception2, ..., ExceptionN

Exceptions themselves are concrete subclasses of Throwable and must be defined and locatable in regular way.

Throwing Exception To throw an Exception, use the throw keyword

followed by an instance of the Exception classvoid foo() throws SomeException{ if (whatever) {...} else{ throw new SomeException(...)}

We’ll talk about passing data via the Exception constructor soon.

Note that if a method foo has a throw clause within it, that the Exception that is thrown (or one of its superclasses) must be claimed after the signature.

Catching Exceptions The third piece of the picture is catching

exceptions. This is what you will do with most commonly,

since many of java’s library methods are defined to throw one or more runtime exception.

Catching exceptions:– When a method is called that throws and Exception e.g

SomeException, it must be called in a try-catch block:try{ foo(); }catch(SomeException se){...}

Catching Exceptions, cont.

Note that if a method throws an Exception that is NOT a RuntimeException, you must do one of two things:– try-catch it (often called handling it)– rethrow it

In the latter case, responsibility then moves up the calling chain to handle it, and so on all the way up to main.

More on try-catchThe general form of the try-catch structure is:

try{ /* any number of lines of code that call any number of methods with any thrown Exceptions */ } catch(Exception1 e1){ /* do anything you want here e.g. change value and try again. print error and quit print stacktrace */ catch (Exception2 e2){ /* any number of exceptions can be handled ... */ }

Example1import java.io.*;

public class Exception1{ public static void main(String[] args){ InputStream f; try{ f = new FileInputStream("foo.txt"); } catch(FileNotFoundException fnfe){ System.out.println(fnfe.getMessage()); } }}

Example2import java.io.*; public class Exception2{ public static void main(String[] args){ InputStream fin; try{ fin = new FileInputStream("foo.txt"); int input = fin.read(); } catch(FileNotFoundException fnfe){ System.out.println(fnfe.getMessage()); } catch(IOException ioe){ System.out.println(ioe.getMessage()); } }}

import java.io.*; public class Exception2{ public static void main(String[] args){ InputStream fin; try{ fin = new FileInputStream("foo.txt"); int input = fin.read(); } catch(FileNotFoundException fnfe){ System.out.println(fnfe.getMessage()); } catch(IOException ioe){ System.out.println(ioe.getMessage()); } }}

Recommendations

Do not use Exceptions to handle normal conditions in the program that can be checked with if statements. For example:– to find the end of an array– to check if an object is null

See other commented examples in course notes.

Creating your own Exceptions

You can follow this procedure exactly when creating your own Exception.

Create a class that subclasses Exception (or RuntimeException).

You may also add functionality so that a relevant message is stored when the error is thrown, and any other customized functionality you choose.

See Exception5.java example

Overview of Swing

Widgets, layouts, events

Miscellaneous tidbits

Creating user interfaces in Java

Three steps for simple GUI– Determine what components that you would

like your “form” to contain. Examples are buttons, checkboxes, text areas, graphics panels, etc.

– Determine how you would like these panels to layout on your form. This is the domain of layout managers.

– Write the event handlers for the GUI.

Creating a GUI application

Must create a JFrame and call the show() method. This is normally done by creating a class that extends JFrame as:

class MyFrame extends JFrame{ MyFrame(){ setSize(300,300); //sets the pixel size } main(){ MyFrame frame = new MyFrame(); frame.show(); }}

Swing Components

Swing components, cont.

Each component is a Java class with a fairly extensive inheritency hierarchy:

Object

Component

Container

JComponent

JPanel

Window

Frame

JFrame

Using Swing Components

Very simple, just create object from appropriate class – examples:– JButton but = new JButton();– JTextField text = new JTextField();– JTextArea text = new JTextArea();– JLabel lab = new JLabel();

Many more classes. Don’t need to know every one to get started.

See ch. 9 Hortsmann

Adding components

Once a component is created, it can be added to a container by calling the container’s add method:

Container cp = getContentPane();

cp.add(new JButton(“cancel”));

cp.add(new JButton(“go”));

How these are laid out is determined by the layout manager.

Laying out components Not so difficult but takes a little practice Do not use absolute positioning – not very portable,

does not resize well, etc. Use layout managers – basically tells form how to

align components when they’re added. Each Container has a layout manager associated with

it. A JPanel is a Container – to have different layout

managers associated with different parts of a form, tile with JPanels and set the desired layout manager for each JPanel, then add components directly to panels.

Layout Managers

Java comes with 7 or 8. Most common and easiest to use are– FlowLayout– BorderLayout– GridLayout

Using just these three it is possible to attain fairly precise layout for most simple applications.

