Advanced Java Workshop
John Cole
Senior Lecturer
Computer Science Department
The University of Texas at Dallas
August 23, 2014
http://www.utdallas.edu/~John.Cole
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Topics
• Programming Environments
• Introduction to classes
• Collection classes and generics: Set, List, Queue,
Vector, Map
• Basic Swing: JFrame, Panels, Labels, Buttons, Text
Fields, Combo Boxes, Lists
• Event-driven programming
• Text and Binary I/O
• Multithreading
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NetBeans
• URL: https://netbeans.org/downloads/7.4/
• Great general-purpose environment for most
Java development
• UTD doesn’t support 8.0 yet because of java
issues, but 7.4 is solid and will do everything
you want
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Eclipse with ADT
• http://developer.android.com/sdk/index.html
#download
• While you can install the Android Developer
Tools in NetBeans, I have had trouble with this
• Eclipse is a great general-purpose java
environment as well as having the Android
tools
• Get this if you’re taking CS6301: User Interface
Design
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Other Resources
• Both NetBeans and Eclipse have IntelliSense,
but you may need more than that.
• Java Docs:
http://docs.oracle.com/javase/7/docs/api/
• Java Tutorials:
http://docs.oracle.com/javase/tutorial/
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Classes
• There are no header files in Java.
• The import statement finds packages that your
program can use.
• When you create a class, the declaration looks
like this:
public class Rectangle
{
private double length;
private double width;
}
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Classes
• Any methods go in the braces. You don’t need to
use prototypes:
public class Rectangle
{
private double length;
private double width;
Public double getArea()
{
return length * width;
}
}
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Class Layout Conventions
• The suggested organization of a source code
file can vary by employer or instructor.
• A common organization is:
– Fields listed first
– Methods listed second
• Accessors and mutators are typically grouped.
• There are tools that can help in formatting
code to specific standards.
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Derived Classes
• You can derive a new class from an existing
one. A common thing to see in a GUI program
is:
public class MyFrame extends Jframe
• You can then write your own constructor and
other methods while using everything in
JFrame
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Interfaces
• An interface in java is like an abstract class
• You must provide concrete implementations of
all methods
• The keyword for using an interface is
implements
• You can implement multiple interfaces
public class c implements ActionListener
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Arrays
• An array in Java is not just a set of memory
locations; it is an actual object
• You create an array the way you would any
object:
int[] numbers = new int[6];
• Note that you can have a variable in place of
6, since this is being allocated dynamically
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Two-Dimensional Arrays
• A 2D array is essentially an array of arrays.
Thus you can have:
String[][] puzzle;
puzzle = new String[5][5];
puzzle[0][0] = “search”;
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Ragged Arrays
• Since a multidimensional array is an array of
arrays, not all rows need contain the same
number of columns:
int[][] triangle = {
{1, 2, 3, 4},
{5, 6, 7},
{8, 9},
{10}};
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Collection Classes
•
•
•
•
•
•
•
•
ArrayList
HashSet
LinkedHashSet
TreeSet
Vector
Stack
Queue
HashMap
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ArrayList
• If you need to support random access through
an index without inserting or removing
elements from any place other than the end,
ArrayList offers the most efficient collection
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HashSet
• The HashSet class is a concrete class that
implements Set. It can be used to store
duplicate-free elements. For efficiency, objects
added to a hash set need to implement the
hashCode method in a manner that properly
disperses the hash code.
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LinkedHashSet
• The elements in the HashSet are not ordered
• LinkedHashSet elements can be retrieved in
the order in which they were inserted
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TreeSet
• SortedSet is a subinterface of Set, which
guarantees that the elements in the set are
sorted.
• TreeSet is a concrete class that implements
the SortedSet interface.
• You can use an iterator to traverse the
elements in the sorted order.
• The elements can be sorted in two ways.
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TreeSet (Continued)
• The elements can be sorted in two ways:
• One way is to use the Comparable interface.
