How to create a Search Crawler in Java

Search Crawler in Java

With Search Crawler shown in this tip, you can enter search criteria and then search the Web in real time, URL by URL, looking for matches to the criteria.

The content of the SearchCrawler.java file is written below:
import java.awt.*;
import java.awt.event.*;
import java.io.*;
import java.net.*;
import java.util.*;
import java.util.regex.*;
import javax.swing.*;
import javax.swing.table.*;

// The Search Web Crawler
public class SearchCrawler extends JFrame {

// Max URLs drop down values.
private static final String[] MAX_URLS =
{"50", "100", "500", "1000"};

// Cache of robot disallow lists.
private HashMap disallowListCache = new HashMap();

// Search GUI controls.
private JTextField startTextField;
private JComboBox maxComboBox;
private JCheckBox limitCheckBox;
private JTextField logTextField;
private JTextField searchTextField;
private JCheckBox caseCheckBox;
private JButton searchButton;

// Search stats GUI controls.
private JLabel crawlingLabel2;
private JLabel crawledLabel2;
private JLabel toCrawlLabel2;
private JProgressBar progressBar;
private JLabel matchesLabel2;

// Table listing search matches.
private JTable table;

// Flag for whether or not crawling is underway.
private boolean crawling;

// Matches log file print writer.
private PrintWriter logFileWriter;

// Constructor for Search Web Crawler.
public SearchCrawler() {
// Set application title.
setTitle("Search Crawler");

// Set window size.
setSize(600, 600);

// Handle window closing events.
addWindowListener(new WindowAdapter() {
public void windowClosing(WindowEvent e) {
actionExit();
}
});

// Set up file menu.
JMenuBar menuBar = new JMenuBar();
JMenu fileMenu = new JMenu("File");
fileMenu.setMnemonic(KeyEvent.VK_F);
JMenuItem fileExitMenuItem = new JMenuItem("Exit",
KeyEvent.VK_X);
fileExitMenuItem.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
actionExit();
}
});
fileMenu.add(fileExitMenuItem);
menuBar.add(fileMenu);
setJMenuBar(menuBar);

// Set up search panel.
JPanel searchPanel = new JPanel();
GridBagConstraints constraints;
GridBagLayout layout = new GridBagLayout();
searchPanel.setLayout(layout);

JLabel startLabel = new JLabel("Start URL:");
constraints = new GridBagConstraints();
constraints.anchor = GridBagConstraints.EAST;
constraints.insets = new Insets(5, 5, 0, 0);
layout.setConstraints(startLabel, constraints);
searchPanel.add(startLabel);

startTextField = new JTextField();
constraints = new GridBagConstraints();
constraints.fill = GridBagConstraints.HORIZONTAL;
constraints.gridwidth = GridBagConstraints.REMAINDER;
constraints.insets = new Insets(5, 5, 0, 5);
layout.setConstraints(startTextField, constraints);
searchPanel.add(startTextField);

JLabel maxLabel = new JLabel("Max URLs to Crawl:");
constraints = new GridBagConstraints();
constraints.anchor = GridBagConstraints.EAST;
constraints.insets = new Insets(5, 5, 0, 0);
layout.setConstraints(maxLabel, constraints);
searchPanel.add(maxLabel);

maxComboBox = new JComboBox(MAX_URLS);
maxComboBox.setEditable(true);
constraints = new GridBagConstraints();
constraints.insets = new Insets(5, 5, 0, 0);
layout.setConstraints(maxComboBox, constraints);
searchPanel.add(maxComboBox);

limitCheckBox =
new JCheckBox("Limit crawling to Start URL site");
constraints = new GridBagConstraints();
constraints.anchor = GridBagConstraints.WEST;
constraints.insets = new Insets(0, 10, 0, 0);
layout.setConstraints(limitCheckBox, constraints);
searchPanel.add(limitCheckBox);

JLabel blankLabel = new JLabel();
constraints = new GridBagConstraints();
constraints.gridwidth = GridBagConstraints.REMAINDER;
layout.setConstraints(blankLabel, constraints);
searchPanel.add(blankLabel);

JLabel logLabel = new JLabel("Matches Log File:");
constraints = new GridBagConstraints();
constraints.anchor = GridBagConstraints.EAST;
constraints.insets = new Insets(5, 5, 0, 0);
layout.setConstraints(logLabel, constraints);
searchPanel.add(logLabel);

String file =
System.getProperty("user.dir") +
System.getProperty("file.separator") +
"crawler.log";
logTextField = new JTextField(file);
constraints = new GridBagConstraints();
constraints.fill = GridBagConstraints.HORIZONTAL;
constraints.gridwidth = GridBagConstraints.REMAINDER;
constraints.insets = new Insets(5, 5, 0, 5);
layout.setConstraints(logTextField, constraints);
searchPanel.add(logTextField);

JLabel searchLabel = new JLabel("Search String:");
constraints = new GridBagConstraints();
constraints.anchor = GridBagConstraints.EAST;
constraints.insets = new Insets(5, 5, 0, 0);
layout.setConstraints(searchLabel, constraints);
searchPanel.add(searchLabel);

searchTextField = new JTextField();
constraints = new GridBagConstraints();
constraints.fill = GridBagConstraints.HORIZONTAL;
constraints.insets = new Insets(5, 5, 0, 0);
constraints.gridwidth= 2;
constraints.weightx = 1.0d;
layout.setConstraints(searchTextField, constraints);
searchPanel.add(searchTextField);

caseCheckBox = new JCheckBox("Case Sensitive");
constraints = new GridBagConstraints();
constraints.insets = new Insets(5, 5, 0, 5);
constraints.gridwidth = GridBagConstraints.REMAINDER;
layout.setConstraints(caseCheckBox, constraints);
searchPanel.add(caseCheckBox);

searchButton = new JButton("Search");
searchButton.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
actionSearch();
}
});
constraints = new GridBagConstraints();
constraints.gridwidth = GridBagConstraints.REMAINDER;
constraints.insets = new Insets(5, 5, 5, 5);
layout.setConstraints(searchButton, constraints);
searchPanel.add(searchButton);

JSeparator separator = new JSeparator();
constraints = new GridBagConstraints();
constraints.fill = GridBagConstraints.HORIZONTAL;
constraints.gridwidth = GridBagConstraints.REMAINDER;
constraints.insets = new Insets(5, 5, 5, 5);
layout.setConstraints(separator, constraints);
searchPanel.add(separator);

JLabel crawlingLabel1 = new JLabel("Crawling:");
constraints = new GridBagConstraints();
constraints.anchor = GridBagConstraints.EAST;
constraints.insets = new Insets(5, 5, 0, 0);
layout.setConstraints(crawlingLabel1, constraints);
searchPanel.add(crawlingLabel1);

crawlingLabel2 = new JLabel();
crawlingLabel2.setFont(
crawlingLabel2.getFont().deriveFont(Font.PLAIN));
constraints = new GridBagConstraints();
constraints.fill = GridBagConstraints.HORIZONTAL;
constraints.gridwidth = GridBagConstraints.REMAINDER;
constraints.insets = new Insets(5, 5, 0, 5);
layout.setConstraints(crawlingLabel2, constraints);
searchPanel.add(crawlingLabel2);

