Checking Character Properties
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! // check if ch is a letter if ((ch >= 'a' && ch <= 'z') || (ch >= 'A' && ch <= 'Z')) // ... // check if ch is a digit if (ch >= '0' && ch <= '9') // ... // check if ch is a whitespace if ((ch == ' ') || (ch =='\n') || (ch == '\t')) // ...
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)) // ...
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.
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
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) // ...