• The other way is to specify a comparator for
the elements in the set if the class for the
elements does not implement the
Comparable interface, or you don’t want to
use the compareTo method in the class that
implements the Comparable interface
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Vector
• In Java 2, Vector is the same as ArrayList,
except that Vector contains the synchronized
methods for accessing and modifying the
vector.
• None of the new collection data structures
introduced so far are synchronized (threadsafe)
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Stack
• The Stack class represents a last-in-first-out
stack of objects
• The elements are accessed only from the top
of the stack.
• You can retrieve, insert, or remove an element
from the top of the stack
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Queue
• A queue is a first-in/first-out data structure
• Elements are appended to the end of the
queue and are removed from the beginning of
the queue
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HashMap
• Efficient for locating a value, inserting a
mapping, and deleting a mapping
• Not sorted
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Generics
• You can specify the data types of the elements
of a collection.
HashMap h<String, double[]>
• This tells the compiler that the HashMap’s key
will be a string and its data will be an array of
doubles
• This is checked at compile time only.
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Data Structure Exercise
• Write a program that asks for the names and
ages of 5 people. Put the information into a
HashMap with the name as the key and the
age as the data.
• Do the same thing using a TreeMap
• Print the data from each structure using an
iterator
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Basic Swing
• import javax.swing.*
• Swing components are “lightweight” and very
portable.
• They don’t depend upon the underlying
operating system
• Don’t use the older AWT (Abstract Windowing
Toolkit) controls unless absolutely necessary
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JFrame
• When writing a GUI program, create a class
derived from JFrame.
• This lets you do initialization in the
constructor
• You can create controls that will show in the
window, for example
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JFrame
• Useful things in the constructor:
this.setTitle(("Graduate Java Workshop"));
this.setSize(500, 600);
setDefaultCloseOperation(JFrame.EXIT_ON_CLO
SE);
this.setVisible(true);
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Layout Managers
• JFrames and other containers can have a
layout that defines how contained graphical
objects appear
• This implies that you can have nested
containers, each with its own layout
• This was done because Java was intended to
be independent of screen resolution
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FlowLayout
• This is the simplest layout manager.
Components are placed left to right, wrapping
around at the edge of the container
• You can change the way the components are
aligned within the flow with constants:
CENTER, LEFT, and RIGHT.
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FlowLayout
Write a program that
adds three labels and
three text fields into
the content pane of a
frame with a
FlowLayout manager.
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The FlowLayout Class
java.awt.FlowLayout
The get and set methods for these data fields are provided in
the class, but omitted in the UML diagram for brevity.
-alignment: int
The alignment of this layout manager (default: CENTER).
-hgap: int
The horizontal gap of this layout manager (default: 5 pixels).
-vgap: int
The vertical gap of this layout manager (default: 5 pixels).
+FlowLayout()
Creates a default FlowLayout manager.
+FlowLayout(alignment: int)
Creates a FlowLayout manager with a specified alignment.
+FlowLayout(alignment: int, hgap:
int, vgap: int)
Creates a FlowLayout manager with a specified alignment,
horizontal gap, and vertical gap.
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GridLayout
• Controls are arranged in a grid. You specify the
number of rows and columns.
frame.setLayout(new GridLayout(3,5));
• One peculiarity is that everything in a grid cell is
sized to fit the cell.
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GridLayout
This example shows
using a GridLayout
with 3 rows and 2
columns to display
the labels and text
fields.
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The GridLayout Class
java.awt.GridLayout
The get and set methods for these data fields are provided in
the class, but omitted in the UML diagram for brevity.
-rows: int
The number of rows in this layout manager (default: 1).
-columns: int
The number of columns in this layout manager (default: 1).
-hgap: int
The horizontal gap of this layout manager (default: 0).
-vgap: int
The vertical gap of this layout manager (default: 0).
+GridLayout()
Creates a default GridLayout manager.
+GridLayout(rows: int, columns: int) Creates a GridLayout with a specified number of rows and columns.
