[Chapter 4] 4.3 Increment/Decrement Operators
Author: Mark Grand
Date: Thu Jul 31 13:10:02 1997

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| Chapter 4 Expressions |

## 4.3 Increment/Decrement Operators

The `++` operator is used to increment the contents of a variable or an array element by one, while the `- -` operator is used to decrement such a value by one. The operand of `++` or `- -` must evaluate to a variable or an array element; it cannot be an expression that produces a pure value. For example, the following operations succeed because the operand of the `++` operator produces a variable:

    int g = 0;
    g++;

However, the following uses of `++` generate error messages:

    final int h = 23;
    h++;
    5++;

The expression `h++` produces an error because `h` is declared `final`, which means that its value cannot be changed. The expression `5++` generates an error message because 5 is a literal value, not a variable.

The increment and decrement operators can be used in both postfix expressions (e.g., `i++` or `i- -`) and in prefix expressions (e.g., `++i` or `- -i`). Although both types of expression have the same side effect of incrementing or decrementing a variable, they differ in the values that they produce. A postfix expression produces a pure value that is the value of the variable before it is incremented or decremented, while a prefix expression produces a pure value that is the value of the variable after it has been incremented or decremented. For example, consider the following code fragment:

    int i = 3, j = 3;
    System.out.println( "i++ produces " + i++);
    System.out.println( "++j produces " + ++j);

The above code fragment produces the following output:

    i++ produces 3
    ++j produces 4

After the code fragment has been evaluated, both `i` and `j` have the value 4.

In essence, what you need to remember is that a prefix expression performs its increment or decrement before producing a value, while a postfix expression performs its increment or decrement after producing a value.

### Postfix Increment/Decrement Operators

A postfix increment/decrement expression is a primary expression that may be followed by either a `++` or a `- -`:

The postfix increment and decrement operators are equal in precedence and are effectively non-associative.

If a postfix expression includes a `++` or `- -`, the primary expression must produce a variable or an array element of an arithmetic type. The postfix increment operator (`++`) has the side effect of incrementing the contents of the variable or array element by one. The postfix decrement operator (`- -`) has the side effect of decrementing the contents of the variable or array element by one.

The data type of the value produced by a postfix increment/decrement operator is the same as the data type of the variable or array element produced by the primary expression. A postfix increment/decrement operator produces the original pure value stored in the variable or array element before it is incremented or decremented.

The following is an example of using a postfix decrement operator:

    char j = '\u0100';
    while (j-- > 0)        // call doit for char values
        doit(j);           // '\u00ff' through '\u0000'

This example works because Java treats `char` as an arithmetic data type.

References Arithmetic Types; Order of Operations; Primary Expressions

### Prefix Increment/Decrement Operators

A prefix increment/decrement expression is a primary expression that may be preceded by either a `++` or a `- -`:

The prefix increment and decrement operators are equal in precedence and are effectively non-associative.

If a prefix expression includes a `++` or `- -`, the primary expression must produce a variable or an array element of an arithmetic type. The prefix increment operator (`++`) has the side effect of incrementing the contents of the variable or array element by one. The prefix decrement operator (`- -`) has the side effect of decrementing the contents of the variable or array element by one.

The data type of the value produced by a prefix increment/decrement operator is the same as the data type of the variable or array element produced by the primary expression. A prefix increment/decrement operator produces the pure value stored in the variable or array element after it has been incremented or decremented.

Here's an example of using a prefix increment operator:

    void foo(int a[]) {
        int j = -1;
        while (++j < a.length)    // call doit for each element
            doit(a[j]);              // of a
        }

References Arithmetic Types; Order of Operations; Primary Expressions

Allocation Expressions |  | Unary Operators