advanced java workshop

Advanced Java Workshop 1

Advanced Java Workshop

John ColeSenior Lecturer

Computer Science DepartmentThe University of Texas at Dallas

August 23, 2014http://www.utdallas.edu/~John.Cole


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


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

https://netbeans.org/downloads/7.4/

https://netbeans.org/downloads/7.4/


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

http://developer.android.com/sdk/index.html#download




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/

http://docs.oracle.com/javase/7/docs/api/

http://docs.oracle.com/javase/7/docs/api/

http://docs.oracle.com/javase/tutorial/


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;

}


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;}

}


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.


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


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 interfacespublic class c implements ActionListener


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


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”;


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}};


Collection Classes

• ArrayList• HashSet• LinkedHashSet• TreeSet• Vector• Stack• Queue• HashMap


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


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.


LinkedHashSet

• The elements in the HashSet are not ordered• LinkedHashSet elements can be retrieved in

the order in which they were inserted


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.


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


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 (thread-safe)


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


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


HashMap

• Efficient for locating a value, inserting a mapping, and deleting a mapping

• Not sorted


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.


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


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


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


JFrame

• Useful things in the constructor:this.setTitle(("Graduate Java Workshop"));this.setSize(500, 600); setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);this.setVisible(true);


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


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.


FlowLayout

Write a program that adds three labels and three text fields into the content pane of a frame with a FlowLayout manager.


The FlowLayout Class

java.awt.FlowLayout

-alignment: int

-hgap: int

-vgap: int

+FlowLayout()

+FlowLayout(alignment: int)

+FlowLayout(alignment: int, hgap: int, vgap: int)

The alignment of this layout manager (default: CENTER).

The horizontal gap of this layout manager (default: 5 pixels).

The vertical gap of this layout manager (default: 5 pixels).

Creates a default FlowLayout manager.

Creates a FlowLayout manager with a specified alignment.

Creates a FlowLayout manager with a specified alignment, horizontal gap, and vertical gap.

The get and set methods for these data fields are provided in the class, but omitted in the UML diagram for brevity.


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.


GridLayout

This example shows using a GridLayout with 3 rows and 2 columns to display the labels and text fields.


The GridLayout Class

java.awt.GridLayout

-rows: int

-columns: int

-hgap: int

-vgap: int

+GridLayout()

+GridLayout(rows: int, columns: int)

+GridLayout(rows: int, columns: int, hgap: int, vgap: int)

The number of rows in this layout manager (default: 1).

The number of columns in this layout manager (default: 1).

The horizontal gap of this layout manager (default: 0).

The vertical gap of this layout manager (default: 0).

Creates a default GridLayout manager.

Creates a GridLayout with a specified number of rows and columns.

Creates a GridLayout manager with a specified number of rows and columns, horizontal gap, and vertical gap.



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.


BorderLayout


The BorderLayout Class

java.awt.BorderLayout

-hgap: int

-vgap: int

+BorderLayout()

+BorderLayout(hgap: int, vgap: int)

The horizontal gap of this layout manager (default: 0).

The vertical gap of this layout manager (default: 0).

Creates a default BorderLayout manager.

Creates a BorderLayout manager with a specified number of horizontal gap, and vertical gap.



Panels

A JPanel is a container that can be put into another container. • Panels have their own layoutJpanel pnlTop = new JPanel();pnlTop.setLayout(new GridLayout(4,3));frame.add(pnlTop);• Note that the panel will be added to the frame’s

layout.


Controls

• The screen objects you see, such as text fields, labels, checkboxes, buttons, and the like are collectively referred to as controls.


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)


JTextFieldA 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.JTextField -columns: int

-horizontalAlignment: int

+JTextField()

+JTextField(column: int)

+JTextField(text: String)

+JTextField(text: String, columns: int)

The number of columns in this text field.

The horizontal alignment of this text field (default: LEFT).

Creates a default empty text field with number of columns set to 0.

Creates an empty text field with specified number of columns.

Creates a text field initialized with the specified text.

Creates a text field initialized with the specified text and columns.

javax.swing.text.JTextComponent -text: String

-editable: boolean

The text contained in this text component.

Indicates whether this text component is editable (default: true).



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 thespecified text and the column size.


JTextField Properties

• text• horizontalAlignment• editable• columns


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.


JTextAreaIf 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.JTextArea -columns: int

-rows: int

-tabSize: int

-lineWrap: boolean

-wrapStyleWord: boolean

+JTextArea()

+JTextArea(rows: int, columns: int)

+JTextArea(text: String)

+JTextArea(text: String, rows: int, columns: int)

+append(s: String): void

+insert(s: String, pos: int): void

+replaceRange(s: String, start: int, end: int): void

+getLineCount(): int

The number of columns in this text area.

The number of rows in this text area.

The number of characters used to expand tabs (default: 8).