+GridLayout(rows: int, columns: int, Creates a GridLayout manager with a specified number of rows and
hgap: int, vgap: int)
columns, horizontal gap, and vertical gap.
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BorderLayout
The BorderLayout
manager divides the
container into five areas:
East, South, West, North,
and Center. Components
are added to a
BorderLayout by using
the add method.
add(Component,
constraint), where
constraint is
BorderLayout.EAST,
BorderLayout.SOUTH,
BorderLayout.WEST,
BorderLayout.NORTH, or
BorderLayout.CENTER.
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BorderLayout
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The BorderLayout Class
java.awt.BorderLayout
The get and set methods for these data fields are provided in
the class, but omitted in the UML diagram for brevity.
-hgap: int
The horizontal gap of this layout manager (default: 0).
-vgap: int
The vertical gap of this layout manager (default: 0).
+BorderLayout()
Creates a default BorderLayout manager.
+BorderLayout(hgap: int, vgap: int) Creates a BorderLayout manager with a specified number of
horizontal gap, and vertical gap.
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Panels
A JPanel is a container that can be put into
another container.
• Panels have their own layout
Jpanel pnlTop = new JPanel();
pnlTop.setLayout(new GridLayout(4,3));
frame.add(pnlTop);
• Note that the panel will be added to the frame’s
layout.
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Controls
• The screen objects you see, such as text fields,
labels, checkboxes, buttons, and the like are
collectively referred to as controls.
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JLabel
• This is used to display text or an image, or both
• The constructors for labels are as follows:
JLabel()
JLabel(String text, int horizontalAlignment)
JLabel(String text)
JLabel(Icon icon)
JLabel(Icon icon, int horizontalAlignment)
JLabel(String text, Icon icon, int
horizontalAlignment)
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JTextField
A text field is an input area where the user can type in
characters. Text fields are useful in that they enable the user to
enter in variable data (such as a name or a description).
javax.swing.text.JTextComponent
The get and set methods for these data fields are provided in
the class, but omitted in the UML diagram for brevity.
-text: String
The text contained in this text component.
-editable: boolean
Indicates whether this text component is editable (default: true).
javax.swing.JTextField
-columns: int
The number of columns in this text field.
-horizontalAlignment: int
The horizontal alignment of this text field (default: LEFT).
+JTextField()
Creates a default empty text field with number of columns set to 0.
+JTextField(column: int)
Creates an empty text field with specified number of columns.
+JTextField(text: String)
Creates a text field initialized with the specified text.
+JTextField(text: String, columns: int)
Creates a text field initialized with the specified text and columns.
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JTextField Constructors
• JTextField(int columns)
Creates an empty text field with the specified
number of columns.
• JTextField(String text)
Creates a text field initialized with the specified
text.
• JTextField(String text, int
columns)
Creates a text field initialized with the
specified text and the column size.
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JTextField Properties
• text
• horizontalAlignment
• editable
• columns
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JTextField Methods
• getText()
Returns the string from the text field.
• setText(String text)
Puts the given string in the text field.
• setEditable(boolean editable)
Enables or disables the text field to be edited. By
default, editable is true.
• setColumns(int)
Sets the number of columns in this text field.
The length of the text field is changeable.
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JTextArea
If you want to let the user enter multiple lines of text, you cannot use
text fields unless you create several of them. The solution is to use
JTextArea, which enables the user to enter multiple lines of text.
javax.swing.text.JTextComponent
The get and set methods for these data fields are provided in
the class, but omitted in the UML diagram for brevity.
javax.swing.JTextArea
-columns: int
The number of columns in this text area.
-rows: int
The number of rows in this text area.
-tabSize: int
The number of characters used to expand tabs (default: 8).
-lineWrap: boolean
Indicates whether the line in the text area is automatically wrapped (default:
false).
-wrapStyleWord: boolean
Indicates whether the line is wrapped on words or characters (default: false).
+JTextArea()
Creates a default empty text area.
+JTextArea(rows: int, columns: int)
Creates an empty text area with the specified number of rows and columns.