JLabel crawledLabel1 = new JLabel("Crawled URLs:");
constraints = new GridBagConstraints();
constraints.anchor = GridBagConstraints.EAST;
constraints.insets = new Insets(5, 5, 0, 0);
layout.setConstraints(crawledLabel1, constraints);
searchPanel.add(crawledLabel1);

crawledLabel2 = new JLabel();
crawledLabel2.setFont(
crawledLabel2.getFont().deriveFont(Font.PLAIN));
constraints = new GridBagConstraints();
constraints.fill = GridBagConstraints.HORIZONTAL;
constraints.gridwidth = GridBagConstraints.REMAINDER;
constraints.insets = new Insets(5, 5, 0, 5);
layout.setConstraints(crawledLabel2, constraints);
searchPanel.add(crawledLabel2);

JLabel toCrawlLabel1 = new JLabel("URLs to Crawl:");
constraints = new GridBagConstraints();
constraints.anchor = GridBagConstraints.EAST;
constraints.insets = new Insets(5, 5, 0, 0);
layout.setConstraints(toCrawlLabel1, constraints);
searchPanel.add(toCrawlLabel1);

toCrawlLabel2 = new JLabel();
toCrawlLabel2.setFont(
toCrawlLabel2.getFont().deriveFont(Font.PLAIN));
constraints = new GridBagConstraints();
constraints.fill = GridBagConstraints.HORIZONTAL;
constraints.gridwidth = GridBagConstraints.REMAINDER;
constraints.insets = new Insets(5, 5, 0, 5);
layout.setConstraints(toCrawlLabel2, constraints);
searchPanel.add(toCrawlLabel2);

JLabel progressLabel = new JLabel("Crawling Progress:");
constraints = new GridBagConstraints();
constraints.anchor = GridBagConstraints.EAST;
constraints.insets = new Insets(5, 5, 0, 0);
layout.setConstraints(progressLabel, constraints);
searchPanel.add(progressLabel);

progressBar = new JProgressBar();
progressBar.setMinimum(0);
progressBar.setStringPainted(true);
constraints = new GridBagConstraints();
constraints.fill = GridBagConstraints.HORIZONTAL;
constraints.gridwidth = GridBagConstraints.REMAINDER;
constraints.insets = new Insets(5, 5, 0, 5);
layout.setConstraints(progressBar, constraints);
searchPanel.add(progressBar);

JLabel matchesLabel1 = new JLabel("Search Matches:");
constraints = new GridBagConstraints();
constraints.anchor = GridBagConstraints.EAST;
constraints.insets = new Insets(5, 5, 10, 0);
layout.setConstraints(matchesLabel1, constraints);
searchPanel.add(matchesLabel1);

matchesLabel2 = new JLabel();
matchesLabel2.setFont(
matchesLabel2.getFont().deriveFont(Font.PLAIN));
constraints = new GridBagConstraints();
constraints.fill = GridBagConstraints.HORIZONTAL;
constraints.gridwidth = GridBagConstraints.REMAINDER;
constraints.insets = new Insets(5, 5, 10, 5);
layout.setConstraints(matchesLabel2, constraints);
searchPanel.add(matchesLabel2);

// Set up matches table.
table =
new JTable(new DefaultTableModel(new Object[][]{},
new String[]{"URL"}) {
public boolean isCellEditable(int row, int column) {
return false;
}
});

// Set up matches panel.
JPanel matchesPanel = new JPanel();
matchesPanel.setBorder(
BorderFactory.createTitledBorder("Matches"));
matchesPanel.setLayout(new BorderLayout());
matchesPanel.add(new JScrollPane(table),
BorderLayout.CENTER);

// Add panels to display.
getContentPane().setLayout(new BorderLayout());
getContentPane().add(searchPanel, BorderLayout.NORTH);
getContentPane().add(matchesPanel, BorderLayout.CENTER);
}

// Exit this program.
private void actionExit() {
System.exit(0);
}

// Handle search/stop button being clicked.
private void actionSearch() {
// If stop button clicked, turn crawling flag off.
if (crawling) {
crawling = false;
return;
}

ArrayList errorList = new ArrayList();

// Validate that start URL has been entered.
String startUrl = startTextField.getText().trim();
if (startUrl.length() < 1) {
errorList.add("Missing Start URL.");
}
// Verify start URL.
else if (verifyUrl(startUrl) == null) {
errorList.add("Invalid Start URL.");
}

// Validate that max URLs is either empty or is a number.
int maxUrls = 0;
String max = ((String) maxComboBox.getSelectedItem()).trim();
if (max.length() > 0) {
try {
maxUrls = Integer.parseInt(max);
} catch (NumberFormatException e) {
}
if (maxUrls < 1) {
errorList.add("Invalid Max URLs value.");
}
}

// Validate that matches log file has been entered.
String logFile = logTextField.getText().trim();
if (logFile.length() < 1) {
errorList.add("Missing Matches Log File.");
}

// Validate that search string has been entered.
String searchString = searchTextField.getText().trim();
if (searchString.length() < 1) {
errorList.add("Missing Search String.");
}

// Show errors, if any, and return.
if (errorList.size() > 0) {
StringBuffer message = new StringBuffer();

// Concatenate errors into single message.
for (int i = 0; i < errorList.size(); i++) {
message.append(errorList.get(i));
if (i + 1 < errorList.size()) {
message.append("\n");
}
}

showError(message.toString());
return;
}

// Remove "www" from start URL if present.
startUrl = removeWwwFromUrl(startUrl);

// Start the search crawler.
search(logFile, startUrl, maxUrls, searchString);
}

private void search(final String logFile, final String startUrl,
final int maxUrls, final String searchString) {
// Start the search in a new thread.
Thread thread = new Thread(new Runnable() {
public void run() {
// Show hour glass cursor while crawling is under way.
setCursor(Cursor.getPredefinedCursor(Cursor.WAIT_CURSOR));

// Disable search controls.
startTextField.setEnabled(false);
maxComboBox.setEnabled(false);
limitCheckBox.setEnabled(false);
logTextField.setEnabled(false);
searchTextField.setEnabled(false);
caseCheckBox.setEnabled(false);

// Switch search button to "Stop."
searchButton.setText("Stop");

// Reset stats.
table.setModel(new DefaultTableModel(new Object[][]{},
new String[]{"URL"}) {
public boolean isCellEditable(int row, int column) {
return false;
}
});
updateStats(startUrl, 0, 0, maxUrls);

