String charAt（）方法在Java中如何工作？

Question

Why does the following code create compile error? Doesn't charAt() method return char datatype?

public static void main (String[] args) throws java.lang.Exception
{
    String a = "abcd";
    char c = a.charAt(2)-'a';
}

error: incompatible types: possible lossy conversion from int to char

Answer 1

When you subtract two char s, they are promoted to int and the subtraction is performed on two int operands, and the result is an int .

You can cast to char to assign the result to a char :

char c = (char) (a.charAt(2)-'a');

Note that you might get unexpected results if the subtraction results in a negative value, since char can't contain negative values.

Besides, I'm not sure it makes any sense to subtract 'a' from a character and store the result in a char . In your example, it will give you the character whose numeric value is 2, which has no relation to the character 'c'.

Answer 2

Java Language Specification says:

• If any of the operands is of a reference type, unboxing conversion (§5.1.8) is performed. Then:
• If either operand is of type double, the other is converted to double.
• Otherwise, if either operand is of type float, the other is converted to float.

• Otherwise, if either operand is of type long, the other is converted to long.

• Otherwise, both operands are converted to type int.

So you need to explicitly cast it to char to get rid of the possible lossy error

char c = (char) (a.charAt(2)-'a');

Otherwise, the result of the binary operation is converted to the type of the left-hand variable, subjected to value set conversion (§5.1.13) to the appropriate standard value set (not an extended-exponent value set), and the result of the conversion is stored into the variable.

Answer 3

charAt() method return char type But you are subtracting two char that will return int value in this case it will be 2 . So receive it into int it will work.

int c = a.charAt(2)-'a';