(Java ) problems with myString.charAt(i) > “1” or myString.charAt(i) < “A”

Question

I have a car Object which is supposed to have a characteristic. The characteristic is supposed to have the requirements: starts with two capital letter followed by a number from 1-9 followed by 4 numbers from 0-9.

public void writeCharacteristic(){


    System.out.println("write down the characteristic for the car.");
    String characteristic = kb.nextLine();

    progress = false; 

    if (characteristic.length() != 7){
        System.out.println("The string is not 7 letter/numbers long");
        progress = false; 
    }

    for(int i = 0; i < 2; ++i){
         if (characteristic.charAt(i) < "A" || characteristic.charAt(i) > "Z"){
        System.out.println(" character number " + i + " is invalid");
        progress = false;
         }

    }

    if (characteristic.charAt(3) < "1" || characteristic.charAt(3) > "9")
        progress = false;

    for (int j = 3; j < 7; ++j){
        if (characteristic.charAt(j) < 0 || characteristic.charAt(j) > 9)
            progress =false;
    }
        if (progress == false){
            System.out.println("characteristic will have the value null.");
            characteristic = null;
        }

        if (progress == true)
                car.setCharacteristic(characteristic);

}

I'm having a problem at the lines "if (characteristic.charAt(i) < "A" || characteristic.charAt(i) > "Z"){"

The compiler is saying "The operator < is undefined for the argument type(s) char, String"

Any help is highly appreciated, thanks.

Answer 1

In Java, you can compare a character ( char ) to a character, but you can't compare a character to a String . charAt returns a character, so you must compare its result to a character.

These are String

"A" "Z" "1" "9"

And these are characters

'A' 'Z' '1' '9'

---

You can compare a character to an integer ( int ), but the result may not be what you want. So in the code below:

for (int j = 3; j < 7; ++j){
    if (characteristic.charAt(j) < 0 || characteristic.charAt(j) > 9)

0 and 9 should be change to '0' and '9' .

---

Note: There is another unrelated logic error in your code:

String characteristic = kb.nextLine();

progress = false; 

Shouldn't progress be set to true here?

Answer 2

I would certainly check out the other answers on this page re. character comparisons. However, I would perhaps suggest a different approach given:

starts with two capital letter followed by a number from 1-9 followed by 4 numbers from 0-9

and investigate regular expressions . Something like:

[A-Z]{2}[1-9][0-9]{4}

would satisfy the above requirement.

Answer 3

Use single quotes for chars, double quotes for Strings.

 characteristic.charAt(3) < '1'

there is meaning for single and double quotes in java

And for your situation best suits is a regex

Answer 4

Replace the double quotes with single quotes.

You'll also have to put single quotes around the numbers when comparing them with chars, even though the compiler doesn't complain.

Answer 5

像这样比较

characteristic.charAt(3) < '1'

Answer 6

First, you can achieve this goal with regexp: [AZ]{2}[1-9][0-9]{4} (Read Pattern article to know how to use it).

If you want to do it as you started - use singleqoutes instead of doublequotes with characters. eg "a" -> 'a' .

Answer 7

If you want to assign value to char use single quote. If it is a String use double quote

 char myChar='a';

 String myString="a";

so

 characteristic.charAt(3) < "1"  should change as characteristic.charAt(3) < '1'