singleton
When several threads access a Singleton is has to be thread-safe and should cause as few locks as possible. Using lazy-loading, this can be achieved through double-checked locking (DCL). Though, in the literature you will find quite sophisticated articles explaining why DCL is not guaranteed by the JVM specs to work correctly.
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public class Singleton
{
  private static Singleton sSingleton;

  private Singleton()
  {
    ...
  }

  public static Singleton instance()
  {
    if(sSingleton == null)
    {
      synchronized(Singleton.class)   // you only have to synchronize once
      {
        if(sSingleton == null)
        {
          sSingleton = new Singleton();
        }
      }
    }
    return sSingleton;
  }
}
singleton unit test
In order to test a Singleton for thread-safety you can start with the follwoing JUnit test case template. There is no guarantee for correctness, though.
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public void testThread()
{
  TestThread[] threads = new TestThread[NUMOFTHREADS];
  for(int i=0; i<NUMOFTHREADS; i++)
  {
    threads[i] = new TestThread();
    threads[i].start();
  }

  for(int i=0; i<NUMOFTHREADS; i++)
  {
    try
    {
      threads[i].join();
    }
    catch(InterruptedException e)
    {
      fail("Interrupted Thread " + i);
    }
  }

  for(int i=0; i<NUMOFTHREADS-1; i++)
  {
    assertSame("thread " + i, threads[i].fSingleton, threads[i+1].fSingleton);
  }
}

class TestThread extends Thread
{
  private Singleton fSingleton;

  public void run()
  {
    fSingleton = Singleton.instance();
  }
}
read from and write to classpath
Sometimes it is necessary to read from a file on the classpath or write to a file that is already on the classpath.
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// read from classpath
InputStream is = CpReadAndWrite.class.getResourceAsStream(FILENAME);
BufferedReader reader = new BufferedReader(new InputStreamReader(is));
content = reader.readLine();
...
// write to classpath
String absolutePath = CpReadAndWrite.class.getResource(FILENAME).getPath();
PrintWriter writer = new PrintWriter((new FileOutputStream(absolutPath)));
writer.println(content);
array casting
In some situation elements of the same class are added to a collection. The collection's elements can then be retrieved as an array of the elements' class (and not only as an array of Object).
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fList = new Vector();
for(int i=0; i<10; i++)
{
  ...
  fList.add("item " + i);
  ...
}
...
return (String[]) fList.toArray(new String[fList.size()]);
reflection
Reflection is very powerful in Java. It allows at runtime to retrieve all kind of information about a class like its methods, its super class, etc...
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Vector vector = new Vector();
Method[] methods = vector.getClass().getDeclaredMethods();
for(int i=0; i<methods.length; i++)
{
  Method method = methods[i];
  String methodName = method.getName();
  String modifiers = Modifier.toString(method.getModifiers());
  String returnType = method.getReturnType().getName();
  Class[] parameters = method.getParameterTypes();
  Class[] exceptions = method.getExceptionTypes();
  ...
  System.out.println(modifiers + " " + returnType + " " + methodName + ... );
}
In addition to retrieving information about a class, it is possible to invoke methods, instantiate classes, and change field values.
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myClass = Class.forName("java.lang.Integer");
parameterTypes = new Class[]{String.class, int.class};
method = myClass.getMethod("parseInt", parameterTypes);
parameters = new Object[]{"FF", new Integer(16)};
Integer number = (Integer) method.invoke(null, parameters);
System.out.println(number.intValue());
There is even the possibility to access fields, constructors, and methods that have only private access.
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// the method getIterator has only private access
Class myClass = Class.forName("java.util.Hashtable");
Method method = myClass.getDeclaredMethod("getIterator", new Class[]{Integer.TYPE});
method.setAccessible(true);
Iterator iterator = (Iterator) method.invoke(new Hashtable(), new Object[]{new Integer(1)});
execute external applications
There is the possibility to execute external applications from within your Java application and wait for the executed application to finish before continuing with your Java program.
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Process p = Runtime.getRuntime().exec("notepad");
try
{
  p.waitFor();
}
catch(InterruptedException ie)
{
  System.out.println(ie);
}
int ret = p.exitValue();
find location of class file
Sometimes it helps to know the location from where a class file has been loaded.
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Class c = "foo".getClass();
String className = c.getName();
String resourceName = "/" + className.replace('.', '/') + ".class";
URL location = c.getResource(resourceName);
System.out.println(location);
serializing into and deserializing from string
An object can be serialized into a string and later be deserialized from that string again. Compression is also easy to include in order to keep the string shorter.
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// serialize object into a string
Object toSerialize = ...

ByteArrayOutputStream arrayStream = new ByteArrayOutputStream();
GZIPOutputStream zipStream = new GZIPOutputStream(arrayStream);
ObjectOutputStream objectStream = new ObjectOutputStream(zipStream);

objectStream.writeObject(toSerialize);
byte[] serialized = arrayStream.toByteArray();
String s = new String(serialized, "ISO-8859-1");
...
// deserialize object from string
String toDeserialize = ...
byte[] serialized = toDeserialize.getBytes("ISO-8859-1");

ByteArrayInputStream arrayStream = new ByteArrayInputStream(serialized);
GZIPInputStream zipStream = new GZIPInputStream(arrayStream);
ObjectInputStream objectStream = new ObjectInputStream(zipStream);

Object o = objectStream.readObject();
serializing into and deserializing from pseudo.xml
There is a library called Java Serialization for XML (JSX) that allows to serialize any object into pseudo-XML and later deserialize it from pseudo-XML again.
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// serialize object into a string
Object toSerialize = ...
StringWriter writer = new StringWriter();
ObjOut out = new ObjOut(false, writer);
out.writeObject(toSerialize);
String s = writer.toString();
...
// deserialize object from string
String toDeserialize = ...
StringReader reader = new StringReader(toDeserialize);
ObjIn in = new ObjIn(reader);
Object o = in.readObject();
number crunching
In high school I often forgot to do my homework and my math teacher always made calculate twenty to the power of four. If there had been Java back then...
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// not exact enough
for(int i = 0; i < 21; i++) {
  double pow = Math.pow(4, i);
  System.out.println("4^" + i + " = " + pow);
}
...
// exact
for(int i = 0; i < 21; i++) {
 BigInteger pow = BigInteger.valueOf(4).pow(i);
 System.out.println("4^" + i + " = " + pow);
}
swing look & feel
You can define the default look & feel of your Swing application on the command line by setting a system property.

java -Dswing.defaultlaf=com.sun.java.swing.plaf.windows.WindowsLookAndFeel MyApplication