+JTextArea(text: String)
Creates a new text area with the specified text displayed.
+JTextArea(text: String, rows: int, columns: int) Creates a new text area with the specified text and number of rows and columns.
Appends the string to text in the text area.
+append(s: String): void
+insert(s: String, pos: int): void
Inserts string s in the specified position in the text area.
+replaceRange(s: String, start: int, end: int):
void
Replaces partial text in the range from position start to end with string s.
+getLineCount(): int
Returns
the Java
actual
number of lines contained in the text area.
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JTextArea Constructors
• JTextArea(int rows, int
columns)
Creates a text area with the specified number
of rows and columns.
• JTextArea(String s, int rows,
int columns)
Creates a text area with the initial text and
the number of rows and columns specified.
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JTextArea Properties
•
•
•
•
•
•
•
•
text
editable
columns
lineWrap
wrapStyleWord
rows
lineCount
tabSize
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JComboBox
A combo box is a simple list of items from which the user can
choose. It performs basically the same function as a list, but
can get only one value.
javax.swing.JComponent
javax.swing.JComboBox
+JComboBox()
Creates a default empty combo box.
+JComboBox(items: Object[])
Creates a combo box that contains the elements in the specified array.
+addItem(item: Object): void
Adds an item to the combo box.
+getItemAt(index: int): Object
Returns the item at the specified index.
+getItemCount(): int
Returns the number of items in the combo box.
+getSelectedIndex(): int
Returns the index of the selected item.
+setSelectedIndex(index: int): void
Sets the selected index in the combo box.
+getSelectedItem(): Object
Returns the selected item.
+setSelectedItem(item: Object): void
Sets the selected item in the combo box.
+removeItem(anObject: Object): void Removes an item from the item list.
Removes the item at the specified index in the combo box.
+removeItemAt(anIndex: int): void
+removeAllItems(): void
Removes all items in the combo box.
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JComboBox Methods
To add an item to a JComboBox jcbo, use
jcbo.addItem(Object item)
To get an item from JComboBox jcbo, use
jcbo.getItem()
To get the index of an item, use
jcbo.getItemIndex()
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JList
• This shows a list of items and allows a single
selection or multiple selections
• The “model” is the data that is to be displayed
• The “view” is how you see that data
• The “controller” mediates between the model
and the view
• You can’t add items to a JLIst; you add them to
the model and they show up in the view
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JList
• The JList does not scroll. If you want it to, you
must place it in a JScrollPane
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Event-Driven Programming
• GUI programs (and Android apps) must be
able to respond to events
• The obvious events are button clicks,
keystrokes, etc.
• There are many others, including every mouse
motion
• In Android, there are touch-screen events and
sensor events, as well
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JButton
• JButton provides a simple button. It can have
text, an icon, or both
javax.swing.AbstractButton
javax.swing.JButton
+JButton()
Creates a default button with no text and icon.
+JButton(icon: javax.swing.Icon)
Creates a button with an icon.
+JButton(text: String)
Creates a button with text.
+JButton(text: String, icon: Icon)
Creates a button with text and an icon.
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JButton Constructors
The following are JButton constructors:
JButton()
JButton(String text)
JButton(String text, Icon icon)
JButton(Icon icon)
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JButton Properties
•
•
•
•
•
•
•
•
text
icon
mnemonic
horizontalAlignment
verticalAlignment
horizontalTextPosition
verticalTextPosition
iconTextGap
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JButton Events
• To respond to button clicks, add an event
handler:
JButton jbt = new Jbutton(“Press”);
Jbt.addActionListener(new ActionListener(){
Public void actionPerformed(ActionEvent e){
// Do something here.
}
});
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JComboBox State Change
Jcbo.AddItemListener(new cboStateChanged());
Class cboStateChanged implements ItemListener
{
public void itemStateChanged(ItemEvent e) {
// Make sure the source is a combo box
if (e.getSource() instanceof JComboBox)
String s = (String)e.getItem();
}
}
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Swing Exercise
• Write a program that displays three labels and
three text fields, a JList, and has two buttons:
Save and Cancel.
