Java算术：为什么它们不是“ 9 = 9”？

Question

I have a question on java arithmetic with integers and strings. For example,

int a = 1;
int b = 3;
int c = 5;
System.out.println(a + b + (c + " = ") + a + (b + c));
System.out.println((a + b) + c + " = " + a + b + c);
System.out.println(a + (b + c) + " = " + (a + b) + c);

The code above outputs "45 = 18", "9 = 135", and "9 = 45" respectively. I don't understand the logic behind this operation. My first instinct was they all output "9 = 9". I would like somebody to help me understand this operation. I appreciate your help!

Answer 1

Addition is left-associative but parenthesis can change the order of execution.

So if we have to break down the first println here, when we write a+b , it results in arithmetic addition (5) , but when we do c + " = " + a + b + c , it results in string concatenation 5=9 , because c + " = " evaluates first and make the expression as String + int operation resulting in string concatenation. Remember, int+int is int and String+int is String

Because of parenthesis () , how the expression evaluates changes. This is how the above expression evaluates if we include parenthesis

(c + " = ") + a + (b + c)
 - First it evaluates (c + " = "), so the expression becomes 5 = + a + (b + c)
 - Now it evaluates b+c because of parenthesis, , so the expression becomes 5 = + a + 8
 - Now as there are not parenthesis, it evaluates the expression from left to 
   right and as the first operand is string, the whole expression becomes a
   string concatenation operation

Complete breakdown of first expression

a + b + (c + " = ") + a + (b + c)
- First precedence is of (b + c), so now it becomes a + b + (c + " = ") + a+8
- Next precedence  is of (c + " = "), so now it becomes a + b + "5 = " + a+8
- Now as there is not (), expression is evaluated from left to right, so
  now it evaluates a + b , so it becomes 4 + "5 = " + a+8
- now it evaluates '4 + "5 = "', so it becomes `45 = a + 8`, because we have 
  now a string in the expression, so it does string concatenation
- and it becomes `45 = 18`

Similarly you can breakdown the other two expression

Answer 2

The point here is that you are mixing int addition + with the string concatenation + operation.

In the you compute 1+3, resulting in 4. Then you put that in front of a string that is "5 = 1" which is followed by the result of 5 + 3 (8).

The different results are then based on the different effects placing the braces.

Answer 3

If you concat int with string then it will result in string, and by adding brackets it changes the execution structure Your example: System.out.println(a + b + (c + " = ") + a + (b + c));

1. (b + c) will resolve to 8
2. (c + " = ") will be resolved to "5="
3. a + b which is at start of statement will result in 4
4. a + b + (c + " = ") this statement will become a string value 4+"5=" and output will be "45="
5. then this will get concatenated with a + b + (c + " = ") + a and result in "45=" + 1 and results in "45=1"
6. so the whole statement will become a + b + (c + " = ") + a + (b + c) resolved to "45=1" + 8 so the final result is "45=18"

Answer 4

There are two meanings for + operation. First meaning is arithmetic addition operation between multiple numbers value. For example

System.out.println(1 + 2 + 5);  

Above print result is 9.
Second meaning is String concatenation between String and others. For example

System.out.println(9 + "=" + 9);  

Above print result is "9=9".
I think you may want to print the commutative law of addition. Following may be what you want.

int a = 1, b = 3, c = 5;
System.out.println( (( a + b ) + c ) + "=" + ( a + ( b + c )) );