String
The String class represents a read-only string of characters . A String object is created by the Java compiler whenever it encounters a string in double quotes; this method of creation is typically simpler than using a constructor. The static valueOf( ) factory methods create new String objects that hold the textual representation of various Java primitive types. There are also valueOf( ) methods, copyValueOf( ) methods and String( ) constructors for creating a String object that holds a copy of the text contained in another String , StringBuffer , StringBuilder , or a char or int array. You can also use the String( ) constructor to create a String object from an array of bytes. If you do this, you may explicitly specify the name of the charset (or character encoding) to be used to decode the bytes into characters, or you can rely on the default charset for your platform. (See java.nio.charset.Charset for more on charset names .) In Java 5.0, the static format( ) methods provide another useful way to create String objects that hold formatted text. These utility methods create and use a new java.util.Formatter object and behave like the sprintf( ) function in the C programming language. length( ) returns the number of characters in a string. charAt( ) extracts a character from a string. You can use these two methods to iterate through the characters of a string. You can obtain a char array that holds the characters of a string with toCharArray( ) , or use getChars( ) to copy just a selected region of the string into an existing array. Use getBytes( ) if you want to obtain an array of bytes that contains the encoded form of the characters in a string, using either the platform's default encoding or a named encoding. This class defines many methods for comparing strings and substrings. equals( ) returns TRue if two String objects contain the same text, and equalsIgnoreCase( ) returns true if two strings are equal when uppercase and lowercase differences are ignored. As of Java 1.4, the contentEquals( ) method compares a string to a specified StringBuffer object, returning true if they contain the same text. startsWith( ) and endsWith( ) return true if a string starts with the specified prefix string or ends with the specified suffix string. A two-argument version of startsWith( ) allows you to specify a position within this string at which the prefix comparison is to be done. The regionMatches( ) method is a generalized version of this startsWith( ) method. It returns TRue if the specified region of the specified string matches the characters that begin at a specified position within this string. The five-argument version of this method allows you to perform this comparison ignoring the case of the characters being compared. The final string comparison method is matches( ) , which, as described below, compares a string to a regular expression pattern. compareTo( ) is another string comparison method, but it is used for comparing the order of two strings, rather than simply comparing them for equality. compareTo( ) implements the Comparable interface and enables sorting of lists and arrays of String objects. See Comparable for more information. compareToIgnoreCase( ) is like compareTo( ) but ignores the case of the two strings when doing the comparison. The CASE_INSENSITIVE_ORDER constant is a Comparator for sorting strings in a way that ignores the case of their characters. (The java.util.Comparator interface is similar to the Comparable interface but allows the definition of object orderings that are different from the default ordering defined by Comparable .) The compareTo( ) and compareToIgnoreCase( ) methods and the CASE_INSENSITIVE_ORDER Comparator object order strings based only on the numeric ordering of the Unicode encoding of their characters. This is not always the preferred "alphabetical ordering" in some languages. See java.text.Collator for a more general technique for collating strings. indexOf( ) and lastIndexOf( ) search forward and backward in a string for a specified character or substring. They return the position of the match, or -1 if there is no match. The one argument versions of these methods start at the beginning or end of the string, and the two-argument versions start searching from a specified character position. Java 5.0 adds new comparison methods that work with any CharSequence . A new version of contentEquals( ) enables the comparison of a string with any CharSequence , including StringBuilder objects. The contains( ) method returns true if the string contains any sequence of characters equal to the specified CharSequence . substring( ) returns a string that consists of the characters from (and including) the specified start position to (but not including) the specified end position. A one-argument version returns all characters from (and including) the specified start position to the end of the string. As of Java 1.4, the String class implements the CharSequence interface and defines the subSequence( ) method, which works just like the two-argument version of substring( ) but returns the specified characters as a CharSequence rather than as a String . Several methods return new strings that contain modified versions of the text held by the original string (the original string remains unchanged). replace( ) creates a new string with all occurrences of one character replaced by another. Java 5.0 adds a generalized version of replace( ) that replaces all occurrences of one CharSequence with another. More general methods, replaceAll( ) and replaceFirst( ) , use regular expression pattern matching; they are described later in this section. toUpperCase( ) and toLowerCase( ) return a new string in which all characters are converted to upper- or lowercase respectively. These case-conversion methods take an optional Locale argument to perform locale-specific case conversion. trim( ) is a utility method that returns a new string in which all leading and trailing whitespace has been removed. concat( ) returns the new string formed by concatenating or appending the specified string to this string. String concatenation is more commonly done, however, with the + operator. Note that String objects are immutable; there is no setCharAt( ) method to change the contents. The methods that return a String do not modify the string they are invoked on but instead return a new String object that holds a modified copy of the text of the original. Use a StringBuffer if you want to manipulate the contents of a string or call toCharArray( ) or getChars( ) to convert a string to an array of char values. Java 1.4 introduced support for pattern matching with regular expressions. matches( ) returns true if this string exactly matches the pattern specified by the regular expression argument. replaceAll( ) and replaceFirst( ) create a new string in which all occurrences or the first occurrence of a substring that matches the specified regular expression is