// Open matches log file.
try {
logFileWriter = new PrintWriter(new FileWriter(logFile));
} catch (Exception e) {
showError("Unable to open matches log file.");
return;
}

// Turn crawling flag on.
crawling = true;

// Perform the actual crawling.
crawl(startUrl, maxUrls, limitCheckBox.isSelected(),
searchString, caseCheckBox.isSelected());

// Turn crawling flag off.
crawling = false;

// Close matches log file.
try {
logFileWriter.close();
} catch (Exception e) {
showError("Unable to close matches log file.");
}

// Mark search as done.
crawlingLabel2.setText("Done");

// Enable search controls.
startTextField.setEnabled(true);
maxComboBox.setEnabled(true);
limitCheckBox.setEnabled(true);
logTextField.setEnabled(true);
searchTextField.setEnabled(true);
caseCheckBox.setEnabled(true);

// Switch search button back to "Search."
searchButton.setText("Search");

// Return to default cursor.
setCursor(Cursor.getDefaultCursor());

// Show message if search string not found.
if (table.getRowCount() == 0) {
JOptionPane.showMessageDialog(SearchCrawler.this,
"Your Search String was not found. Please try another.",
"Search String Not Found",
JOptionPane.WARNING_MESSAGE);
}
}
});
thread.start();
}

// Show dialog box with error message.
private void showError(String message) {
JOptionPane.showMessageDialog(this, message, "Error",
JOptionPane.ERROR_MESSAGE);
}

// Update crawling stats.
private void updateStats(
String crawling, int crawled, int toCrawl, int maxUrls) {
crawlingLabel2.setText(crawling);
crawledLabel2.setText("" + crawled);
toCrawlLabel2.setText("" + toCrawl);

// Update progress bar.
if (maxUrls == -1) {
progressBar.setMaximum(crawled + toCrawl);
} else {
progressBar.setMaximum(maxUrls);
}
progressBar.setValue(crawled);

matchesLabel2.setText("" + table.getRowCount());
}

// Add match to matches table and log file.
private void addMatch(String url) {
// Add URL to matches table.
DefaultTableModel model =
(DefaultTableModel) table.getModel();
model.addRow(new Object[]{url});

// Add URL to matches log file.
try {
logFileWriter.println(url);
} catch (Exception e) {
showError("Unable to log match.");
}
}

// Verify URL format.
private URL verifyUrl(String url) {
// Only allow HTTP URLs.
if (!url.toLowerCase().startsWith("http://"))
return null;

// Verify format of URL.
URL verifiedUrl = null;
try {
verifiedUrl = new URL(url);
} catch (Exception e) {
return null;
}

return verifiedUrl;
}

// Check if robot is allowed to access the given URL.
private boolean isRobotAllowed(URL urlToCheck) {
String host = urlToCheck.getHost().toLowerCase();

// Retrieve host's disallow list from cache.
ArrayList disallowList =
(ArrayList) disallowListCache.get(host);

// If list is not in the cache, download and cache it.
if (disallowList == null) {
disallowList = new ArrayList();

try {
URL robotsFileUrl =
new URL("http://" + host + "/robots.txt");

// Open connection to robot file URL for reading.
BufferedReader reader =
new BufferedReader(new InputStreamReader(
robotsFileUrl.openStream()));

// Read robot file, creating list of disallowed paths.
String line;
while ((line = reader.readLine()) != null) {
if (line.indexOf("Disallow:") == 0) {
String disallowPath =
line.substring("Disallow:".length());

// Check disallow path for comments and
// remove if present.
int commentIndex = disallowPath.indexOf("#");
if (commentIndex != - 1) {
disallowPath =
disallowPath.substring(0, commentIndex);
}

// Remove leading or trailing spaces from
// disallow path.
disallowPath = disallowPath.trim();

// Add disallow path to list.
disallowList.add(disallowPath);
}
}

// Add new disallow list to cache.
disallowListCache.put(host, disallowList);
} catch (Exception e) {
/* Assume robot is allowed since an exception
is thrown if the robot file doesn't exist. */
return true;
}
}

/* Loop through disallow list to see if the
crawling is allowed for the given URL. */
String file = urlToCheck.getFile();
for (int i = 0; i < disallowList.size(); i++) {
String disallow = (String) disallowList.get(i);
if (file.startsWith(disallow)) {
return false;
}
}

return true;
}

// Download page at given URL.
private String downloadPage(URL pageUrl) {
try {
// Open connection to URL for reading.
BufferedReader reader =
new BufferedReader(new InputStreamReader(
pageUrl.openStream()));

// Read page into buffer.
String line;
StringBuffer pageBuffer = new StringBuffer();
while ((line = reader.readLine()) != null) {
pageBuffer.append(line);
}

return pageBuffer.toString();
} catch (Exception e) {
}

return null;
}

// Remove leading "www" from a URL's host if present.
private String removeWwwFromUrl(String url) {
int index = url.indexOf("://www.");
if (index != -1) {
return url.substring(0, index + 3) +
url.substring(index + 7);
}

return (url);
}

// Parse through page contents and retrieve links.
private ArrayList retrieveLinks(
URL pageUrl, String pageContents, HashSet crawledList,
boolean limitHost) {
// Compile link matching pattern.
Pattern p =
Pattern.compile("]",
Pattern.CASE_INSENSITIVE);
Matcher m = p.matcher(pageContents);

// Create list of link matches.
ArrayList linkList = new ArrayList();
while (m.find()) {
String link = m.group(1).trim();

// Skip empty links.
if (link.length() < 1) {
continue;
}

// Skip links that are just page anchors.
if (link.charAt(0) == '#') {
continue;
}

// Skip mailto links.
if (link.indexOf("mailto:") != -1) {
continue;
}

// Skip JavaScript links.
if (link.toLowerCase().indexOf("javascript") != -1) {
continue;
}

// Prefix absolute and relative URLs if necessary.
if (link.indexOf("://") == -1) {
// Handle absolute URLs.
if (link.charAt(0) == '/') {
link = "http://" + pageUrl.getHost() + link;
// Handle relative URLs.
} else {
String file = pageUrl.getFile();
if (file.indexOf('/') == -1) {
link = "http://" + pageUrl.getHost() + "/" + link;
} else {
String path =
file.substring(0, file.lastIndexOf('/') + 1);
link = "http://" + pageUrl.getHost() + path + link;
}
}
}

// Remove anchors from link.
int index = link.indexOf('#');
if (index != -1) {
link = link.substring(0, index);
}

// Remove leading "www" from URL's host if present.
link = removeWwwFromUrl(link);

// Verify link and skip if invalid.
URL verifiedLink = verifyUrl(link);
if (verifiedLink == null) {
continue;
}

/* If specified, limit links to those
having the same host as the start URL. */
if (limitHost &&
!pageUrl.getHost().toLowerCase().equals(
verifiedLink.getHost().toLowerCase())) {
continue;
}

// Skip link if it has already been crawled.
if (crawledList.contains(link)) {
continue;
}

