Prefix Increment Operator

The increment operator ++ can be put in front of a variable. When this is done, it is a prefix operator. When it is put behind a variable it is a postfix operator. Both ways increment the variable. However:

++counter	means increment the value before using it.
counter++	means increment the value after using it.

When the increment operator is used as part of an arithmetic expression you must distinguish between prefix and postfix operators.

int sum = 0;
int counter = 10;

sum = ++counter ;

System.out.println("sum: "+ sum " + counter: " + counter );

This program fragment will print:

sum: 11  counter: 11 

This fragment requires careful inspection:

• The ++ operator is now a prefix operator.
• Now, counter is incremented before the value it holds is used.
• The assignment statement is executed in two steps:
  1. Evaluate the expression on the right of the =
     • the value is 11 (because counter is incremented before use.)
  2. Assign the value to the variable on the left of the =
     • sum gets 11.
• The next statement writes out: sum: 11 counter: 11

The Java AP Examination does not test students on using the prefix operator for any purpose (not even to increment an isolated variable).

int x = 99;
int y = 10;

y = ++x ; // prefix increment operator

System.out.println("x: " + x + "  y: " + y );