Precedence

When expressions contain more than one operator, Java must determine what to evaluate first. Each operator's precedence determines which operator executes first.

precedence

Rules specifying the order in which operators are evaluated when there are multiple operators in an expression.

Operators with higher precedence are evaluated first.

c = b + 7; // The + has higher precedence than = so it's
           // evaluated first, and its result is the
           // right-hand operand of the =.

c = 5 + b * 7; // The + operator looks for a right-hand operand.
               // b is shared between + and *. The * has higher
               // precedence than + so b * 7 is evaluated first,
               // and its result is used as the right-hand operand of the +.

Associativity and Precedence

Associativity determines which operator gets evaluated first when an expression contains more than one operator with the same precedence.

int a = 5;
int b = 10;
int c = 19;
a = b + c + 10; // +'s have the same precedence,
                // associativity is left to right. Take
                // b and add it to to c first, then add the
                // result to 10.

Parentheses ( ) and Precedence

We can change the order of precedence by grouping expressions in parentheses.

int b = 5;
int c = 2;
a = b * (c + 10); // * looks right for a right-hand operand.
                  // parentheses cause c + 10 to execute first

Imagine Java narrating this statement evaluation.

• "a ... A variable. What follows it?"
• "= ... Look to the right for something to put in this variable."
• "b ... A variable. Substitute its value."
• "5 ... Is this part of another expression?"
• "* ... Yes. What is the other operand for * ? I'll keep reading to the right."
• "( ... I have to evaluate whatever is in this () group before doing the multiplication"
• "c ... A variable. Substitute its value."
• "2 ... Is this part of another expression? I'll keep reading to the right."
• "+ ... Yes. What is the other operand for + ? I'll keep reading to the right."
• "10 ... Is this part of another expression? I'll keep reading to the right."
• ") ... No. And it's the end of a () group. I'll evaluate it."
• "12 ... Is this part of another expression? I'll keep reading to the right."
• "; ... It's the end of the statement so I should now evaluate the previous expression."
• "5 * 12 evaluates to 60."
• "Take 60 from the right side of the = and assign it to a."

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