Indicates whether the line in the text area is automatically wrapped (default: false).

Indicates whether the line is wrapped on words or characters (default: false).

Creates a default empty text area.

Creates an empty text area with the specified number of rows and columns.

Creates a new text area with the specified text displayed.

Creates a new text area with the specified text and number of rows and columns.

Appends the string to text in the text area.

Inserts string s in the specified position in the text area.

Replaces partial text in the range from position start to end with string s.

Returns the actual number of lines contained in the text area.

javax.swing.text.JTextComponent



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 andthe number of rows and columns specified.


JTextArea Properties• text• editable• columns• lineWrap• wrapStyleWord• rows• lineCount• tabSize


JComboBoxA 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.JComboBox

+JComboBox()

+JComboBox(items: Object[])

+addItem(item: Object): void

+getItemAt(index: int): Object

+getItemCount(): int

+getSelectedIndex(): int

+setSelectedIndex(index: int): void

+getSelectedItem(): Object

+setSelectedItem(item: Object): void

+removeItem(anObject: Object): void

+removeItemAt(anIndex: int): void

+removeAllItems(): void

Creates a default empty combo box.

Creates a combo box that contains the elements in the specified array.

Adds an item to the combo box.

Returns the item at the specified index.

Returns the number of items in the combo box.

Returns the index of the selected item.

Sets the selected index in the combo box.

Returns the selected item.

Sets the selected item in the combo box.

Removes an item from the item list.

Removes the item at the specified index in the combo box.

Removes all items in the combo box.

javax.swing.JComponent


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()


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


JList

• The JList does not scroll. If you want it to, you must place it in a JScrollPane


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


JButton

• JButton provides a simple button. It can have text, an icon, or both

javax.swing.JButton

+JButton()

+JButton(icon: javax.swing.Icon)

+JButton(text: String)

+JButton(text: String, icon: Icon)

Creates a default button with no text and icon.

Creates a button with an icon.

Creates a button with text.

Creates a button with text and an icon.

javax.swing.AbstractButton


JButton Constructors

The following are JButton constructors:

JButton()

JButton(String text)

JButton(String text, Icon icon)

JButton(Icon icon)


JButton Properties

• text• icon• mnemonic• horizontalAlignment• verticalAlignment• horizontalTextPosition• verticalTextPosition• iconTextGap


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.}});


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();}}


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.


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);


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 non-digit.

• nextDouble() reads a double


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”);


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


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.


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.


Binary I/O Classes

InputStream

OutputStream

Object

ObjectOutputStream

FilterOutputStream

FileOutputStream

BufferedInputStream

DataInputStream

BufferedOutputStream

DataOutputStream

PrintStream

ObjectInputStream

FilterInputStream

FileInputStream


InputStream

java.io.InputStream

+read(): int

+read(b: byte[]): int

+read(b: byte[], off: int, len: int): int

+available(): int

+close(): void

+skip(n: long): long

+markSupported(): boolean

+mark(readlimit: int): void

+reset(): void

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.

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.

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.

Returns the number of bytes that can be read from the input stream.

Closes this input stream and releases any system resources associated with the stream.

Skips over and discards n bytes of data from this input stream. The actual number of bytes skipped is returned.

Tests if this input stream supports the mark and reset methods.

Marks the current position in this input stream.

Repositions this stream to the position at the time the mark method was last called on this input stream.


OutputStream

The value is a byte as an int type.

java.io.OutputStream

+write(int b): void

+write(b: byte[]): void

+write(b: byte[], off: int, len: int): void

+close(): void

+flush(): void

Writes the specified byte to this output stream. The parameter b is an int value. (byte)b is written to the output stream.

Writes all the bytes in array b to the output stream.

Writes b[off], b[off+1], …, b[off+len-1] into the output stream.

Closes this input stream and releases any system resources associated with the stream.

Flushes this output stream and forces any buffered output bytes to be written out.


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.


Creates a RandomAccessFile stream with the specified File object and mode.

Creates a RandomAccessFile stream with the specified file name string and mode.

Closes the stream and releases the resource associated with the stream.

Returns the offset, in bytes, from the beginning of the file to where the next read or write occurs.

Returns the length of this file.

Reads a byte of data from this file and returns –1 an the end of stream.

Reads up to b.length bytes of data from this file into an array of bytes.

Reads up to len bytes of data from this file into an array of bytes.

Sets the offset (in bytes specified in pos) from the beginning of the stream to where the next read or write occurs.

Sets a new length of this file.

Skips over n bytes of input discarding the skipped bytes.

Writes b.length bytes from the specified byte array to this file, starting at the current file pointer.

Writes len bytes from the specified byte array starting at offset off to this file.

