Operators and Assignments - Prefix and Postfix Operators

• the operators ++ and -- are used to increment or decrement a variable value by 1
• binary numeric promotion is applied on both the 1 and the variable value before the addition or subtraction occurs (ie at a minimum both values are promoted to an int) BUT the type of the expression is the type of the variable so narrowing conversion is applied if necessary ie if the original variable is a byte, short, or char the result is narrowed to the corresponding type
  

  byte b = 2;
  byte b1;

  b1 = ++b;  // no error although promotion occurs
  b = 127;
  b1 = ++b;  // result: -128 (no error as fits within byte type)

• the expression has the same type as the variable
• they can appear before a variable (prefix) or after a variable (postfix)
• cannot be used with final variables

Prefix (++x) and Postfix (x++) increment operators

• 1 is added to the value of the variable and the result is stored back in the variable
• both operators have the same effect as x = x + 1;

      int x;
      
      x = 0;
      ++x;                    // result: 1
      
      x = 0;
      x++;                    // result: 1
  

Prefix (--x) and Postfix (x--) decrement operators

• 1 is subtracted from the value of the variable and the result is stored back in the variable
• both operators have the same effect as x = x - 1

      int x;
      
      x = 0;
      --x;                    // result: -1
      
      x = 0;
      x--;                    // result: -1
      
  

Using prefix and postfix operators in expressions

• when a prefix expression (++x or --x) is used as part of an expression, the value returned is the value calculated after the prefix operator is applied

      int x = 0;
      int y = 0;
      y = ++x;            // result: y=1, x=1
      
      x is incremented by 1 and the result is assigned to y
  

• when a postfix expression (x++ or x--) is used as part of an expression, the value returned is the value calculated before the postfix operator is applied

      int x = 0;
      int y = 0;
      y = x++;            // result: y=0, x=1
      
      original value of x is stored, x is incremented, 
      original value of x is assigned to y
  

• when using the postfix form of the operators do not try constructs like

      int x = 0;
      x = x++;   // result: 0, x is not incremented    
    
      original value of x is saved (x0rig)
      x is incremented
      x0rig is assigned to x
      therefore, x will always equal original value
  

Effect on 'char' type

• both the prefix and postfix forms may be used on char types

      char c = 'a';
      c++;                // result: b
      --c;                // result: a
  

• the type of the variable does not change

Example Code

• TestPrefixAndPostfix.java

Tips

• postfix/prefix operators have the highest level of precedence
• remember that when the postfix operator is used in an expression, the current value of the variable is used