// Add link to list.
linkList.add(link);
}

return (linkList);
}

/* Determine whether or not search string is
matched in the given page contents. */
private boolean searchStringMatches(
String pageContents, String searchString,
boolean caseSensitive) {
String searchContents = pageContents;

/* If case sensitive search, lowercase
page contents for comparison. */
if (!caseSensitive) {
searchContents = pageContents.toLowerCase();
}

// Split search string into individual terms.
Pattern p = Pattern.compile("[\\s]+");
String[] terms = p.split(searchString);

// Check to see if each term matches.
for (int i = 0; i < terms.length; i++) {
if (caseSensitive) {
if (searchContents.indexOf(terms[i]) == -1) {
return false;
}
} else {
if (searchContents.indexOf(terms[i].toLowerCase()) == -1) {
return false;
}
}
}

return true;
}

// Perform the actual crawling, searching for the search string.
public void crawl(
String startUrl, int maxUrls, boolean limitHost,
String searchString, boolean caseSensitive) {
// Setup crawl lists.
HashSet crawledList = new HashSet();
LinkedHashSet toCrawlList = new LinkedHashSet();

// Add start URL to the to crawl list.
toCrawlList.add(startUrl);

/* Perform actual crawling by looping
through the to crawl list. */
while (crawling && toCrawlList.size() > 0) {
/* Check to see if the max URL count has
been reached, if it was specified.*/
if (maxUrls != -1) {
if (crawledList.size() == maxUrls) {
break;
}
}

// Get URL at bottom of the list.
String url = (String) toCrawlList.iterator().next();

// Remove URL from the to crawl list.
toCrawlList.remove(url);

// Convert string url to URL object.
URL verifiedUrl = verifyUrl(url);

// Skip URL if robots are not allowed to access it.
if (!isRobotAllowed(verifiedUrl)) {
continue;
}

// Update crawling stats.
updateStats(url, crawledList.size(), toCrawlList.size(),
maxUrls);

// Add page to the crawled list.
crawledList.add(url);

// Download the page at the given url.
String pageContents = downloadPage(verifiedUrl);

/* If the page was downloaded successfully, retrieve all of its
links and then see if it contains the search string. */
if (pageContents != null && pageContents.length() > 0) {
// Retrieve list of valid links from page.
ArrayList links =
retrieveLinks(verifiedUrl, pageContents, crawledList,
limitHost);

// Add links to the to crawl list.
toCrawlList.addAll(links);

/* Check if search string is present in
page and if so record a match. */
if (searchStringMatches(pageContents, searchString,
caseSensitive)) {
addMatch(url);
}
}

// Update crawling stats.
updateStats(url, crawledList.size(), toCrawlList.size(),
maxUrls);
}
}

// Run the Search Crawler.
public static void main(String[] args) {
SearchCrawler crawler = new SearchCrawler();
crawler.show();
}
}

How to create a download manager in Java

download manager in java

This example shows how to create a simple download manager in Java. It contains four classes in foru Java source files:

* Download.java: Contains Download class which downloads a file from a URL.
* DownloadManager.java: Contains the main class for download manager application.
* DownloadsTableModel.java: Contains the class which manages the download table's data.
* ProgressRenderer.java: Contains the class which is responsible to render a JProgressBar in a table cell.

The contents of the listed files are written below.

Download.java
import java.io.*;
import java.net.*;
import java.util.*;

// This class downloads a file from a URL.
class Download extends Observable implements Runnable {

// Max size of download buffer.
private static final int MAX_BUFFER_SIZE = 1024;

// These are the status names.
public static final String STATUSES[] = {"Downloading",
"Paused", "Complete", "Cancelled", "Error"};

// These are the status codes.
public static final int DOWNLOADING = 0;
public static final int PAUSED = 1;
public static final int COMPLETE = 2;
public static final int CANCELLED = 3;
public static final int ERROR = 4;

private URL url; // download URL
private int size; // size of download in bytes
private int downloaded; // number of bytes downloaded
private int status; // current status of download

// Constructor for Download.
public Download(URL url) {
this.url = url;
size = -1;
downloaded = 0;
status = DOWNLOADING;

// Begin the download.
download();
}

// Get this download's URL.
public String getUrl() {
return url.toString();
}

// Get this download's size.
public int getSize() {
return size;
}

// Get this download's progress.
public float getProgress() {
return ((float) downloaded / size) * 100;
}

// Get this download's status.
public int getStatus() {
return status;
}

// Pause this download.
public void pause() {
status = PAUSED;
stateChanged();
}

// Resume this download.
public void resume() {
status = DOWNLOADING;
stateChanged();
download();
}

// Cancel this download.
public void cancel() {
status = CANCELLED;
stateChanged();
}

// Mark this download as having an error.
private void error() {
status = ERROR;
stateChanged();
}

// Start or resume downloading.
private void download() {
Thread thread = new Thread(this);
thread.start();
}

// Get file name portion of URL.
private String getFileName(URL url) {
String fileName = url.getFile();
return fileName.substring(fileName.lastIndexOf('/') + 1);
}

// Download file.
public void run() {
RandomAccessFile file = null;
InputStream stream = null;

try {
// Open connection to URL.
HttpURLConnection connection =
(HttpURLConnection) url.openConnection();

// Specify what portion of file to download.
connection.setRequestProperty("Range",
"bytes=" + downloaded + "-");

// Connect to server.
connection.connect();

// Make sure response code is in the 200 range.
if (connection.getResponseCode() / 100 != 2) {
error();
}

// Check for valid content length.
int contentLength = connection.getContentLength();
if (contentLength < 1) {
error();
}

/* Set the size for this download if it
hasn't been already set. */
if (size == -1) {
size = contentLength;
stateChanged();
}

// Open file and seek to the end of it.
file = new RandomAccessFile(getFileName(url), "rw");
file.seek(downloaded);

stream = connection.getInputStream();
while (status == DOWNLOADING) {
/* Size buffer according to how much of the
file is left to download. */
byte buffer[];
if (size - downloaded > MAX_BUFFER_SIZE) {
buffer = new byte[MAX_BUFFER_SIZE];
} else {
buffer = new byte[size - downloaded];
}

// Read from server into buffer.
int read = stream.read(buffer);
if (read == -1)
break;

// Write buffer to file.
file.write(buffer, 0, read);
downloaded += read;
stateChanged();
}

/* Change status to complete if this point was
reached because downloading has finished. */
if (status == DOWNLOADING) {
status = COMPLETE;
stateChanged();
}
} catch (Exception e) {
error();
} finally {
// Close file.
if (file != null) {
try {
file.close();
} catch (Exception e) {}
}

// Close connection to server.
if (stream != null) {
try {
stream.close();
} catch (Exception e) {}
}
}
}

// Notify observers that this download's status has changed.
private void stateChanged() {
setChanged();
notifyObservers();
}
}

