Best practices in Java and Swing

Best practices in Java and Swing
Arne Bachmann

Folie 1
Vortrag > Autor > Dokumentname > 09.11.2005

Topics

Arrays, Collections and Loops

Enumerations

The final keyword

Threading
In Swing

Singleton

Tools

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"Foreach" style loop

void listLines(String[] lines) {
PrintStream ps = new PrintStream(
new FileOutputStream("C:test.out"));

for (String line: lines) {
ps.println(line);
}

ps.close();
}

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"Map-Entry" style loop – similar to ".items()" in Python

String listItems(Map<String, Long> collection) {
StringBuilder sb = new StringBuilder();

for (Entry<String, Long> entry: collection.entrySet())
{
sb.append(entry.getKey())
.append(" maps to ")
.append(entry.getValue());
.append("n");
}

return sb.toString();
}

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Enumerations

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Enumerations

Don't try to serialize enums
don't provide a private static final long serialVersionUID

Use enum instead of true/false where possible

import static my.package.Visibility.VISIBLE;
import static my.package.Visibility.HIDDEN;

display.setVisible(VISIBLE);
display.setVisible(HIDDEN);

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Enumerations

Don't try to serialize enums
don't provide a private static final long serialVersionUID

Use enum instead of true/false where possible

import static my.package.Visibility.VISIBLE;
import static my.package.Visibility.HIDDEN;
Import static my.package.Visibility.SEMI_TRANSPARENT;

display.setVisible(SEMI_TRANSPARENT);

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Enumerations

Combine settings by using enumSet

enum ELogCapability {
Console,
Graphical,
File;
}

// Constructor
MyLogger(EnumSet<ELogCapability> logTypes) {
if (logTypes.contains(ELogCapability.Console))
{
// ...
} else if ...
}
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Enumerations

Powerful enumerations – Part 1: The less known constructor

enum EVariableDataType {

Integer ("long"),
Real ("double"),
File ("file");

private final String type;

private EVariableDataType(String aType) {
type = aType;
}

}
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Enumerations

Powerful enumerations – Part 2: The trick

private static final Map<String, EVariableDataType>
TYPES = new Hashtable<String, EVariableDataType>();

static {
TYPES.put(Integer.type, Integer);
TYPES.put(Real.type, Real);
TYPES.put(File.type, File);
}

public static EVariableDataType typeOf(String forName) {
return TYPES.get(forName.toLowerCase());
}

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Final

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Final

Use final as often as possible

final class ExampleClass {

void example(final File source) {
final File[] files = source.listFiles();
for (final File file: files) {
final String name =
file.getAbsolutePath();
try {
file.delete();
} catch (final IOException e) {
logger.error(e);
}
}
}

}
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Final

Constant value initialization from the constructor

protected class MyClass {

final private String param;

MyClass(final String p) {
param = p;
}

}

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Variable number of arguments

Beware of varargs methods:
Can create very hard to find errors
Long[] createLongArray(final long first) {
return new Long[] { first };
}

Long[] createLongArray(final long first, final long second) {
return new Long[] { first, second };
}

Long[] createLongArray(long... args) {
final List<Long> list = new ArrayList<Long>();
for (final Long l: args) {
list.add(l);
}
return list.toArray(new Long[0]); // or: new Long[] {}
}
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Collections

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Collections

Always type collections (we're in Java SE 5, which already came 2004)
e.g.: List<Pair<String, Wrapper<ELogCapability>[]>> list;

Hashtable<T, U>
Allows only non-null keys and values
Synchronized
HashMap<T, U>
Allows null for both keys and values
Unsynchronized

Use new ConcurrentHashMap() for Collections.synchronizedMap()
Internally synchronized

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Singleton

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Singleton

Singleton – Often implemented wrong:
public class MySingleton {

private static MySingleton instance;

private MySingleton() {
// initialization
}

public static MySingleton getInstance() {
if (instance == null) {
instance = new MySingleton();
}
return instance;
}

}

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Singleton

Singleton – This works
public class MySingleton {

private static MySingleton instance;

private MySingleton() {
// initialization
}

public static synchronized MySingleton getInstance() {
if (instance == null) {
instance = new MySingleton();
}
return instance;
}

}

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Singleton

Singleton – Use synchronization of the classloader

public class MySingleton2 {

final static private MySingleton2 instance = new MySingleton2();

private MySingleton2() {
// initialization
}

public MySingleton2 getInstance() {
return instance;
}

}

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Threading

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Threading

Useful: Set UncaughtExceptionHandler

class MyHandler implements UncaughtExceptionHandler {
public void uncaughtException(final Thread t, final Throwable e) {
logger.error(t.getName() + ": " + e.getMessage());
}
}

void newThread() {
final Thread thread = new Thread(new Runnable() {
public void run() {
// do something in the thread
}
});
thread.setUncaughtExceptionHandler(new MyHandler.getInstance());
}

