As you know, Strings are objects in Java code. These objects, however, are immutable. That is, their value, once assigned, can never be changed

Polymorphism means "any forms." In object-oriented programming, it refers to the capability of objects to react differently to the same method. Polymorphism can be implemented in the Java language in the form of multiple methods having the same name. Java code uses a late-binding technique to support polymorphism; the method to be invoked is decided at runtime. Overloaded methods are methods that have the same name, but different argument lists. Overriding, on the other hand, occurs when a subclass method has the same name, same return type, and same argument list as the superclass method.

If the variable passed is a primitive, only a copy of the variable is actually passed to the method. So modifying the variable within the method has no effect on the actual variable. When you pass an object variable into a method, a copy of the reference variable is actually passed. In this case, both variables refer to the same object. If the object is modified inside the method, the change is visible in the original variable also. Though the called method can change the object referred by the variable passed, it cannot change the actual variable in the calling method. In other words, you cannot reassign the original reference variable to some other value.

This free book is an excellent tutorial book for beginners. It is a collection of notes and sample codes written by the author while he was learning JDK himself. Topics are only related to JDK packages other than java.lang.*: Blowfish, Calendar, certificate, character set, cipher, client authentication, collections, datagram, Date, decryption, DES, digital signature, encryption, DOM, DSA, DTD, example, HTTP, HTTPS, J2SDK, Java, JCA, JDK, JDK 1.3.1, JDK 1.4.1, JDK 1.5.0, key store, keytool, Locale, notes, message digests, MD5, online, private key, public key, RSA, SAX, secret key, SHA, socket, SSL, TLS, unnamed package, Unicode, UTF-8, X.509, XML, XSD, XSL.

The sleep() method causes the current thread to sleep for a given time in milliseconds. The yield() method causes the current thread to move from the running state to the runnable state, so that other threads may get a chance to run. However, the next thread chosen for running might not be a different thread. The isAlive() method returns true if the thread upon which it is called has been started but not moved to the dead state. When a thread calls join() on another thread, the currently running thread will wait until the thread it joins with has completed.

Collections are objects used to group together and manipulate multiple data elements. They can dynamically grow and shrink, which is their advantage over arrays. Collections provide methods to add objects to a collection, remove objects from it, check if an object is present in it, retrieve objects from it, and iterate through it

List of available thread constructors: Thread(), Thread(String name), Thread(Runnable runnable), Thread(Runnable runnable, String name), Thread(ThreadGroup g, Runnable runnable), Thread(ThreadGroup g, Runnable runnable, String name), Thread(ThreadGroup g, String name)

This tiny tutorial is for those who are familiar with regular expressions in Perl and want to use them in Java too. First I recommend you to read Scott A. Hommel's excellent tutorial, and then you can come back here to find those things summed up. I won't explain everything, to get acquainted with the basics, start with the other.