The unary operator '++expr' is a prefix operator and 'expr++' is a postfix operator.

When used in a assignment or print context (like within a print statement), a prefix operator (e.g. ++a) first increments a and then return the value of a, whereas the postfix operator (e.g. a++) returns the value of a and then increments a.

Expansions for the prefix and postfix shorthand forms

Below expansions will help you understand these operators well.

1. a++ expands to

   • a=a+1;

2. a-- expands to

   • a = a-1;

3. ++a expands to

   • a = a+1;

4. --a expands to 

   • a = a - 1;

5. b = a++ expands to

   • temp = a;

   • a = a + 1;

   • b = temp;

6. b=a-- expands to

   • temp = a;

   • a = a - 1;

   • b = temp;

7. b=++a expands to

   • a = a + 1;

   • b = a;

8. b=--a expands to

   • a = a - 1;

   • b = a;

9. print(a++)

   • print(a)

   • a = a + 1;

10. print(++a)

    • a = a + 1;

    • print(a);

Note: Ones that use temp can be very tricky to solve. Refer to this question and try to answer: http://javajee.com/javaquizzes/question-49.

Example: prefix and postfix operator

int a=4;

System.out.println(a--);

System.out.println(--a);

System.out.println(a);

This will print:

4

2

2

• The variable 'a' is assigned 4 initially.

• First println with a-- prints the value of a and then decrements a to 3. So 4 is printed first.

• Second println with --a first decrements a and then prints a, so value is now 2 and 2 is printed.  

• Last println prints the current value of a, which is 2.