replaced with the specified replacement string. The split( ) methods return an array of substrings of this string, formed by splitting this string at positions that match the specified regular expression. These regular expression methods are all convenience methods that simply call methods of the same name in the java.util.regex package. See the Pattern and Matcher classes in that package for further details. Many programs use strings as commonly as they use Java primitive values. Because the String type is an object rather than a primitive value, however, you cannot in general use the = = operator to compare two strings for equality. Instead, even though strings are immutable, you must use the more expensive equals( ) method. For programs that perform a lot of string comparison, the intern( ) provides a way to speed up those comparisons. The String class maintains a set of String objects that includes all double-quoted string literals and all compile-time constant strings defined in a Java program. The set is guaranteed not to contain duplicates, and the set is used to ensure that duplicate String objects are not created unnecessarily. The intern( ) method looks up a string in or adds a new string to this set of unique strings. It searches the set for a string that contains exactly the same characters as the string you invoked the method on. If such a string is found, intern( ) returns it. If no matching string is found, the string you invoked intern( ) on is itself stored in the set ("interned") and becomes the return value of the method. What this means is that you can safely compare any strings returned by the intern( ) method using the = = and != operators instead of equals( ) . You can also successfully compare any string returned by intern( ) to any string constant with = = and != . In Java 5.0, Unicode supplementary characters may be represented as a single int codepoint value or as a sequence of two char values known as a "surrogate pair." See Character for more on supplementary characters and methods for working with them. String methods for working with supplementary characters, such as codePointAt( ) , codePointCount( ) , and offsetByCodePoints( ) , are similar to those defined by Character . Figure 10-58. java.lang.String public final class String implements Serializable, Comparable<String>, CharSequence { // Public Constructors public String ( ); 5.0 public String (StringBuilder builder ); public String (StringBuffer buffer ); public String (char[ ] value ); public String (String original ); 1.1 public String (byte[ ] bytes ); 1.1 public String (byte[ ] bytes , String charsetName ) throws java.io.UnsupportedEncodingException; # public String (byte[ ] ascii , int hibyte ); public String (char[ ] value , int offset , int count ); 1.1 public String (byte[ ] bytes , int offset , int length ); 5.0 public String (int[ ] codePoints , int offset , int count ); # public String (byte[ ] ascii , int hibyte , int offset , int count ); 1.1 public String (byte[ ] bytes , int offset , int length , String charsetName ) throws java.io.UnsupportedEncodingException; // Public Constants 1.2 public static final java.util.Comparator<String> CASE_INSENSITIVE_ORDER ; // Public Class Methods public static String copyValueOf (char[ ] data ); public static String copyValueOf (char[ ] data , int offset , int count ); 5.0 public static String format (String format , Object... args ); 5.0 public static String format (java.util.Locale l , String format , Object... args ); public static String valueOf (float f ); public static String valueOf (long l ); public static String valueOf (Object obj ); public static String valueOf (double d ); public static String valueOf (boolean b ); public static String valueOf (char[ ] data ); public static String valueOf (int i ); public static String valueOf (char c ); public static String valueOf (char[ ] data , int offset , int count ); // Public Instance Methods public char charAt (int index ); Implements: CharSequence 5.0 public int codePointAt (int index ); 5.0 public int codePointBefore (int index ); 5.0 public int codePointCount (int beginIndex , int endIndex ); public int compareTo (String anotherString ); Implements: Comparable 1.2 public int compareToIgnoreCase (String str ); public String concat (String str ); 5.0 public boolean contains (CharSequence s ); 1.4 public boolean contentEquals (StringBuffer sb ); 5.0 public boolean contentEquals (CharSequence cs ); public boolean endsWith (String suffix ); public boolean equalsIgnoreCase (String anotherString ); 1.1 public byte[ ] getBytes ( ); 1.1 public byte[ ] getBytes (String charsetName ) throws java.io. UnsupportedEncodingException; public void getChars (int srcBegin , int srcEnd , char[ ] dst , int dstBegin ); public int indexOf (int ch ); public int indexOf (String str ); public int indexOf (int ch , int fromIndex ); public int indexOf (String str , int fromIndex ); public String intern ( ); native public int lastIndexOf (String str ); public int lastIndexOf (int ch ); public int lastIndexOf (String str , int fromIndex ); public int lastIndexOf (int ch , int fromIndex ); public int length ( ); Implements: CharSequence 1.4 public boolean matches (String regex ); 5.0 public int offsetByCodePoints (int index , int codePointOffset ); public boolean regionMatches (int toffset , String other , int ooffset , int len ); public boolean regionMatches (boolean ignoreCase , int toffset , String other , int ooffset , int len ); public String replace (char oldChar , char newChar ); 5.0 public String replace (CharSequence target , CharSequence replacement ); 1.4 public String replaceAll (String regex , String replacement ); 1.4 public String replaceFirst (String regex , String replacement ); 1.4 public String[ ] split (String regex ); 1.4 public String[ ] split (String regex , int limit ); public boolean startsWith (String prefix ); public boolean startsWith (String prefix , int toffset ); public String substring (int beginIndex ); public String substring (int beginIndex , int endIndex ); public char[ ] toCharArray ( ); public String toLowerCase ( ); 1.1 public String toLowerCase (java.util.Locale locale ); public String toString ( ); Implements: CharSequence public String toUpperCase ( ); 1.1 public String toUpperCase (java.util.Locale locale ); public String trim ( ); // Methods Implementing CharSequence public char charAt (int index ); public int length ( ); 1.4 public CharSequence subSequence (int beginIndex , int endIndex ); public String toString ( ); // Methods Implementing Comparable public int compareTo (String anotherString ); // Public Methods Overriding Object public boolean equals (Object anObject ); public int hashCode ( ); // Deprecated Public Methods # public void getBytes (int srcBegin , int srcEnd , byte[ ] dst , int dstBegin ); } Passed ToToo many methods to list. Returned ByToo many methods to list. Type OfToo many fields to list. |
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