He must increase, but I must decrease.
—John 3:30
A unary expression preceded by a -- operator is a prefix decrement expression.
The result of the unary expression must be a variable of a numeric type, or a compile-time error occurs. The type of the prefix decrement expression is the type of
the variable. The result of the prefix decrement expression is not a variable, but a
value.
At run time, if evaluation of the operand expression completes abruptly, then the prefix decrement expression completes abruptly for the same reason and no decrementation occurs. Otherwise, the value 1 is subtracted from the value of the variable and the difference is stored back into the variable. Before the subtraction, binary numeric promotion (§5.6.2) is performed on the value 1 and the value of the variable. If necessary, the difference is narrowed by a narrowing primitive conversion (§5.1.3) to the type of the variable before it is stored. The value of the prefix decrement expression is the value of the variable after the new value is stored.
A variable that is declared final cannot be decremented, because when an access of a final variable is used as an expression, the result is a value, not a variable. Thus, it cannot be used as the operand of a prefix decrement operator.