DataInput

DataInput

java.io.RandomAccessFile

+RandomAccessFile(file: File, mode: String)

+RandomAccessFile(name: String, mode: String)

+close(): void

+getFilePointer(): long

+length(): long

+read(): int

+read(b: byte[]): int

+read(b: byte[], off: int, len: int) : int

+seek(long pos): void

+setLength(newLength: long): void

+skipBytes(int n): int

+write(b: byte[]): void

+write(byte b[], int off, int len) +write(b: byte[], off: int, len: int):

void


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.


DataInputStream

java.io.DataInput

+readBoolean(): boolean

+readByte(): byte

+readChar(): char

+readFloat(): float

+readDouble(): float

+readInt(): int

+readLong(): long

+readShort(): short

+readLine(): String

+readUTF(): String

Reads a Boolean from the input stream.

Reads a byte from the input stream.

Reads a character from the input stream.

Reads a float from the input stream.

Reads a double from the input stream.

Reads an int from the input stream.

Reads a long from the input stream.

Reads a short from the input stream.

Reads a line of characters from input.

Reads a string in UTF format.

InputStream

FilterInputStream

DataInputStream

+DataInputStream( in: InputStream)


DataOutputStream java.io.DataOutput

+writeBoolean(b: Boolean): void

+writeByte(v: int): void

+writeBytes(s: String): void

+writeChar(c: char): void

+writeChars(s: String): void

+writeFloat(v: float): void

+writeDouble(v: float): void

+writeInt(v: int): void

+writeLong(v: long): void

+writeShort(v: short): void

+writeUTF(s: String): void

Writes a Boolean to the output stream.

Writes to the output stream the eight low-order bits of the argument v.

Writes the lower byte of the characters in a string to the output stream.

Writes a character (composed of two bytes) to the output stream.

Writes every character in the string s, to the output stream, in order, two bytes per character.

Writes a float value to the output stream.

Writes a double value to the output stream.

Writes an int value to the output stream.

Writes a long value to the output stream.

Writes a short value to the output stream.

Writes two bytes of length information to the output stream, followed by the UTF representation of every character in the string s.

OutputStream

FilterOutputStream

DataOutputStream

+DataOutputStream( out: OutputStream)


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.


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.


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.


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.


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


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.



• void seek(long pos) throws IOException;

Sets the offset from the beginning of the RandomAccessFile stream to where the next reador write occurs.

• long getFilePointer() IOException;

Returns the current offset, in bytes, from thebeginning of the file to where the next reador write occurs.



• 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 ofcharacters.


RandomAccessFile Constructor

RandomAccessFile raf = new RandomAccessFile("test.dat", "rw"); //allows read and write

RandomAccessFile raf = new RandomAccessFile("test.dat","r");

//read only


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 four-character 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.


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”


Creating Tasks and Threads

// Custom task class public class TaskClass implements Runnable { ... public TaskClass(...) { ... } // Implement the run method in Runnable public void run() { // Tell system how to run custom thread ... } ... }

// 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); // Start a thread thread.start(); ... } ... }

java.lang.Runnable

TaskClass


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


The Thread Class

java.lang.Thread

+Thread()

+Thread(task: Runnable)

+start(): void

+isAlive(): boolean

+setPriority(p: int): void

+join(): void

+sleep(millis: long): void

+yield(): void

+interrupt(): void

Creates a default thread.

Creates a thread for a specified task.

Starts the thread that causes the run() method to be invoked by the JVM.

Tests whether the thread is currently running.

Sets priority p (ranging from 1 to 10) for this thread.

Waits for this thread to finish.

Puts the runnable object to sleep for a specified time in milliseconds.

Causes this thread to temporarily pause and allow other threads to execute.

Interrupts this thread.

«interface» java.lang.Runnable


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.


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.


The join() MethodYou 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:

The numbers after 50 are printed after thread printA is finished.

printA.join() -char token +getToken +setToken +paintComponet +mouseClicked

Thread print100

-char token +getToken +setToken +paintComponet +mouseClicked

Wait for printA to finish

+getToken +setToken +paintComponet

Thread printA

-char token +getToken +setToken +paintComponet +mouseClicked

printA finished -char token

public void run() { Thread thread4 = new Thread(

new PrintChar('c', 40)); thread4.start(); try { for (int i = 1; i <= lastNum; i++) { System.out.print(" " + i); if (i == 50) thread4.join(); } } catch (InterruptedException ex) { } }


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


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.


Thread SynchronizationA 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 0 newBalance = bank.getBalance() + 1;

2 0 newBalance = bank.getBalance() + 1;

3 1 bank.setBalance(newBalance);

4 1 bank.setBalance(newBalance);


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)


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.

synchronized (object1) { // do something here synchronized (object2) { // do something here } }

Thread 1

synchronized (object2) { // do something here synchronized (object1) { // do something here } }

Thread 2

Step

1 2 3 4 5 6

Wait for Thread 2 to release the lock on object2

Wait for Thread 1 to release the lock on object1

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