• The program should accept data into the text
fields, and display it in the JList when you
press the Save button.
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Text I/O
• You can use the Scanner object to read text
files.
• This requires two objects: File and Scanner,
per the following code:
Scanner fRead = null;
File fInput = null;
fInput = new File(“Filename”);
fRead = new Scanner(fInput);
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Text I/O
• The Scanner object has various methods:
• nextLine() is probably the most useful. It
reads a line of text up to the newline
character.
• nextInt() reads an integer up to the next nondigit.
• nextDouble() reads a double
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Text I/O
• You can write text to a file with the PrintWriter
class
File fOut = new File(“File.txt”);
PrintWriter pw = new PrintWriter(fOut);
pw.writeLine(“This will be written”);
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Binary I/O
• Not all data is text
• You may need to write code to deal with raw
data, such as various data structures
• Such data is not human-readable
• For example, Java source code is text, but
.class files are binary
• Serialization is good too, but sometimes not
sufficient
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Binary I/O
• When you write 65535 to a text file, Java
converts this binary number to a sequence of
characters and writes 10 bytes. (Unicode
requires 2 bytes per character.)
• When you write 65535 to a binary file, there is
no conversion to characters. Exactly 4 bytes
are written.
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Binary I/O
• A File object encapsulates the properties of a
file or a path, but does not contain the
methods for reading/writing data from/to a
file. In order to perform I/O, you need to
create objects using appropriate Java I/O
classes.
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Binary I/O Classes
FileInputStream
DataInputStream
InputStream
FilterInputStream
BufferedInputStream
ObjectInputStream
Object
OutputStream
FileOutputStream
BufferedOutputStream
FilterOutputStream
DataOutputStream
ObjectOutputStream
PrintStream
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InputStream
java.io.InputStream
+read(): int
Reads the next byte of data from the input stream. The value byte is returned as
an int value in the range 0 to 255. If no byte is available because the end of
the stream has been reached, the value –1 is returned.
+read(b: byte[]): int
Reads up to b.length bytes into array b from the input stream and returns the
actual number of bytes read. Returns -1 at the end of the stream.
+read(b: byte[], off: int,
len: int): int
Reads bytes from the input stream and stores into b[off], b[off+1], …,
b[off+len-1]. The actual number of bytes read is returned. Returns -1 at the
end of the stream.
+available(): int
Returns the number of bytes that can be read from the input stream.
+close(): void
Closes this input stream and releases any system resources associated with the
stream.
+skip(n: long): long
Skips over and discards n bytes of data from this input stream. The actual
number of bytes skipped is returned.
+markSupported(): boolean Tests if this input stream supports the mark and reset methods.
+mark(readlimit: int): void Marks the current position in this input stream.
+reset(): void
Repositions this stream to the position at the time the mark method was last
called on this input stream.
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OutputStream
The value is a byte as an int type.
java.io.OutputStream
+write(int b): void
Writes the specified byte to this output stream. The parameter b is an int value.
(byte)b is written to the output stream.
+write(b: byte[]): void
Writes all the bytes in array b to the output stream.
+write(b: byte[], off: int, Writes b[off], b[off+1], …, b[off+len-1] into the output stream.
len: int): void
+close(): void
Closes this input stream and releases any system resources associated with the
stream.
+flush(): void
Flushes this output stream and forces any buffered output bytes to be written out.
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RandomAccessFile
• This is probably the most useful class
• All of the streams you have used so far are known
as read-only or write-only streams. The external
files of these streams are sequential files that
cannot be updated without creating a new file. It
is often necessary to modify files or to insert new
records into files. Java provides the
RandomAccessFile class to allow a file to be read
from and write to at random locations.
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DataInput
DataInput
java.io.RandomAccessFile
+RandomAccessFile(file: File, mode:
String)
Creates a RandomAccessFile stream with the specified File object and
mode.
