Associativity

Java reads statements left to right. When it sees operators, it looks for data to use with the operators. Associativity determines in which order a given operator looks for its operands.

associativity

Where an operator looks for data.

b = 4; // The = has right-to-left associativity. This means it
       // looks at the right-hand side for a value,
       // and assigns it to the variable on the left-hand side.

int c = 5;
int d = c + 8; // The + has left-to-right associativity. It will
               // look at the left hand side and evaluate it,
               // then evaluate the right hand side, and then add the two.
               // In this example it substitutes the value 5 for c on the left,
               // finds the literal 8 on the right, and adds the two.

Associativity Example

Java knows the associativity of operators. Let's imagine Java narrating what it "thinks" as it evaluates a statement. (Assume all variables have been declared.)

b = 5 + 7;

• "b ... A variable. What follows it?"
• "= ... Look to the right for something to put in this variable."
• "5 ... Is this part of another expression?"
• "+ ... Yes. What is the other operand for + ? I'll keep reading to the right."
• "7 ... 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 + 7 evaluates to 12."
• "Take 12 from the right side of the = and assign it to b."

Java will substitute variable values as it reads left to right. Look at the following example.

int a = 9;
b = a + 7;

• "b ... A variable. What follows it?"
• "= ... Look to the right for something to put in this variable."
• "a ... A variable. Substitute its value."
• "9 ... 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."
• "7 ... 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."
• "9 + 7 evaluates to 16."
• "Take 16 from the right side of the = and assign it to b."

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