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Threading

How to create standard threads
Dont' use new Thread(…) anymore!

final ExecutorService single;

single = Executors.newSingleThreadExecutor();

single.execute(new Runnable() {
public void run() {
// anonymous code
}
});

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Threading

How to create standard threads
Dont' use new Thread(…) anymore!

final ExecutorService single;
final ExecutorService pool;
final ExecutorService timed;

single = Executors.newSingleThreadExecutor();
pool = Executors.newFixedThreadPool(3);
timed = Executors.newSingleThreadScheduledExecutor();

single.execute(new Runnable() {
public void run() {
// anonymous code
}
});

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Threading

Use volatile and synchronize wisely
Not easy to understand, find yourself some articles on that topic!

Volatile ensures visibility of
Variable reference (for reference types)
Variable value (for simple types)
Use atomic types for long and double

Because Java usually doesn't guarantee visibility of other
threads' memory changes

Synchronize explicitly uses JVM's object locking mechanism to
synchronize (linearize) variable accesses → deadlocks possible

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Threading

Avoid using anonymous classes where applicable
Refactor to named classes step-by-step

How to avoid the "synthetic accessor method" problem?
Named inner classes, see next slide

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Threading

"Read access is emulated by a synthetic accessor" (Eclipse warning)

public class WrongSyntheticAccessorExample extends JDialog {

private String name;

protected JPanel createPanel() {
final JPanel panel = new JPanel();
final JButton button = new JButton("Huh?");
button.addActionListener(new ActionListener() {
public void actionPerformed(final ActionEvent evt) {
button.setText(button.getText() + name);
}
});
return panel;
}

}
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Threading

Solution: Inner classes
public class SyntheticAccessorExample extends JDialog {

private class InnerClass implements ActionListener {
private final String copyOfName;
private final JButton buttonRef;
InnerClass(final String name, final JButton button) {
copyOfName = new String(name); // shallow clone
buttonRef = button; // keep reference
}
buttonRef.setText(buttonRef.getText() + copyOfName);
}
}

protected JPanel createPanel() {
final JPanel panel = new JPanel();
final JButton button = new JButton("Huh?");
button.addActionListener(new InnerClass("Done!", button));
return panel;
}

}

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Threading in Swing

The Event Dispatch Thread (EDT)
Almost everything GUI related must happen in this thread!
Otherwise: GUI not updated, blocking or unintelligible results
Exception: first static call from main or init
Exception: repaint() … and listener (de-) registration

Events by GUI components are always automatically on the EDT

Put everything CPU or time consuming to other threads (executors)
And provide callbacks for GUI updates

Q: How to build up a complex GUI while staying responsive?

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Threading in Swing

Events by GUI components are always automatically on the EDT, e.g.
public void actionPerformed(final ActionEvent evt) {}
public void mouseClicked(final MouseEvent e) {}

Use javax.swing.Timer to create repeating events on the EDT:

final Timer timer = new Timer(200 /* ms */,
new ActionListener() {
button.setEnabled(!button.isEnabled());
}
});
timer.setInitialDelay(0); // start immediately
timer.start();

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Threading in Swing

Put everything CPU or time consuming to other threads (executors)
And provide callbacks for GUI updates
final JButton okB = new JButton("OK");
okB.setEnabled(false);
final ExecutorService thread =
Executors.newSingleThreadExecutor();

thread.execute(new Runnable() {
public void run() {
// do some time-consuming calculations here
SwingUtilities.invokeLater(new Runnable() {
public void run() {
okB.setEnabled(true);
}
});
thread.shutdown(); // commence own shutdown
}
}); Folie 31

Threading in Swing

Initialization during class load or construction is possible
OK if class loaded on EDT (e.g. new GUI(); in invokeLater())
OK if initialization is only simple, fast and failsafe
BUT if exceptions occur here, they are hard to track down!

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Threading in Swing

Question: How to build up a complex GUI while staying responsive?
A: Ensure that a splash screen is shown before building GUI?
A: Break down GUI creation into single work tasks queued in EDT?
A: Interrupt the thread somehow?

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Tools

VisualVM
Based on Netbeans platform
Connects to VMs via JMX (really useful only since Java SE 6)
Allows profiling, monitoring, recording, snapshotting
Many plugins available

Swing Explorer Library
Detect errors in UI applications

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References

Enumerations
http://www.javaworld.com/community/node/3125

Singleton
http://www.theserverside.de/singleton-pattern-in-java/

Threading
http://java.sun.com/products/jfc/tsc/articles/threads/threads1.html

Java Best practices:
Joshua Bloch (2008): "Effective Java. Second edition."

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Questions?

http://www.walle-derfilm.de/

Best practices in Java and Swing

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