Java算术: why aren't they "9 = 9"?

Question

我有一个关于 Java 整数和字符串算术的问题。例如，

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);

上面的代码分别输出“45 = 18”、“9 = 135”和“9 = 45”。我不明白这个操作背后的逻辑。我的第一直觉是它们都输出“9 = 9”。我希望有人帮助我理解这个操作。感谢您的帮助!

Answer 1

加法是左关联的，但括号可以改变执行顺序。

因此，如果我们必须在这里分解第一个 println，当我们写入 a+b 时，它会导致算术加法(5)，但是当我们执行 c + "= "+ a + b + c 时，会导致字符串连接 5=9，因为 c + "= " 首先求值，并将表达式作为 String + int 运算，从而产生字符串连接。请记住，int+int 是 int，String+int 是 String

由于括号()，表达式的求值方式发生了变化。如果我们包含括号，这就是上面表达式的计算方式

(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

第一个表达式的完整分解

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`

类似地，你可以分解其他两个表达式