Character and String APIs
The Character class encapsulates the char data type. For the J2SE release 5, many methods were added to the Character class to support supplementary characters. This API falls into two categories: methods that convert between char and code point values and methods that verify the validity of or map code points.
This section describes a subset of the available methods in the Character class. For the complete list of available APIs, see the Character class specification.
Conversion Methods and the Character Class
The following table includes the most useful conversion methods, or methods that facilitate conversion, in the Character class. The codePointAt and codePointBefore methods are included in this list because text is generally found in a sequence, such as a String , and these methods can be used to extract the desired substring.
| Method(s) | Description |
|---|---|
| toChars(int codePoint, char[] dst, int dstIndex) toChars(int codePoint) | Converts the specified Unicode code point to its UTF-16 representation and places it in a char array. Sample usage: Character.toChars(0x10400) |
| toCodePoint(char high, char low) | Converts the specified surrogate pair to its supplementary code point value. |
| codePointAt(char[] a, int index) codePointAt(char[] a, int index, int limit) codePointAt(CharSequence seq, int index) | Returns the Unicode code point at the specified index. The third method takes a CharSequence and the second method enforces an upper limit on the index. |
| codePointBefore(char[] a, int index) codePointBefore(char[] a, int index, int start) codePointBefore(CharSequence seq, int index) codePointBefore(char[], int, int) | Returns the Unicode code point before the specified index. The third method accepts a CharSequence and the other methods accept a char array. The second method enforces a lower limit on the index. |
| charCount(int codePoint) | Returns the value 1 for characters that can be represented by a single char . Returns the value 2 for supplementary characters that require two char s. |
Verification and Mapping Methods in the Character Class
Some of the previous methods that used the char primitive data type, such as isLowerCase(char) and isDigit(char) , were supplanted by methods that support supplementary characters, such as isLowerCase(int) and isDigit(int) . The previous methods are supported but do not work with supplementary characters. To create a global application and ensure that your code works seamlessly with any language, it is recommended that you use the newer forms of these methods.
Note that, for performance reasons, most methods that accept a code point do not verify the validity of the code point parameter. You can use the isValidCodePoint method for that purpose.
The following table lists some of the verification and mapping methods in the Character class.
| Method(s) | Description |
|---|---|
| isValidCodePoint(int codePoint) | Returns true if the code point is within the range of 0x0000 to 0x10FFFF, inclusive. |
| isSupplementaryCodePoint(int codePoint) | Returns true if the code point is within the range of 0x10000 to 0x10FFFF, inclusive. |
| isHighSurrogate(char) | Returns true if the specified char is within the high surrogate range of \uD800 to \uDBFF, inclusive. |
| isLowSurrogate(char) | Returns true if the specified char is within the low surrogate range of \uDC00 to \uDFFF, inclusive. |
| isSurrogatePair(char high, char low) | Returns true if the specified high and low surrogate code values represent a valid surrogate pair. |
| codePointCount(CharSequence, int, int) codePointCount(char[], int, int) | Returns the number of Unicode code points in the CharSequence , or char array. |
| isLowerCase(int) isUpperCase(int) | Returns true if the specified Unicode code point is a lowercase or uppercase character. |
| isDefined(int) | Returns true if the specified Unicode code point is defined in the Unicode standard. |
| isJavaIdentifierStart(char) isJavaIdentifierStart(int) | Returns true if the specified character or Unicode code point is permissible as the first character in a Java identifier. |
| isLetter(int) isDigit(int) isLetterOrDigit(int) | Returns true if the specified Unicode code point is a letter, a digit, or a letter or digit. |
| getDirectionality(int) | Returns the Unicode directionality property for the given Unicode code point. |
| Character.UnicodeBlock.of(int codePoint) | Returns the object representing the Unicode block that contains the given Unicode code point or returns null if the code point is not a member of a defined block. |
Methods in the String Classes
The String , StringBuffer , and StringBuilder classes also have constructors and methods that work with supplementary characters. The following table lists some of the commonly used methods. For the complete list of available APIs, see the javadoc for the String , StringBuffer , and StringBuilder classes.
