Java

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• JNI

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• Generics in Java 5.0

java.exe Command-Line

-Djava.library.path=<dllpath>

Set path to find DLL Libraries

-jar <jarfile>

Set jar file to load (should contain main())

java -Djava.library.path=.\var -jar "$(MYROOT)\var\main.jar"

Sample Program

• Create a file HelloWorld.java, with content:

public class HelloWorld {
  public static void main(String[] args) {
    System.out.println("Hello, World!");
    for (int i = 0; i < args.length; i++)
      System.out.print(0 == i ? args[i] : " " + args[i]);
    System.out.println();
  }
}

• Compile (javac must be in the path):

javac HelloWorld.java

• Run:

java HelloWorld

Syntax 1.4

Arrays

• Index is 0-based
• Array base type can be any primitive type or reference types.
• new allocates room for primitive types, or for references (not for object instance themselves).
• After creation, arrays cannot grow or shrink. For dynamic arrays, consider using ArrayList class.

// Declaration
int [] arrayOfInts;                                // preferred declaration style
int arrayOfInts [];                                // ... c-style

// Creation and Initialization
int arrayOfInts [] = new int [42];                 // declaration and creation (set to 0)
arrayOfInts[0] = 69;

String [] someStrings = new String [ 3 ];          // ... idem (set to null)
String [] someStrings = { null, null, null };      // ... same as above
String [] someStrings = { "hello", new String(), someStuff.toString(), null };

// Anonymous arrays
setPets ( new Animal [] { new Dog("gray"), new Cat("grey"), new Cat("orange") });

// Get array length
int aLen = arrayOfInts.length

Syntax 1.5

For loops

Foreach loops (available since 1.5) [1]:

• More readable
• Series of values
• Arrays and Collections
• Applies to Iterable<E>.

But

• Only access.
• Only single structure.
• Only single element.
• Only forward.
• At least Java 5.

FOR EACH loop	Equivalent FOR loop
for (type var : arr) { body-of-loop }	for (int i = 0; i < arr.length; i++) { type var = arr[i]; body-of-loop }
for (type var : coll) { body-of-loop }	for (Iterator<type> iter = coll.iterator(); iter.hasNext(); ) { type var = iter.next(); body-of-loop }

Miscellaneous

Class Literal

A class literal is an expression consisting of the name of a class, interface, array, or primitive type followed by a . and the token class. The type of a class literal is Class. It evaluates to the Class object for the named type (or for void) as defined by the defining class loader of the class of the current instance.

Eg:

public class MyClass //...
//...
printf ("Class name is %s\n",MyClass.class.getName());

Throwable.getStackTrace()

Could be used to get the name of current class / methods for debug...

How-To

Generate a string of repeating characters

On Java 1.4.2:

  String repeatchar(char c, int len)
  {
    char [] a = new char[len];
    Arrays.fill(a, c);
    return new String(a);
  } /* repeatchar */

Other pointers:

• Check java.util.Formatter
• Apache StringUtils has several methods: leftPad, rightPad, center and repeat. http://www.jdocs.com/lang/2.1/org/apache/commons/lang/StringUtils.html

Pad a string

On Java 1.4.2 (see [2]):

  public static String padString(String s, int n, char c, boolean padLeft)
  {
    if ( null == s ) {
      return null;
    }

    int  npad = n - s.length();
    if ( npad < 0 ) {
      return s;
    }

    char [] pad = new char [npad];
    Arrays.fill(pad, c);
    return ( padLeft ) ? new String(pad) + s : s + new String(pad);  // TODO: Faster to use StringBuffer, and insert/append?
  } /* padString */

On Java 1.5, use String.format:

public static String padRight(String s, int n) {
     return String.format("%1$-" + n + "s", s);  
}

public static String padLeft(String s, int n) {
    return String.format("%1$#" + n + "s", s);  
}

Other pointers:

• For formatting numbers, use java.text.DecimalFormat (see [3])
• Since Java 1.5, one should prefer using StringBuilder over StringBuffer.
• rosettacode.org contains more code snippets for any language ([4])

Debug

References

• http://blog.zvikico.com/2007/11/five-ways-for-t.html

Logging:

• http://java.sun.com/j2se/1.4.2/docs/guide/util/logging/
• http://commons.apache.org/logging/
• http://logging.apache.org/log4j/1.2/index.html

Dynamic Proxy:

• http://java.sun.com/j2se/1.3/docs/guide/reflection/proxy.html

Java Trace

• cfr [5]

Profilers

• http://www.eclipse.org/tptp/

Aspect-Oriented Programming

• http://en.wikipedia.org/wiki/Aspect-oriented_programming

Some sample code:

  private static final String ANSI_PURPLE = (char) 27 + "[35m";
  private static final String ANSI_NONE   = (char) 27 + "[0m";

  public static String getMethodName()
  {
    return getMethodName(1);
  }

  public static String getMethodName(int idx)
  {
    return new Throwable().getStackTrace()[1].getMethodName();
  }

  public static String getFileMethodLine()
  {
    return getFileMethodLine(1);
  }

  public static String getFileMethodLine(int idx)
  {
    StackTraceElement e = new Throwable().getStackTrace()[idx];

    //return e.getClassName() + "." + e.getMethodName() + "(" + e.getFileName() + ":" + e.getLineNumber() + ")";    // Java like
    return e.getFileName() + " " + e.getMethodName() + ":" + e.getLineNumber();                                   // C like
  }

  public static void debug(String s)
  {
    System.out.println(ANSI_PURPLE + getFileMethodLine(2) + ANSI_NONE + " " + s);
  }

References

String Integer.toHexString(byte);                    // Convert an integer to string in hexadecimal

"00" + Integer.toHexString( i )                      // Pad with leading zeros (then take rightmost characters)

String.format("%04x",0x2a);

//Prepend 0 if needed
String hex = Integer.toHexString(abyte & 0xff);
StringBuffer hexbuf = new StringBuffer();
if (hex.length() == 1) hexbuf.append('0');
hexbuf.append(hex);

//Prepend 0 if needed
String hex=Integer.toHexString(aInt);                  // Assume 0<= aInt <= 255 (i.e. positive!)
System.out.print((hex.length()>1? "" : "0") + hex + ", ");