+RandomAccessFile(name: String,
mode: String)
Creates a RandomAccessFile stream with the specified file name
string and mode.
+close(): void
Closes the stream and releases the resource associated with the stream.
+getFilePointer(): long
Returns the offset, in bytes, from the beginning of the file to where the
next read or write occurs.
+length(): long
Returns the length of this file.
+read(): int
Reads a byte of data from this file and returns –1 an the end of stream.
+read(b: byte[]): int
Reads up to b.length bytes of data from this file into an array of bytes.
+read(b: byte[], off: int, len: int) : int
Reads up to len bytes of data from this file into an array of bytes.
+seek(long pos): void
Sets the offset (in bytes specified in pos) from the beginning of the
stream to where the next read or write occurs.
+setLength(newLength: long): void
Sets a new length of this file.
+skipBytes(int n): int
Skips over n bytes of input discarding the skipped bytes.
+write(b: byte[]): void
+write(byte b[], int off, int len)
Writes b.length bytes from the specified byte array to this file, starting
at the current file pointer.
+write(b: byte[], off: int, len: int):
void
Writes len bytes from the specified byte array starting at offset off to
this file.
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DataInputStream/DataOutputStream
• DataInputStream reads bytes from the stream
and converts them into appropriate primitive
type values or strings.
• DataOutputStream converts primitive type
values or strings into bytes and output the
bytes to the stream.
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DataInputStream
InputStream
FilterInputStream
DataInputStream
+DataInputStream(
in: InputStream)
java.io.DataInput
+readBoolean(): boolean Reads a Boolean from the input stream.
+readByte(): byte
Reads a byte from the input stream.
+readChar(): char
Reads a character from the input stream.
+readFloat(): float
Reads a float from the input stream.
+readDouble(): float
Reads a double from the input stream.
+readInt(): int
Reads an int from the input stream.
+readLong(): long
Reads a long from the input stream.
+readShort(): short
Reads a short from the input stream.
+readLine(): String
Reads a line of characters from input.
+readUTF(): String
Reads a string in UTF format.
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DataOutputStream
OutputStream
FilterOutputStream
DataOutputStream
+DataOutputStream(
out: OutputStream)
java.io.DataOutput
+writeBoolean(b: Boolean): void Writes a Boolean to the output stream.
+writeByte(v: int): void
Writes to the output stream the eight low-order bits
of the argument v.
+writeBytes(s: String): void
Writes the lower byte of the characters in a string to
the output stream.
+writeChar(c: char): void
Writes a character (composed of two bytes) to the
output stream.
+writeChars(s: String): void
Writes every character in the string s, to the output
stream, in order, two bytes per character.
+writeFloat(v: float): void
Writes a float value to the output stream.
+writeDouble(v: float): void
Writes a double value to the output stream.
+writeInt(v: int): void
Writes an int value to the output stream.
+writeLong(v: long): void
Writes a long value to the output stream.
+writeShort(v: short): void
Writes a short value to the output stream.
+writeUTF(s: String): void
Writes two bytes of length information to the output
stream, followed by the UTF representation of
every character in the string s.
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Characters and Strings in Binary I/O
• A Unicode consists of two bytes. The
writeChar(char c) method writes the Unicode
of character c to the output. The
writeChars(String s) method writes the
Unicode for each character in the string s to
the output.
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What is UTF-8?
• UTF-8 is a coding scheme that allows systems to
operate with both ASCII and Unicode efficiently. Most
operating systems use ASCII. Java uses Unicode. The
ASCII character set is a subset of the Unicode character
set. Since most applications need only the ASCII
character set, it is a waste to represent an 8-bit ASCII
character as a 16-bit Unicode character. The UTF-8 is an
alternative scheme that stores a character using 1, 2, or
3 bytes. ASCII values (less than 0x7F) are coded in one
byte. Unicode values less than 0x7FF are coded in two
bytes. Other Unicode values are coded in three bytes.