DownloadManager.java
import java.awt.*;
import java.awt.event.*;
import java.net.*;
import java.util.*;
import javax.swing.*;
import javax.swing.event.*;

// The Download Manager.
public class DownloadManager extends JFrame
implements Observer {

// Add download text field.
private JTextField addTextField;

// Download table's data model.
private DownloadsTableModel tableModel;

// Table listing downloads.
private JTable table;

// These are the buttons for managing the selected download.
private JButton pauseButton, resumeButton;
private JButton cancelButton, clearButton;

// Currently selected download.
private Download selectedDownload;

// Flag for whether or not table selection is being cleared.
private boolean clearing;

// Constructor for Download Manager.
public DownloadManager() {
// Set application title.
setTitle("Download Manager");

// Set window size.
setSize(640, 480);

// Handle window closing events.
addWindowListener(new WindowAdapter() {
public void windowClosing(WindowEvent e) {
actionExit();
}
});

// Set up file menu.
JMenuBar menuBar = new JMenuBar();
JMenu fileMenu = new JMenu("File");
fileMenu.setMnemonic(KeyEvent.VK_F);
JMenuItem fileExitMenuItem = new JMenuItem("Exit",
KeyEvent.VK_X);
fileExitMenuItem.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
actionExit();
}
});
fileMenu.add(fileExitMenuItem);
menuBar.add(fileMenu);
setJMenuBar(menuBar);

// Set up add panel.
JPanel addPanel = new JPanel();
addTextField = new JTextField(30);
addPanel.add(addTextField);
JButton addButton = new JButton("Add Download");
addButton.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
actionAdd();
}
});
addPanel.add(addButton);

// Set up Downloads table.
tableModel = new DownloadsTableModel();
table = new JTable(tableModel);
table.getSelectionModel().addListSelectionListener(new
ListSelectionListener() {
public void valueChanged(ListSelectionEvent e) {
tableSelectionChanged();
}
});
// Allow only one row at a time to be selected.
table.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);

// Set up ProgressBar as renderer for progress column.
ProgressRenderer renderer = new ProgressRenderer(0, 100);
renderer.setStringPainted(true); // show progress text
table.setDefaultRenderer(JProgressBar.class, renderer);

// Set table's row height large enough to fit JProgressBar.
table.setRowHeight(
(int) renderer.getPreferredSize().getHeight());

// Set up downloads panel.
JPanel downloadsPanel = new JPanel();
downloadsPanel.setBorder(
BorderFactory.createTitledBorder("Downloads"));
downloadsPanel.setLayout(new BorderLayout());
downloadsPanel.add(new JScrollPane(table),
BorderLayout.CENTER);

// Set up buttons panel.
JPanel buttonsPanel = new JPanel();
pauseButton = new JButton("Pause");
pauseButton.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
actionPause();
}
});
pauseButton.setEnabled(false);
buttonsPanel.add(pauseButton);
resumeButton = new JButton("Resume");
resumeButton.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
actionResume();
}
});
resumeButton.setEnabled(false);
buttonsPanel.add(resumeButton);
cancelButton = new JButton("Cancel");
cancelButton.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
actionCancel();
}
});
cancelButton.setEnabled(false);
buttonsPanel.add(cancelButton);
clearButton = new JButton("Clear");
clearButton.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
actionClear();
}
});
clearButton.setEnabled(false);
buttonsPanel.add(clearButton);

// Add panels to display.
getContentPane().setLayout(new BorderLayout());
getContentPane().add(addPanel, BorderLayout.NORTH);
getContentPane().add(downloadsPanel, BorderLayout.CENTER);
getContentPane().add(buttonsPanel, BorderLayout.SOUTH);
}

// Exit this program.
private void actionExit() {
System.exit(0);
}

// Add a new download.
private void actionAdd() {
URL verifiedUrl = verifyUrl(addTextField.getText());
if (verifiedUrl != null) {
tableModel.addDownload(new Download(verifiedUrl));
addTextField.setText(""); // reset add text field
} else {
JOptionPane.showMessageDialog(this,
"Invalid Download URL", "Error",
JOptionPane.ERROR_MESSAGE);
}
}

// Verify download URL.
private URL verifyUrl(String url) {
// Only allow HTTP URLs.
if (!url.toLowerCase().startsWith("http://"))
return null;

// Verify format of URL.
URL verifiedUrl = null;
try {
verifiedUrl = new URL(url);
} catch (Exception e) {
return null;
}

// Make sure URL specifies a file.
if (verifiedUrl.getFile().length() < 2)
return null;

return verifiedUrl;
}

// Called when table row selection changes.
private void tableSelectionChanged() {
/* Unregister from receiving notifications
from the last selected download. */
if (selectedDownload != null)
selectedDownload.deleteObserver(DownloadManager.this);

/* If not in the middle of clearing a download,
set the selected download and register to
receive notifications from it. */
if (!clearing) {
selectedDownload =
tableModel.getDownload(table.getSelectedRow());
selectedDownload.addObserver(DownloadManager.this);
updateButtons();
}
}

// Pause the selected download.
private void actionPause() {
selectedDownload.pause();
updateButtons();
}

// Resume the selected download.
private void actionResume() {
selectedDownload.resume();
updateButtons();
}

// Cancel the selected download.
private void actionCancel() {
selectedDownload.cancel();
updateButtons();
}

// Clear the selected download.
private void actionClear() {
clearing = true;
tableModel.clearDownload(table.getSelectedRow());
clearing = false;
selectedDownload = null;
updateButtons();
}

/* Update each button's state based off of the
currently selected download's status. */
private void updateButtons() {
if (selectedDownload != null) {
int status = selectedDownload.getStatus();
switch (status) {
case Download.DOWNLOADING:
pauseButton.setEnabled(true);
resumeButton.setEnabled(false);
cancelButton.setEnabled(true);
clearButton.setEnabled(false);
break;
case Download.PAUSED:
pauseButton.setEnabled(false);
resumeButton.setEnabled(true);
cancelButton.setEnabled(true);
clearButton.setEnabled(false);
break;
case Download.ERROR:
pauseButton.setEnabled(false);
resumeButton.setEnabled(true);
cancelButton.setEnabled(false);
clearButton.setEnabled(true);
break;
default: // COMPLETE or CANCELLED
pauseButton.setEnabled(false);
resumeButton.setEnabled(false);
cancelButton.setEnabled(false);
clearButton.setEnabled(true);
}
} else {
// No download is selected in table.
pauseButton.setEnabled(false);
resumeButton.setEnabled(false);
cancelButton.setEnabled(false);
clearButton.setEnabled(false);
}
}

/* Update is called when a Download notifies its
observers of any changes. */
public void update(Observable o, Object arg) {
// Update buttons if the selected download has changed.
if (selectedDownload != null && selectedDownload.equals(o))
updateButtons();
}

