Chapter 1 Principles of Multithreading
1.1 Thread class
1.2 Ways to create threads
1.3 The difference between Thread and Runnable
Chapter 2 Thread Safety
2.1 Thread safety
When sharing the same resources, often different results than expected.
2.2 Thread synchronization (There are three ways to solve the problem of thread synchronization)
About lock
Chapter 3 Thread Status
3.1 Time waiting
sleep（long a）
wait (long a)
3.2 Infinite wait
object.wait()
object.notify()
