Java - Order of Operations - Using Two Assignment Operators in a Single Line

问题

What are the order of operations when using two assignment operators in a single line?

public static void main(String[] args){
    int i = 0;
    int[] a = {3, 6};
    a[i] = i = 9; // this line in particular
    System.out.println(i + " " + a[0] + " " + a[1]);
}

Edit: Thanks for the posts. I get that = takes values from the right, but when I compile this I get:

9 9 6

I thought it would have been and ArrayOutOfBounds exception, but it is assigning 'a[i]' before it's moving over the 9. Does it just do that for arrays?

回答1:

= is parsed as right-associative, but order of evaluation is left-to-right.

So: The statement is parsed as a[i] = (i = 9). However, the expression i in a[i] is evaluated before the right hand side (i = 9), when i is still 0.

It's the equivalent of something like:

int[] #0 = a;
int #1 = i;
int #2 = 9;
i = #2;
#0[#1] = #2;

回答2:

As per the specs:

• http://docs.oracle.com/javase/specs/jls/se5.0/html/expressions.html

15.26 Assignment Operators There are 12 assignment operators; all are syntactically right-associative (they group right-to-left). Thus, a=b=c means a=(b=c), which assigns the value of c to b and then assigns the value of b to a.

So, a[i] = i = 9; is the same as i = 9; a[i] = i;

Edit

Actually, that's not the case. Sample test class:

import java.util.Arrays;

public class Mkt {
  public static void main(String[] args) {
    int[] a = new int[10];
    int i = 5;
    a[i] = i = 9;

    System.out.println(Arrays.toString(a));
  }
}

Sample run:

$ javac Mkt.java && java Mkt
[0, 0, 0, 0, 0, 9, 0, 0, 0, 0]

Please refer to the other answer for more information. Basically:

• a[i] = i = 9 is the same as a[i] = (i = 9), as = is right-associative

• However, operand evaluation is left-to-right, as per this:

  15.7. Evaluation Order

  The Java programming language guarantees that the operands of operators appear to be evaluated in a specific evaluation order, namely, from left to right.

  It is recommended that code not rely crucially on this specification. Code is usually clearer when each expression contains at most one side effect, as its outermost operation, and when code does not depend on exactly which exception arises as a consequence of the left-to-right evaluation of expressions.

I copied the second paragraph which is very instructive here - rarely does it make sense to write confusing code like that.

I also find this worth checking out.

回答3:

If I remember correctly, = operator is right-associative; so i will be assigned first, then a[i].

回答4:

The = operator is right-associative(as others have already said). This can be easily proven with this test:

int i = 2;
int j = 3;
int x = i = j;
System.out.println(x); // This prints out 3.

This works with all types, Objects and primitives.

The way I have heard this referred to is "dual assignment", since, using the example above, you are assigning the value of j to both i and x.

来源：https://stackoverflow.com/questions/9440844/java-order-of-operations-using-two-assignment-operators-in-a-single-line