Character

In Java a char is a value, much like an int. Often in programs we need to test for ranges (classes) of characters—as for digits, letters, spaces.

Character notes

Instead of acting upon chars directly, we can use built-in static methods on Character. The Character class includes this logic—we just access it.

IsDigit

This method returns true if the char passed to it is "0" through "9." It also supports Unicode so is more complex (and likely slower) than an if-statement and range check.

public class Program {
    public static void main(String[] args) {

        char value1 = '9';
        char value2 = 'Y';
        char value3 = ' ';

        // See if these characters are digits.
        System.out.println(Character.isDigit(value1));
        System.out.println(Character.isDigit(value2));
        System.out.println(Character.isDigit(value3));
    }
}
true
false
false

IsLetter, for-loop

With Character.isLetter we detect lowercase and uppercase letters. Digits, spaces and punctuation are not considered letters. IsLetter returns true or false.

public class Program {
    public static void main(String[] args) {

        String value = "abc123";
        // Loop through characters in this String.
        for (int i = 0; i < value.length(); i++) {
            char c = value.charAt(i);

            // See if the character is a letter or not.
            if (Character.isLetter(c)) {
                System.out.println(c + " = LETTER");
            } else {
                System.out.println(c);
            }
        }
    }
}
a = LETTER
b = LETTER
c = LETTER
1
2
3

IsLetterOrDigit

In my experience, this method is often useful. Most tokens are composed of letters and digits. So with isLetterOrDigit we can easily skip past punctuation and spaces.

public class Program {
    public static void main(String[] args) {

        char value1 = 'U';
        char value2 = '9';
        char value3 = 'w';
        char value4 = '*';
        System.out.println(Character.isLetterOrDigit(value1));
        System.out.println(Character.isLetterOrDigit(value2));
        System.out.println(Character.isLetterOrDigit(value3));
        System.out.println(Character.isLetterOrDigit(value4)); // [False]
    }
}
true
true
true
false

IsLowerCase, isUpperCase

These return true or false if the character is lowercase (or uppercase). This can be implemented (for ASCII) with a simple if-check, but that is harder to read.

public class Program {
    public static void main(String[] args) {

        char[] values = { 'C', 'a', 't', '5' };
        // See if the chars are lower or uppercase.
        for (char value : values) {
            System.out.println(value);
            System.out.println(Character.isLowerCase(value));
            System.out.println(Character.isUpperCase(value));
        }
    }
}
C
false
true
a
true
false
t
true
false
5
false
false

IsWhitespace

This method is a new version of Character.isSpace. It returns true if the character is whitespace (like a space, tab, or newline). It returns false on other chars.

public class Program {
    public static void main(String[] args) {

        // Test these characters for whitespace.
        System.out.println(Character.isWhitespace(' '));
        System.out.println(Character.isWhitespace('\n'));
        System.out.println(Character.isWhitespace('Q'));
    }
}
true
true
false

Invert case

More complex logic is possible. This program applies toLowerCase and toUpperCase to invert the casing of letters in a String.

public class Program {

    static String invertCase(String value) {
        // Convert to a char array.
        char[] array = value.toCharArray();

        for (int i = 0; i < array.length; i++) {
            // Change uppercase letters to lowercase ones.
            // ... And change lower to upper.
            if (Character.isUpperCase(array[i])) {
                array[i] = Character.toLowerCase(array[i]);
            } else if (Character.isLowerCase(array[i])) {
                array[i] = Character.toUpperCase(array[i]);
            }
        }
        return new String(array);
    }

    public static void main(String[] args) {
        // Use invertCase method.
        System.out.println(invertCase("Dog"));
        System.out.println(invertCase(invertCase("Cat")));
    }
}
dOG
Cat

ASCII table

With the Character class, we can generate a table of all 128 ASCII values. Programs can manipulate ASCII values to improve performance.

Lookup table

For optimization, we can store character transformations in a table. This reduces any character modification to a simple array element access.

Handle sentence characters

One algorithm that uses isLetter and toLowerCase is a palindrome-detecting algorithm. It skips over whitespace and non-letter chars to detect palindromes.

Character-based algorithms are often faster than String-based ones. Fewer allocations are needed: a char is simpler than an entire String. With the Character class, we test char types.