java 方法 Character.isDigit(char) 返回 true，输入为 'O'

Question

I made a for loop to iterate through a given string and used the Character.isDigit(char) to return true ONlY if all of the letters in the string were digits. For example isInteger("412") returns true. BUT, with an input of the string "1O1" it returns true, which should be false since O is a letter.

public boolean isInteger(String str)
{
    for(int i = 0; i < str.length(); i++){
        if(Character.isDigit(str.charAt(i))){
            return true;
        }
    }
return false;
}

I also tried making a condition putting:

if(str.charAt(i) != 'O' && Character.isDigit(str.charAt(i))){
            return true;
        }

But that did not work.

Answer 1

Your method is returning true if ANY of the characters in the string are digits. Two of the characters in 1O1 are digits, so the method will return true.

You need to check if any of the characters is not a digit:

for(int i = 0; i < str.length(); i++){
    if(!Character.isDigit(str.charAt(i))){
        return false;
    }
}
return true;

Answer 2

All of the characters have to be numbers for the condition to be true. If any character is not a number, the condition is false.

public boolean isInteger(String str)
{
    if (str == null || str.length() == 0) return false;

    for (int i = 0; i < str.length(); i++) {
        if(!Character.isDigit(str.charAt(i))) {  // not condition
            return false;
        }
    }

    return true;
}