// Run the Download Manager.
public static void main(String[] args) {
DownloadManager manager = new DownloadManager();
manager.show();
}
}

DownloadTableModel.java
import java.util.*;
import javax.swing.*;
import javax.swing.table.*;

// This class manages the download table's data.
class DownloadsTableModel extends AbstractTableModel
implements Observer {

// These are the names for the table's columns.
private static final String[] columnNames = {"URL", "Size",
"Progress", "Status"};

// These are the classes for each column's values.
private static final Class[] columnClasses = {String.class,
String.class, JProgressBar.class, String.class};

// The table's list of downloads.
private ArrayList downloadList = new ArrayList();

// Add a new download to the table.
public void addDownload(Download download) {

// Register to be notified when the download changes.
download.addObserver(this);

downloadList.add(download);

// Fire table row insertion notification to table.
fireTableRowsInserted(getRowCount() - 1, getRowCount() - 1);
}

// Get a download for the specified row.
public Download getDownload(int row) {
return (Download) downloadList.get(row);
}

// Remove a download from the list.
public void clearDownload(int row) {
downloadList.remove(row);

// Fire table row deletion notification to table.
fireTableRowsDeleted(row, row);
}

// Get table's column count.
public int getColumnCount() {
return columnNames.length;
}

// Get a column's name.
public String getColumnName(int col) {
return columnNames[col];
}

// Get a column's class.
public Class getColumnClass(int col) {
return columnClasses[col];
}

// Get table's row count.
public int getRowCount() {
return downloadList.size();
}

// Get value for a specific row and column combination.
public Object getValueAt(int row, int col) {

Download download = (Download) downloadList.get(row);
switch (col) {
case 0: // URL
return download.getUrl();
case 1: // Size
int size = download.getSize();
return (size == -1) ? "" : Integer.toString(size);
case 2: // Progress
return new Float(download.getProgress());
case 3: // Status
return Download.STATUSES[download.getStatus()];
}
return "";
}

/* Update is called when a Download notifies its
observers of any changes */
public void update(Observable o, Object arg) {
int index = downloadList.indexOf(o);

// Fire table row update notification to table.
fireTableRowsUpdated(index, index);
}
}

ProgressRenderer.java
import java.awt.*;
import javax.swing.*;
import javax.swing.table.*;

// This class renders a JProgressBar in a table cell.
class ProgressRenderer extends JProgressBar
implements TableCellRenderer {

// Constructor for ProgressRenderer.
public ProgressRenderer(int min, int max) {
super(min, max);
}

/* Returns this JProgressBar as the renderer
for the given table cell. */
public Component getTableCellRendererComponent(
JTable table, Object value, boolean isSelected,
boolean hasFocus, int row, int column) {
// Set JProgressBar's percent complete value.
setValue((int) ((Float) value).floatValue());
return this;
}
}

source: java-tips.org

Invoke method using Reflection

Reflection is used to invoke a method when name of the method is supplied at run time. This tip will show a sample code to do that.
import java.lang.reflect.Method;

public class RunMthdRef {
public int add(int a, int b) {
return a+b;
}

public int sub(int a, int b) {
return a-b;
}

public int mul(int a, int b) {
return a*b;
}

public int div(int a, int b) {
return a/b;
}

public static void main(String[] args) {
try {
Integer[] input={new Integer(2),new Integer(6)};
Class cl=Class.forName("RunMthdRef");
Class[] par=new Class[2];
par[0]=Integer.TYPE;
par[1]=Integer.TYPE;
Method mthd=cl.getMethod("add",par);
Integer output=(Integer)mthd.invoke(new RunMthdRef(),input);
System.out.println(output.intValue());
} catch (Exception e) {
e.printStackTrace();
}
}
}

How to copy a directory from one location to another location

This Java tip demonstrates a method of copying a directory from one location to another. Copying is done from sourcedirectory to targetdirectory.

// If targetLocation does not exist, it will be created.
public void copyDirectory(File sourceLocation , File targetLocation)
throws IOException {

if (sourceLocation.isDirectory()) {
if (!targetLocation.exists()) {
targetLocation.mkdir();
}

String[] children = sourceLocation.list();
for (int i=0; i copyDirectory(new File(sourceLocation, children[i]),
new File(targetLocation, children[i]));
}
} else {

InputStream in = new FileInputStream(sourceLocation);
OutputStream out = new FileOutputStream(targetLocation);

// Copy the bits from instream to outstream
byte[] buf = new byte[1024];
int len;
while ((len = in.read(buf)) > 0) {
out.write(buf, 0, len);
}
in.close();
out.close();
}
}

source: java-tips.org

Encryption and Decryption using Symmetric Keys

This Tech Tip reprinted with permission by java.sun.com

Encryption and decryption can be done symmetrically -- here the same key is used to encrypt and decrypt the data. Because both parties have the same key, the decryption essentially is performed by reversing some part of the encryption process. The Blowfish algorithm is an example of a symmetric key. It is supported by the Java Cryptography Extension (JCE). You can find the appropriate APIs in the javax.crypto.* packages. In addition to Blowfish, examples of cipher algorithms currently supported by the JCE are the Digital Encryption Standard (DES), Triple DES Encryption (DESede), and Password-based encryption algorithm (PBEWithMD5AndDES).

Symmetric key algorithms tend to be be much faster than asymmetric key algorithms. In addition, as you saw in the first tip, the size of the text that can be encrypted depends on the size of the product of the two primes used to generate the public and private keys. With symmetric key algorithms you do not have a limitation on the total size of what can be encrypted. Although, depending on the symmetric cipher algorithms, the total input size has to be a multiple of block sizes and might require padding. A problem with symmetric keys is that keys must be shared among parties involved in encryption or decryption. So there is the danger of interception or unauthorized sharing.

You create a symmetric key much as you create a key pair. You use a factory method from the KeyGenerator class and pass in the algorithm as a String. When you call the generateKey() method, you get back an object that implements the Key interface instead of the KeyPair interface. The call looks something like this:
SecretKey key =
KeyGenerator.getInstance("DES").generateKey();

Next you need to create a Cipher. This is the workhorse for JCE. You again use a factory method of the Cipher class so that you can take advantage of different providers without changing the application. You create a Cipher like this:
Cipher cipher = Cipher.getInstance("DES");

A Cipher is used to encrypt and decrypt data that is passed in as byte arrays. The two essential methods you must use are init(), to specify which operation will be called, and doFinal(), to perform that operation. For example, the following two lines use the cipher and key instances you created to encrypt a byte array called textBytes. The result is stored in a byte array called encryptedBytes.
cipher.init(Cipher.ENCRYPT_MODE, key);
byte[] encryptedBytes =
cipher.doFinal( textBytes );

