Java如何处理表达式x = x - （x = x - 1）？

Question

I just tested the following code:

int x = 90;

x = x - (x = x - 1);
System.out.print(x);

It prints 1.

As far as I understand, things go in the following order:

1. x - 1 is computed and stored to a temporary variable in memory.
2. x is assigned the result from the temporary variable from item 1.
3. Then x - the new value of x is calculated.
4. The result is assigned to x ;

I don't understand why x from which we subtract the result of item 2 still has initial value after item 2. What am I missing?

Answer 1

From https://docs.oracle.com/javase/tutorial/java/nutsandbolts/operators.html

All binary operators except for the assignment operators are evaluated from left to right; assignment operators are evaluated right to left.

You are doing 90 - (90 - 1) => 1

It's important to not confuse precedence with order of evaluation. They are related but not the same.

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EDIT

As @ruakh points out, the JLS spec put it differently to the tutorial above.

http://docs.oracle.com/javase/specs/jls/se8/html/jls-15.html#jls-15 ‌​.7.

The left-hand operand of a binary operator appears to be fully evaluated before any part of the right-hand operand is evaluated.

If the operator is a compound-assignment operator (§15.26.2), then evaluation of the left-hand operand includes both remembering the variable that the left-hand operand denotes and fetching and saving that variable's value for use in the implied binary operation.

If evaluation of the left-hand operand of a binary operator completes abruptly, no part of the right-hand operand appears to have been evaluated.

Rather than say the assignment is evaluated right to left, it treats assignment as first a store of the variable to be updated, then an evaluation of the value and finally an assignment.

Answer 2

We start with:

int x = 90;
x = x - (x = x - 1);

The first assignment operator = has low precedence, so the right side is evaluated first.

The right side of x = x - (x = x - 1) is x - b where b is (x = x - 1)

This right side is evaluated left to right, so it becomes 90 - b

Then b, which is (x = x - 1), is evaluated. Once again the assignment operator has lowest precedence, so it becomes (x = 90 - 1) or (x = 89)

Substituting back, we have 90 - (x = 89) which is 90 - 89 which is 1. Finally, the first assignment operator is evaluated and x becomes 1. You'll notice that the other assignment operator (x = 89) had no effect on the overall operation.

Answer 3

   int x = 90;
   x = x - (x = x - 1);
   System.out.print(x);`
Here
x = 90 Goes in two section in your code.
First x=90 - , Second  (x=90-1);
Then x=90  - , x = 89 
Then x= 90 - 89
       System.out.Print(x);
that is x=1;