4.14 Order of Operations

In an expression that contains multiple operators, Java uses a number of rules to decide the order in which the operators are evaluated. The first and most important rule is called operator precedence. Operators in an expression that have higher precedence are executed before operators with lower precedence. For example, multiplication has a higher precedence than addition. In the expression 2+3*4, the multiplication is done before the addition, producing a result of 14.

If consecutive operators in an expression have the same precedence, a rule called associativity is used to decide the order in which those operators are evaluated. An operator can be left-associative, right-associative, or non-associative:

• Left-associative operators of the same precedence are evaluated in order from left to right. For example, addition and subtraction have the same precedence and they are left-associative. In the expression 10-4+2, the subtraction is done first because it is to the left of the addition, producing a value of 8.

• Right-associative operators of the same precedence are evaluated in order from right to left. For example, assignment is right-associative. Consider the following code fragment:

  int a = 3;
  int b = 4;
  a = b = 5;
  

  After the code has been evaluated, both a and b contain 5 because the assignments are evaluated from right to left.

• A non-associative operator cannot be combined with other operators of the same precedence.

Table 4-2 shows the precedence and associativity of all the operators in Java.[7]

[7] Although the precedence of operators in Java is similar to that in C++, there are some differences. For example, new has a higher precedence in Java than it does in C++. Another difference is that the ++ and - - operators are effectively non-associative in Java.

As in C/C++, the order in which operators are evaluated can be modified by the use of parentheses.

The rest of the rules that concern order of operations have to do with the evaluation of operands or arguments in a single expression.

• The left operand of a binary operator is evaluated before its right operand.

• The operands of an operator are evaluated before the operator is evaluated. Consider the following expression:

  ((x=4) * x)
  

  First, the left operand of * is evaluated; it produces the value 4. Then the right operand of * is evaluated. Since evaluation of the left operand set x to 4, evaluation of the right operand produces 4. Finally, the * operator itself is evaluated, producing the value 16.

• In an index expression, the expression to the left of the square brackets is evaluated before the expression inside the square brackets.

• In an expression that calls a method through an object reference, the object reference is evaluated before the argument expressions.

• In any expression that calls a method or constructor, the expressions supplied as the actual arguments are evaluated from left to right.

• In an array allocation expression, the expressions that appear in square brackets and provide the dimensions of the array are evaluated from left to right.

The intent of all of these rules is to guarantee that every implementation of Java evaluates any given expression in the same way.[8] In order to produce optimized code, a Java compiler is allowed to deviate from the rules governing the order in which operations are performed, provided that the result is the same as if it had followed the rules.

[8] This is different than C/C++, which leaves a number of details of expression evaluation up to an implementation, such as the order in which the actual parameters of a function call are evaluated.

References Array Allocation Expressions; Index Expressions; Method Call Expression; Object Allocation Expressions