Putting this together, the following program takes an input String and encrypts it. The encrypted String is then decrypted.
import javax.crypto.Cipher;
import javax.crypto.BadPaddingException;
import javax.crypto.IllegalBlockSizeException;
import javax.crypto.KeyGenerator;
import java.security.Key;
import java.security.InvalidKeyException;

public class LocalEncrypter {

private static String algorithm = "DESede";
private static Key key = null;
private static Cipher cipher = null;

private static void setUp() throws Exception {
key = KeyGenerator.getInstance(algorithm).generateKey();
cipher = Cipher.getInstance(algorithm);
}

public static void main(String[] args)
throws Exception {
setUp();
if (args.length !=1) {
System.out.println(
"USAGE: java LocalEncrypter " +
"[String]");
System.exit(1);
}
byte[] encryptionBytes = null;
String input = args[0];
System.out.println("Entered: " + input);
encryptionBytes = encrypt(input);
System.out.println(
"Recovered: " + decrypt(encryptionBytes));
}

private static byte[] encrypt(String input)
throws InvalidKeyException,
BadPaddingException,
IllegalBlockSizeException {
cipher.init(Cipher.ENCRYPT_MODE, key);
byte[] inputBytes = input.getBytes();
return cipher.doFinal(inputBytes);
}

private static String decrypt(byte[] encryptionBytes)
throws InvalidKeyException,
BadPaddingException,
IllegalBlockSizeException {
cipher.init(Cipher.DECRYPT_MODE, key);
byte[] recoveredBytes =
cipher.doFinal(encryptionBytes);
String recovered =
new String(recoveredBytes);
return recovered;
}
}

You can enter any text you like as a command line parameter. For example, if you submit the following on the command line:

java LocalEncrypter "Whatever phrase we would like to
input at this point"

You should see something like this as output:

Entered: Whatever phrase we would like to
input at this point
Recovered: Whatever phrase we would like to
input at this point

In this example, both the encryption and the decryption were done with the same Key object. Encryption and decryption ordinarily occur on different VMs at different times, so you need a method for securely transporting the key.

In the first tip you learned how to generate key pairs for asymmetric cipher algorithms. In the second tip, symmetric keys were used. Here's another technique, one that combines asymmetric and symmetric keys. In this technique a symmetric key is chosen at random and used to encrypt some data. The key itself is then encrypted using the other party's public key. The recipient then uses their private key to decrypt the symmetric key and then uses that decrypted key to decrypt the message. The modulus used in the asymmetric technique need only be large enough to encrypt the symmetric key. The symmetric key is used for a single transmission and then discarded. In this way, the weaknesses of each type are mitigated.

You can find out more about:

* DES Encryption
* Triple DES
* AES encryption (FIPS-197), which is proposed as a replacement for DES
* Blowfish

Copyright (c) 2004-2005 Sun Microsystems, Inc.
All Rights Reserved.

Priority Queue - Binary Heap - Implementation in Java

A priority queue is an abstract data type (ADT) supporting the following three operations:

* add an element to the queue with an associated priority
* remove the element from the queue that has the highest priority, and return it
* (optionally) peek at the element with highest priority without removing it

The simplest way to implement a priority queue data type is to keep an associative array mapping each priority to a list of elements with that priority. If association lists or hash tables are used to implement the associative array, adding an element takes constant time but removing or peeking at the element of highest priority takes linear (O(n)) time, because we must search all keys for the largest one. If a self-balancing binary search tree is used, all three operations take O(log n) time; this is a popular solution in environments that already provide balanced trees but nothing more sophisticated. The van Emde Boas tree, another associative array data structure, can perform all three operations in O(log log n) time, but at a space cost for small queues of about O(2^(m/2)), where m is the number of bits in the priority value, which may be prohibitive.

There are a number of specialized heap data structures that either supply additional operations or outperform the above approaches. The binary heap uses O(log n) time for both operations, but allows peeking at the element of highest priority without removing it in constant time. Binomial heaps add several more operations, but require O(log n) time for peeking. Fibonacci heaps can insert elements, peek at the maximum priority element, and decrease an element's priority in amortized constant time (deletions are still O(log n)).

The following code shows how to implement a priority queue in Java:
// BinaryHeap class
//
// CONSTRUCTION: empty or with initial array.
//
// ******************PUBLIC OPERATIONS*********************
// void insert( x ) --> Insert x
// Comparable deleteMin( )--> Return and remove smallest item
// Comparable findMin( ) --> Return smallest item
// boolean isEmpty( ) --> Return true if empty; else false
// void makeEmpty( ) --> Remove all items
// ******************ERRORS********************************
// Throws UnderflowException for findMin and deleteMin when empty

/**
* Implements a binary heap.
* Note that all "matching" is based on the compareTo method.
* @author Mark Allen Weiss
*/
public class BinaryHeap implements PriorityQueue {
/**
* Construct the binary heap.
*/
public BinaryHeap( ) {
currentSize = 0;
array = new Comparable[ DEFAULT_CAPACITY + 1 ];
}

/**
* Construct the binary heap from an array.
* @param items the inital items in the binary heap.
*/
public BinaryHeap( Comparable [ ] items ) {
currentSize = items.length;
array = new Comparable[ items.length + 1 ];

for( int i = 0; i < items.length; i++ )
array[ i + 1 ] = items[ i ];
buildHeap( );
}

/**
* Insert into the priority queue.
* Duplicates are allowed.
* @param x the item to insert.
* @return null, signifying that decreaseKey cannot be used.
*/
public PriorityQueue.Position insert( Comparable x ) {
if( currentSize + 1 == array.length )
doubleArray( );

// Percolate up
int hole = ++currentSize;
array[ 0 ] = x;

for( ; x.compareTo( array[ hole / 2 ] ) < 0; hole /= 2 )
array[ hole ] = array[ hole / 2 ];
array[ hole ] = x;

return null;
}

/**
* @throws UnsupportedOperationException because no Positions are returned
* by the insert method for BinaryHeap.
*/
public void decreaseKey( PriorityQueue.Position p, Comparable newVal ) {
throw new UnsupportedOperationException( "Cannot use decreaseKey for binary heap" );
}

/**
* Find the smallest item in the priority queue.
* @return the smallest item.
* @throws UnderflowException if empty.
*/
public Comparable findMin( ) {
if( isEmpty( ) )
throw new UnderflowException( "Empty binary heap" );
return array[ 1 ];
}

/**
* Remove the smallest item from the priority queue.
* @return the smallest item.
* @throws UnderflowException if empty.
*/
public Comparable deleteMin( ) {
Comparable minItem = findMin( );
array[ 1 ] = array[ currentSize-- ];
percolateDown( 1 );

return minItem;
}

/**
* Establish heap order property from an arbitrary
* arrangement of items. Runs in linear time.
*/
private void buildHeap( ) {
for( int i = currentSize / 2; i > 0; i-- )
percolateDown( i );
}

/**
* Test if the priority queue is logically empty.
* @return true if empty, false otherwise.
*/
public boolean isEmpty( ) {
return currentSize == 0;
}

