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問題:

So I am writing code that will take a character and return its alpha-numeric keypad equivalent as a character. The problem is, I am getting a question mark in a box as a return. I have checked that the input is correct. For example, with a char 'h' input, i should get a char '4' return. Hoping someone is able to spot my error. Code is below:

public char getDigit(char letter) throws Exception{

switch (letter) {

case 'a': case 'b': case 'c': case '2':

return 2;

case 'd': case 'e': case 'f': case '3':

return 3;

case 'g': case 'h': case 'i': case '4':

return 4;

case 'j': case 'k': case 'l': case '5':

return 5;

case 'm': case 'n': case 'o': case '6':

return 6;

case 'p': case 'q': case 'r': case 's': case '7':

return 7;

case 't': case 'u': case 'v': case '8':

return 8;

case 'w': case 'x': case 'y': case 'z': case '9':

return 9;

default:

throw new IllegalArgumentException("Must be a letter or number on the Alpha-Numeric Keypad.");

}

}

回答1:

The return type of your method is char.

Now take your switch statement. You return char values in the range from 2 to 9. Now look at an ASCII table.

Surprise: these chars are all "unprintable" control characters. Thus your console gives you "?" when you print them!

If you want '4', your code has to return '4', not 4! Or 52, because that entry in the ASCII table represents '4'.

回答2:

You are not getting proper output because you are using char as a return type of getDigit(..) function. You should use int as a return type instead of char as in switch case you are comparing with characters and returning digit values So replace your code with the following code, this will work:

public int getDigit(char letter) throws Exception{

switch (letter) {

case 'a': case 'b': case 'c': case '2':

return 2;

case 'd': case 'e': case 'f': case '3':

return 3;

case 'g': case 'h': case 'i': case '4':

return 4;

case 'j': case 'k': case 'l': case '5':

return 5;

case 'm': case 'n': case 'o': case '6':

return 6;

case 'p': case 'q': case 'r': case 's': case '7':

return 7;

case 't': case 'u': case 'v': case '8':

return 8;

case 'w': case 'x': case 'y': case 'z': case '9':

return 9;

default:

throw new IllegalArgumentException("Must be a letter or number on the Alpha-Numeric Keypad.");

}

}

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