You can categorize characters according to their properties. For instance, X is an uppercase letter and 4 is a decimal digit. Checking character properties is a common way to verify the data entered by end users. If you are selling books online, for example, your order entry screen should verify that the characters in the quantity field are all digits.
Developers who aren't used to writing global software might determine a character's properties by comparing it with character constants. For instance, they might write code like this:
char ch; //... // This code is WRONG! if ((ch >= 'a' && ch <= 'z') || (ch >= 'A' && ch <= 'Z')) // ch is a letter // ... if (ch >= '0' && ch <= '9') // ch is a digit // ... if ((ch == ' ') || (ch =='\n') || (ch == '\t')) // ch is a whitespace
The preceding code is wrong because it works only with English and a few other languages. To internationalize the previous example, replace it with the following statements:
char ch; // ... // This code is OK! if (Character.isLetter(ch)) // ... if (Character.isDigit(ch)) // ... if (Character.isSpaceChar(ch))
The
Character
methods rely on the Unicode Standard for determining the properties of a character. Unicode is a 16-bit character encoding that supports the world's major languages. In the Java programming language char
values represent Unicode characters. If you check the properties of a char
with the appropriate Character
method, your code will work with all major languages. For example, the Character.isLetter
method returns true
if the character is a letter in Chinese, German, Arabic, or another language.
The following list gives some of the most useful Character
comparison methods. The Character
API documentation fully specifies the methods.
isDigit
isLetter
isLetterOrDigit
isLowerCase
isUpperCase
isSpaceChar
isDefined
The Character.getType
method returns the Unicode category of a character. Each category corresponds to a constant defined in the Character
class. For instance, getType
returns the Character.UPPERCASE_LETTER
constant for the character A. For a complete list of the category constants returned by getType
, see the
Character
API documentation. The following example shows how to use getType
and the Character
category constants. All of the expressions in these if
statements are true
:
if (Character.getType('a') == Character.LOWERCASE_LETTER) // ... if (Character.getType('R') == Character.UPPERCASE_LETTER) // ... if (Character.getType('>') == Character.MATH_SYMBOL) // ... if (Character.getType('_') == Character.CONNECTOR_PUNCTUATION)