| Constructor or Methods | Description |
|---|---|
| String(int[] codePoints, int offset, int count) | Allocates a new String instance that contains characters from a subarray of a Unicode code point array. |
| String.codePointAt(int index) StringBuffer.codePointAt(int index) StringBuilder.codePointAt(int index) | Returns the Unicode code point at the specified index. |
| String.codePointBefore(int index) StringBuffer.codePointBefore(int index) StringBuilder.codePointBefore(int index) | Returns the Unicode code point before the specified index. |
| String.codePointCount(int beginIndex, int endIndex) StringBuffer.codePointCount(int beginIndex, int endIndex) StringBuilder.codePointCount(int beginIndex, int endIndex) | Returns the number of Unicode code points in the specified range. |
| StringBuffer.appendCodePoint(int codePoint) StringBuilder.appendCodePoint(int codePoint) | Appends the string representation of the specified code point to the sequence. |
| String.offsetByCodePoints(int index, int codePointOffset) StringBuffer.offsetByCodePoints(int index, int codePointOffset) StringBuilder.offsetByCodePoints(int index, int codePointOffset) | Returns the index that is offset from the given index by the given number of code points. |
Characters
Most of the time, if you are using a single character value, you will use the primitive char type. For example:
char ch = 'a'; // Unicode for uppercase Greek omega character char uniChar = '\u03A9'; // an array of chars char[] charArray = < 'a', 'b', 'c', 'd', 'e' >;
There are times, however, when you need to use a char as an objectfor example, as a method argument where an object is expected. The Java programming language provides a wrapper class that «wraps» the char in a Character object for this purpose. An object of type Character contains a single field, whose type is char . This Character class also offers a number of useful class (that is, static) methods for manipulating characters.
You can create a Character object with the Character constructor:
Character ch = new Character('a'); The Java compiler will also create a Character object for you under some circumstances. For example, if you pass a primitive char into a method that expects an object, the compiler automatically converts the char to a Character for you. This feature is called autoboxingor unboxing, if the conversion goes the other way. For more information on autoboxing and unboxing, see Autoboxing and Unboxing.
Note: The Character class is immutable, so that once it is created, a Character object cannot be changed.
The following table lists some of the most useful methods in the Character class, but is not exhaustive. For a complete listing of all methods in this class (there are more than 50), refer to the java.lang.Character API specification.
| Method | Description |
|---|---|
| boolean isLetter(char ch) boolean isDigit(char ch) | Determines whether the specified char value is a letter or a digit, respectively. |
| boolean isWhitespace(char ch) | Determines whether the specified char value is white space. |
| boolean isUpperCase(char ch) boolean isLowerCase(char ch) | Determines whether the specified char value is uppercase or lowercase, respectively. |
| char toUpperCase(char ch) char toLowerCase(char ch) | Returns the uppercase or lowercase form of the specified char value. |
| toString(char ch) | Returns a String object representing the specified character value that is, a one-character string. |
Escape Sequences
A character preceded by a backslash (\) is an escape sequence and has special meaning to the compiler. The following table shows the Java escape sequences:
| Escape Sequence | Description |
|---|---|
| \t | Insert a tab in the text at this point. |
| \b | Insert a backspace in the text at this point. |
| \n | Insert a newline in the text at this point. |
| \r | Insert a carriage return in the text at this point. |
| \f | Insert a form feed in the text at this point. |
| \’ | Insert a single quote character in the text at this point. |
| \» | Insert a double quote character in the text at this point. |
| \\ | Insert a backslash character in the text at this point. |
When an escape sequence is encountered in a print statement, the compiler interprets it accordingly. For example, if you want to put quotes within quotes you must use the escape sequence, \», on the interior quotes. To print the sentence
System.out.println("She said \"Hello!\" to me.");