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Order and Format
• CAUTION: You have to read the data in the
same order and same format in which they
are stored. For example, if names are written
in UTF-8 using writeUTF, you must read names
using readUTF.
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Checking End of File
• TIP: If you keep reading data at the end of a
stream, an EOFException would occur. So how
do you check the end of a file? You can use
input.available() to check it. input.available()
== 0 indicates that it is the end of a file.
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File Pointer
• The File Pointer is the position in the file
where the next read or write will occur
• When you open the file, this is set to zero
• It is incremented by reading or writing
• For example, if you read an int, the file
pointer moves forward by 4 bytes
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RandomAccessFile Methods
• Many methods in RandomAccessFile are
the same as those in DataInputStream
and DataOutputStream. For example,
readInt(), readLong(),
writeDouble(), readLine(),
writeInt(), and writeLong() can be
used in data input stream or data output
stream as well as in RandomAccessFile
streams.
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RandomAccessFile Methods
• void seek(long pos) throws IOException;
Sets the offset from the beginning of the
RandomAccessFile stream to where the next read
or write occurs.
• long getFilePointer() IOException;
Returns the current offset, in bytes, from the
beginning of the file to where the next read
or write occurs.
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RandomAccessFile Methods
• long length()throws IOException
Returns the length of the file.
• final void writeChar(int v) throws
IOException
Writes a character to the file as a two-byte
Unicode, with the high byte written first.
• final void writeChars(String s)
throws IOException
Writes a string to the file as a sequence of
characters.
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RandomAccessFile Constructor
RandomAccessFile raf = new
RandomAccessFile("test.dat", "rw");
//allows read and write
RandomAccessFile raf = new
RandomAccessFile("test.dat","r");
//read only
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Random Access File Exercise
• Write a program that, upon initialization,
writes the integers 1-100 into the first 400
bytes of a file. It should then write 100 fourcharacter sequences from A001 through A100
into the next 400 bytes.
• The program should then ask the user for a
number from 1-100 and read the
corresponding four-character sequence. The
program terminates when the user enters 0.
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Multithreading
• Your program can have multiple threads
running at the same time. They can run on
the same CPU and time-slice, or run on
different CPUs.
• Threads must be careful not to interfere with
each other.
• See Edsger Dijkstra, “Cooperating Sequential
Processes”
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Creating Tasks and Threads
java.lang.Runnable
TaskClass
// Custom task class
public class TaskClass implements Runnable {
...
public TaskClass(...) {
...
}
// Client class
public class Client {
...
public void someMethod() {
...
// Create an instance of TaskClass
TaskClass task = new TaskClass(...);
// Create a thread
Thread thread = new Thread(task);
// Implement the run method in Runnable
public void run() {
// Tell system how to run custom thread
...
}
...
}
// Start a thread
thread.start();
...
}
...
}
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Multithreading
• In the previous example, the Run method in
the program fragment on the left is started by
the thread.start() function on the right.
• The function on the right that started the
thread continues execution after the
thread.start() call
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The Thread Class
«interface»
java.lang.Runnable
java.lang.Thread
+Thread()
Creates a default thread.
+Thread(task: Runnable)
Creates a thread for a specified task.
+start(): void
Starts the thread that causes the run() method to be invoked by the JVM.
+isAlive(): boolean
Tests whether the thread is currently running.
+setPriority(p: int): void
Sets priority p (ranging from 1 to 10) for this thread.
+join(): void
Waits for this thread to finish.
+sleep(millis: long): void
Puts the runnable object to sleep for a specified time in milliseconds.
+yield(): void
Causes this thread to temporarily pause and allow other threads to execute.
+interrupt(): void
Interrupts this thread.
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The Static yield() Method
You can use the yield() method to temporarily release time
for other threads. For example, suppose you modify the
code in Lines 53-57 in TaskThreadDemo.java as follows:
public void run() {
for (int i = 1; i <= lastNum; i++) {
System.out.print(" " + i);
Thread.yield();
}
}
Every time a number is printed, the print100 thread is
yielded. So, the numbers are printed after the characters.
