1 // 16.6 How do you explain the "synchronized" keyword?
 2 There‘re three properties about a member method of a class:
 3     1. static or not
 4     2. synchronized or not
 5     3. of same object instance of a class or not
 6 For example:
 7 public class FooBar {
 8     public void f() {};
 9     public synchronized void  g() {};
10     public static synchronized void h() {};
11 }
12 
13 There‘re three member methods in the class FooBar:
14     1. f() is non-static and unsynchronized.
15     2. g() is non-static and synchronized.
16     3. h() is static and synchronized.
17 
18 The keyword "synchronized" makes sure that an instance of code segment is executed by only one thread at a time.
19 The instance of the code segment may be a static one, or a non-static one belonging to one object instance.
20 
21 Unsynchronized methods is free of these constraints. They can be called by arbitrary numbers of threads at the same time.
22 Synchronized methods of different object instances is independent from each other. They can be called by different threads at the same time.
23 
24 Here‘s a truth table for your information:
25 ----------------------------------------------------------------
26 synchronized|static    |sameinstance    |can be called at same time?
27 0            |0        |0                |1
28 0            |0        |1                |1
29 0            |1        |0                |1
30 0            |1        |1                |1
31 1            |0        |0                |1
32 1            |0        |1                |0
33 1            |1        |0                |1
34 1            |1        |1                |0
35 ----------------------------------------------------------------
36 Plus: Static method can be regarded as singleton, thus it‘s always one instance only.