Setting layout managers

Very easy to associate a layout manager with a component. Simply call the setLayout method on the Container:

JPanel p1 = new JPanel();p1.setLayout(new FlowLayout(FlowLayout.LEFT));

JPanel p2 = new JPanel();p2.setLayout(new BorderLayout());

As Components are added to the container, the layout manager determines their size and positioning.

Event handling

What are events?

All components can listen for one or more events. Typical examples are:

– Mouse movements

– Mouse clicks

– Hitting any key

– Hitting return key

– etc.

Telling the GUI what to do when a particular event occurs is the role of the event handler.

ActionEvent

In Java, most components have a special event called an ActionEvent.

This is loosely speaking the most common or canonical event for that component.

A good example is a click for a button. To have any component listen for

ActionEvents, you must register the component with an ActionListener. e.g.– button.addActionListener(new MyAL());

Delegation, cont.

This is referred to as the Delegation Model. When you register an ActionListener with a

component, you must pass it the class which will handle the event – that is, do the work when the event is triggered.

For an ActionEvent, this class must implement the ActionListener interface.

This is simple a way of guaranteeing that the actionPerformed method is defined.

actionPerformed The actionPerformed method has the following

signature:void actionPerformed(ActionEvent)

The object of type ActionEvent passed to the event handler is used to query information about the event.

Some common methods are:– getSource()

• object reference to component generating event

– getActionCommand()• some text associated with event (text on button, etc).

actionPerformed, cont.

These methods are particularly useful when using one eventhandler for multiple components.

Graphics Programming

Using Graphics object

Use JPanels – They have a surface on which you can draw.– They are containers and thus can hold other

components

How to draw on a JpanelClass MyPanel extends JPanel{ public void paintComponent(Graphics g){ //use Graphics methods to draw }}

Drawing on a JPanel

How does this work?– Framework calls paintComponent

automatically whenever the application needs to be redrawn.

– Can force this yourself, but do not call paintComponent directly.

– Instead, call repaint(), which will cause paintComponent to be called for all components.

Graphics object method

Framework hand over an object that implements the abstract class Graphics.

Contains functions for drawing shapes to panel.

Simplest Example:

Class HelloWorld extends Jpanel{ public void paintComponent(Graphics g){ g.drawString(“Hello World, 50,50”); }}

More Graphics Object Methods

Many more methods. Consult API as always.

Typical examples:– drawOval– drawCircle– drawImage– drawPolyLine– fillArc

Graphics2D Object

Much more sophisticated rendering capabilites.

To access, use same technique and then downcast Graphics object to Graphics2D object (latter is subclass of former).

See Graphics2D API for additional methods See DrawTest.java and

Colors Use Color class

– 13 standard colors stored as static variables• Color.red, Color.blue, Color.yellow, etc.

– To specify rbg value, create Color object:• new Color(int red, int green, int blue);

Use setPaint(Color) method of Graphics object to specify a Color.

Also setBackground(Color) method Useful: brighter() and darker() methods for Color

objects

Filling shapes

Can fill the interior of any closed shape. Use fill() method. See FillTest.java

Drawing images

Very fun Supports standard GIF images.

Example

Toolkit tk = getDefaultToolkit();

Image img = tk.getImage(“foo.gif);

drawImage(img, 0, 0, null);

Drawing images

Very fun Supports standard GIF images.

Example

Toolkit tk = getDefaultToolkit();

Image img = tk.getImage(“foo.gif);

drawImage(img, 0, 0, null);

Odds and Ends

Timing, system commands, toString and .equals

Timing Java code

Simplest is to use static method in System class double currentTimeMillis();

Example:

public static void main (String args[]){ double begin = System.CurrentTimeMillis();

doWork();

double end = System.CurrentTimeMillis();

double time = (end – begin)/(1000.*60);

System.out.println(“Total time minutes: “ + time);

}

Spawning an OS process Very simple but a little different architecturally Every java program keeps a single copy of a class

Runtime which allows the user to interact with the OS

This is obtained with a static method call (“Singleton pattern”) and returns a Process object:

Runtime rt = Runtime.getRuntime(); Process p = rt.exec(“ls *.java”); InputStream in = p.getInputStream(); //question: how to wrap in a reader??

Overriding equals method

A method boolean equals(Object) also exists in the object class.

This method by default determines whether two Objects point to the same memory location:

SomeObject o1, o2;

o1 = new SomeObject(); o2 = o1;

if (o1.equals(o2)) // yes, same loc in mem This is not very useful in general. Good idea to

override with more useful comparison metric

.equals, cont.

Very good example is String class. .equals is overriden for String class to

actually do a character by character comparison.

Thus, two String which occupy different memory but have the same contents are considered equal.

Note that using == always does a memory comparison! Very import!!