/**
* Returns size.
* @return current size.
*/
public int size( ) {
return currentSize;
}

/**
* Make the priority queue logically empty.
*/
public void makeEmpty( ) {
currentSize = 0;
}

private static final int DEFAULT_CAPACITY = 100;

private int currentSize; // Number of elements in heap
private Comparable [ ] array; // The heap array

/**
* Internal method to percolate down in the heap.
* @param hole the index at which the percolate begins.
*/
private void percolateDown( int hole ) {
int child;
Comparable tmp = array[ hole ];

for( ; hole * 2 <= currentSize; hole = child ) {
child = hole * 2;
if( child != currentSize &&
array[ child + 1 ].compareTo( array[ child ] ) < 0 )
child++;
if( array[ child ].compareTo( tmp ) < 0 )
array[ hole ] = array[ child ];
else
break;
}
array[ hole ] = tmp;
}

/**
* Internal method to extend array.
*/
private void doubleArray( ) {
Comparable [ ] newArray;

newArray = new Comparable[ array.length * 2 ];
for( int i = 0; i < array.length; i++ )
newArray[ i ] = array[ i ];
array = newArray;
}

// Test program
public static void main( String [ ] args ) {
int numItems = 10000;
BinaryHeap h1 = new BinaryHeap( );
Integer [ ] items = new Integer[ numItems - 1 ];

int i = 37;
int j;

for( i = 37, j = 0; i != 0; i = ( i + 37 ) % numItems, j++ ) {
h1.insert( new Integer( i ) );
items[ j ] = new Integer( i );
}

for( i = 1; i < numItems; i++ )
if( ((Integer)( h1.deleteMin( ) )).intValue( ) != i )
System.out.println( "Oops! " + i );

BinaryHeap h2 = new BinaryHeap( items );
for( i = 1; i < numItems; i++ )
if( ((Integer)( h2.deleteMin( ) )).intValue( ) != i )
System.out.println( "Oops! " + i );
}
}

// PriorityQueue interface
//
// ******************PUBLIC OPERATIONS*********************
// Position insert( x ) --> Insert x
// Comparable deleteMin( )--> Return and remove smallest item
// Comparable findMin( ) --> Return smallest item
// boolean isEmpty( ) --> Return true if empty; else false
// void makeEmpty( ) --> Remove all items
// int size( ) --> Return size
// void decreaseKey( p, v)--> Decrease value in p to v
// ******************ERRORS********************************
// Throws UnderflowException for findMin and deleteMin when empty

/**
* PriorityQueue interface.
* Some priority queues may support a decreaseKey operation,
* but this is considered an advanced operation. If so,
* a Position is returned by insert.
* Note that all "matching" is based on the compareTo method.
* @author Mark Allen Weiss
*/
public interface PriorityQueue {
/**
* The Position interface represents a type that can
* be used for the decreaseKey operation.
*/
public interface Position {
/**
* Returns the value stored at this position.
* @return the value stored at this position.
*/
Comparable getValue( );
}

/**
* Insert into the priority queue, maintaining heap order.
* Duplicates are allowed.
* @param x the item to insert.
* @return may return a Position useful for decreaseKey.
*/
Position insert( Comparable x );

/**
* Find the smallest item in the priority queue.
* @return the smallest item.
* @throws UnderflowException if empty.
*/
Comparable findMin( );

/**
* Remove the smallest item from the priority queue.
* @return the smallest item.
* @throws UnderflowException if empty.
*/
Comparable deleteMin( );

/**
* Test if the priority queue is logically empty.
* @return true if empty, false otherwise.
*/
boolean isEmpty( );

/**
* Make the priority queue logically empty.
*/
void makeEmpty( );

/**
* Returns the size.
* @return current size.
*/
int size( );

/**
* Change the value of the item stored in the pairing heap.
* This is considered an advanced operation and might not
* be supported by all priority queues. A priority queue
* will signal its intention to not support decreaseKey by
* having insert return null consistently.
* @param p any non-null Position returned by insert.
* @param newVal the new value, which must be smaller
* than the currently stored value.
* @throws IllegalArgumentException if p invalid.
* @throws UnsupportedOperationException if appropriate.
*/
void decreaseKey( Position p, Comparable newVal );
}


/**
* Exception class for access in empty containers
* such as stacks, queues, and priority queues.
* @author Mark Allen Weiss
*/
public class UnderflowException extends RuntimeException {
/**
* Construct this exception object.
* @param message the error message.
*/
public UnderflowException( String message ) {
super( message );
}
}

How to use a slider with tick marks and labels

This Java Swing tip illustrates a method of implementing a slider with tick marks and labels. The JSlider used in tip used in a similar manner to Scrollbar: the zero-argument constructor creates a horizontal slider with a range from 0 to 100 and an initial value of 50. You can also supply the orientation (via JSlider.HORIZONTAL or JSlider.VERTICAL) and the range and initial value to the constructor. Developer may handle events by attaching a ChangeListener. Its stateChanged method normally calls getValue to look up the current JSlider value.
import java.awt.*;
import javax.swing.*;

public class JSliders extends JFrame {
public static void main(String[] args) {
new JSliders();
}

public JSliders() {
super("Using JSlider");
// Comment out next line for Java LAF
WindowUtilities.setNativeLookAndFeel();
addWindowListener(new ExitListener());
Container content = getContentPane();
content.setBackground(Color.white);

JSlider slider1 = new JSlider();
slider1.setBorder(BorderFactory.createTitledBorder(
"JSlider without Tick Marks"));

content.add(slider1, BorderLayout.NORTH);

JSlider slider2 = new JSlider();
slider2.setBorder(BorderFactory.createTitledBorder(
"JSlider with Tick Marks"));

slider2.setMajorTickSpacing(20);
slider2.setMinorTickSpacing(5);
slider2.setPaintTicks(true);
content.add(slider2, BorderLayout.CENTER);

JSlider slider3 = new JSlider();
slider3.setBorder(BorderFactory.createTitledBorder(
"JSlider with Tick Marks & Labels"));

slider3.setMajorTickSpacing(20);
slider3.setMinorTickSpacing(5);
slider3.setPaintTicks(true);
slider3.setPaintLabels(true);
content.add(slider3, BorderLayout.SOUTH);

pack();
setVisible(true);
}
}

source: java-tips.org

How to get the hexadecimal value of an int

You can print the hexadecimal value of an int in Java with the following code:
int i = 0xf1;
System.out.println("i is hex " + Integer.toHexString(i) );

Changing the Cursor

The component's cursor property can be changed at any time by calling Component.setCursor() in Java. See the java.awt.Cursor class for available predefined cursor shapes.
// Create a component
Component component = new Button("OK");

// By default, the component's cursor is Cursor.DEFAULT_CURSOR
Cursor cursor = component.getCursor();

// Change the component's cursor to another shape
component.setCursor(Cursor.getPredefinedCursor(Cursor.HAND_CURSOR));