java beginner, operator precedence table

Question

I look at the operator precedence table and ++ operator comes before = operator. But to compute this expression, b=a++; first, a is assigned to b and then a is incremented. This is confusing. Which one comes first, ++ or =?

Answer 1

Yes, ++ has higher precedence than =. But you need to think of the ++ post-increment operator as 2 operations -- increment the variable and yield the old value -- that happen before the operator with lower precedence, =.

Here is what happens:

1. Post-increment Operator ++: Increment a.
2. Post-increment Operator ++: Yield the old value of a.
3. Operator =: Assign the old value of a to b.

It's equivalent to this code.

int resultOfPostIncrement = a;
a = a + 1;
b = resultOfPostIncrement;

Answer 2

You can read, for example this tutorial which says that increment/decrement operators have the highest precedence, while assignment operators have the lowest precedence. Thus in your example first a++ will be executed, then the previous value of a will be assigned to b.

Note that you cannot change the precedence in your case even using the parentheses: (b=a)++ is the compilation error.

Answer 3

As opposed to what many people seem to think, b is NOT assigned before incrementing a when we write int b = a++;. It is assigned the former value of a, but later in the timeline than the actual increment of a.

Therefore, it does not contradict what precedence tells you.

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To convince you, here is a little example that convinced me:

int a = 1;
int b = a++ + a++;

At the end, b equals 3, not 2, and a is also 3. What happens because of precedence is:

• the left a++ is evaluated first, incrementing a, but still evaluated to the former value 1 by definition of the operator
• the right a++ is evaluated, reading the new value 2 (proving that the first increment already happened), incrementing it to 3, but still evaluating to 2
• the sum is calculated 1 + 2, and assigned to b

This is because you should see a++ as both an instruction and an expression.

• the instruction increments a
• the expression evaluates to the former value of a

It's the same kind of things as in b = a, which also is an instruction and an expression:

• the instruction assigns the value of a to b
• the expression evaluates to the assigned value