问题
Possible Duplicate:
Java - Order of Operations - Using Two Assignment Operators in a Single Line
If we assign a variable a value in a chain like the following,
int x=10, y=15;
int z=x=y;
System.out.println(x+" : "+y+" : "+z);
then the value of all three variables x, y and z becomes 15.
I however don't understand the following phenomenon with an array.
int array[]={10, 20, 30, 40, 50};
int i = 4;
array[i] = i = 0;
System.out.println(array[0]+" : "+array[1]+" : "+array[2]+" : "+array[3]+" : "+array[4]);
It outputs 10 : 20 : 30 : 40 : 0. It replaces the value of the last element which is array[4] with 0.
Regarding previous assignment statement - int z=x=y;, I expect the value of the first element means array[0] to be replaced with 0. Why is it not the case? It's simple but I can't figure it out. Could you please explain?
By the way, this assignment statement array[i] = i = 0; is dummy and it has no value of its own in this code and should no longer be used but I just wanted to know how thing actually works in this case.
回答1:
int i = 4; when i equals to 4 the array[i] equals to array[4] so array[i] = i = 0; is equivalent to array[4] = i = 0;. That is way it change the value of index 4 with 0.
回答2:
The separators [] and () change precedence. Everything within these separators will be computed before Java looks outside them.
array[i] = i = 0;
During compiler phases, the first change to this line will happen as follows:
array[4] = i = 0; // subscript separator is evaluated first.
Now, assignment operation is right-associative, So, i is assigned value 0 and then array[4] is assigned value of i i.e. 0.
Check following links:
- http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.7
- http://docs.oracle.com/javase/specs/jls/se7/html/jls-3.html#jls-3.12
回答3:
Let me break it....
Your statement:
int array[]={10, 20, 30, 40, 50};
Implementation:
array[0] => 10
array[1] => 20
array[2] => 30
array[3] => 40
array[4] => 50
Your statement:
int i = 4;
Implementation:
i => 4
Your statement:
array[i] = i = 0;
Implementation:
array[4] = i = 0;
array[4] = 0
Well if you want array[0] => 0, then do this...
int i = 0;
array[i] = i = 0
回答4:
because array[i] is evaluated before assignment opeartion
来源：https://stackoverflow.com/questions/13116833/assigning-a-value-to-a-variable-in-a-chain-in-java