Defining, Instantiating, and Starting Threads

• Threads can be created by extending Thread and overriding the public void run() method.
• Thread objects can also be created by calling the Thread constructor that takes a Runnable argument. The Runnable object is said to be the target of the thread.
• You can call start() on a Thread object only once. If start() is called more than once on a Thread object, it will throw a RuntimeException.
• It is legal to create many Thread objects using the same Runnable object as the target.
• When a Thread object is created, it does not become a thread of execution until its start() method is invoked. When a Thread object exists but hasn't been started, it is in the new state and is not considered alive.