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The Static sleep(milliseconds) Method
The sleep(long mills) method puts the thread to sleep for the specified
time in milliseconds. For example, suppose you modify the code in
Lines 53-57 in TaskThreadDemo.java as follows:
public void run() {
for (int i = 1; i <= lastNum; i++) {
System.out.print(" " + i);
try {
if (i >= 50) Thread.sleep(1);
}
catch (InterruptedException ex) {
}
}
}
Every time a number (>= 50) is printed, the print100 thread is put to
sleep for 1 millisecond.
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The join() Method
You can use the join() method to force one thread to wait for another
thread to finish. For example, suppose you modify the code in Lines
53-57 in TaskThreadDemo.java as follows:
Thread
public void run() {
print100
Thread thread4 = new Thread(
new PrintChar('c', 40));
-char token
thread4.start();
try {
+getToken
for (int i = 1; i <= lastNum; i++) {
printA.join()
+setToken
System.out.print(" " + i);
+paintCompo
Wait for printA
-char
if (i == 50) thread4.join();
net token
to finish
+mouseClicke
}
+getToken
d
}
+getToken +setToken
catch (InterruptedException ex) {
+setToken +paintCompone
}
t
+paintComponet
}
+mouseClicked
Thread
printA
-char token
+getToken
+setToken
+paintCompo
net
+mouseClicke
d
printA finished
-char token
The numbers after 50 are printed after thread printA is finished.
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isAlive(), interrupt(), and
isInterrupted()
• The isAlive() method is used to find out the state
of a thread. It returns true if a thread is in the
Ready, Blocked, or Running state; it returns false
if a thread is new and has not started or if it is
finished.
• The interrupt() method interrupts a thread in the
following way: If a thread is currently in the
Ready or Running state, its interrupted flag is set;
if a thread is currently blocked, it is awakened
and enters the Ready state, and an
java.io.InterruptedException is thrown.
• The isInterrupted() method tests whether the
thread is interrupted
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GUI Event Dispatcher Thread
• GUI event handling and painting code
executes in a single thread, called the event
dispatcher thread. This ensures that each
event handler finishes executing before the
next one executes and the painting isn’t
interrupted by events.
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Thread Synchronization
A shared resource may be corrupted if it is
accessed simultaneously by multiple threads. For
example, two unsynchronized threads accessing
the same bank account may cause conflict.
Step
balance
thread[i]
thread[j]
1
2
3
4
0
0
1
1
newBalance = bank.getBalance() + 1;
newBalance = bank.getBalance() + 1;
bank.setBalance(newBalance);
bank.setBalance(newBalance);
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The synchronized keyword
To avoid race conditions, more than one thread must be
prevented from simultaneously entering certain part of the
program, known as critical section. The critical section in the
program is the entire deposit method. You can use the
synchronized keyword to synchronize the method so that only
one thread can access the method at a time. There are several
ways to correct the problem. One approach is to make Account
thread-safe by adding the synchronized keyword in the deposit
method in Line 45 as follows:
public synchronized void deposit(double amount)
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Deadlock
Sometimes two or more threads need to acquire the locks on several shared
objects. This could cause deadlock, in which each thread has the lock on one of
the objects and is waiting for the lock on the other object. Consider the scenario
with two threads and two objects, as shown in Figure 29.15. Thread 1 acquired a
lock on object1 and Thread 2 acquired a lock on object2. Now Thread 1 is waiting
for the lock on object2 and Thread 2 for the lock on object1. The two threads wait
for each other to release the in order to get the lock, and neither can continue to
run.
Step
Thread 2
Thread 1
1
2
3
4
5
6
synchronized (object1) {
synchronized (object2) {
// do something here
// do something here
synchronized (object2) {
synchronized (object1) {
// do something here
}
// do something here
}
}
Wait for Thread 2 to
release the lock on object2
}
Wait for Thread 1 to
release the lock on object1
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