Checking Character Properties - Java Tutorial 5.0 英文版

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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!

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)

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