Strings are constant; their values cannot be changed after they are created. String buffers support mutable strings. Because String objects are immutable they can be shared. For example:

 
String str = "abc"; 

is equivalent to:

 
char data[] = {'a', 'b', 'c'}; 
String str = new String(data); 

Here are some more examples of how strings can be used:

 
System.out.println("abc"); 
String cde = "cde"; 
System.out.println("abc" + cde); 
String c = "abc".substring(2,3); 
String d = cde.substring(1, 2); 

The class String includes methods for examining individual characters of the sequence, for comparing strings, for searching strings, for extracting substrings, and for creating a copy of a string with all characters translated to uppercase or to lowercase. Case mapping relies heavily on the information provided by the Unicode Consortium's Unicode 3.0 specification. The specification's UnicodeData.txt and SpecialCasing.txt files are used extensively to provide case mapping.

The Java language provides special support for the string concatenation operator ( + ), and for conversion of other objects to strings. String concatenation is implemented through the StringBuffer class and its append method. String conversions are implemented through the method toString, defined by Object and inherited by all classes in Java. For additional information on string concatenation and conversion, see Gosling, Joy, and Steele, The Java Language Specification.

Unless otherwise noted, passing a null argument to a constructor or method in this class will cause a NullPointerException to be thrown.

Note that this Comparator does not take locale into account, and will result in an unsatisfactory ordering for certain locales. The java.text package provides Collators to allow locale-sensitive ordering.

The behavior of this constructor when the given bytes are not valid in the default charset is unspecified. The java.nio.charset.CharsetDecoder class should be used when more control over the decoding process is required.


String
public String( byte[] ascii, int hibyte )

Allocates a new String containing characters constructed from an array of 8-bit integer values. Each character cin the resulting string is constructed from the corresponding component b in the byte array such that:

 
c == (char)(((hibyte & 0xff) << 8) 
| (b & 0xff)) 

The behavior of this constructor when the given bytes are not valid in the default charset is unspecified. The java.nio.charset.CharsetDecoder class should be used when more control over the decoding process is required.


String
public String( byte[] ascii, int hibyte, int offset, int count )

Allocates a new String constructed from a subarray of an array of 8-bit integer values.

The offset argument is the index of the first byte of the subarray, and the count argument specifies the length of the subarray.

Each byte in the subarray is converted to a char as specified in the method above.


String
public String( byte[] bytes, int offset, int length, String charsetName ) throws UnsupportedEncodingException

Constructs a new String by decoding the specified subarray of bytes using the specified charset. The length of the new String is a function of the charset, and hence may not be equal to the length of the subarray.

The behavior of this constructor when the given bytes are not valid in the given charset is unspecified. The java.nio.charset.CharsetDecoder class should be used when more control over the decoding process is required.


String
public String( byte[] bytes, String charsetName ) throws UnsupportedEncodingException

Constructs a new String by decoding the specified array of bytes using the specified charset. The length of the new String is a function of the charset, and hence may not be equal to the length of the byte array.

The behavior of this constructor when the given bytes are not valid in the given charset is unspecified. The java.nio.charset.CharsetDecoder class should be used when more control over the decoding process is required.


String
public String( char[] value )

Allocates a new String so that it represents the sequence of characters currently contained in the character array argument. The contents of the character array are copied; subsequent modification of the character array does not affect the newly created string.


String
public String( char[] value, int offset, int count )

Allocates a new String that contains characters from a subarray of the character array argument. The offset argument is the index of the first character of the subarray and the count argument specifies the length of the subarray. The contents of the subarray are copied; subsequent modification of the character array does not affect the newly created string.


String
public String( String original )

Initializes a newly created String object so that it represents the same sequence of characters as the argument; in other words, the newly created string is a copy of the argument string. Unless an explicit copy of original is needed, use of this constructor is unnecessary since Strings are immutable.


String
public String( StringBuffer buffer )

Allocates a new string that contains the sequence of characters currently contained in the string buffer argument. The contents of the string buffer are copied; subsequent modification of the string buffer does not affect the newly created string.

The offset argument is the index of the first character of the subarray. The count argument specifies the length of the subarray. The contents of the subarray are copied; subsequent modification of the character array does not affect the newly created string.


valueOf
public static String valueOf( double d )

Returns the string representation of the double argument.

The representation is exactly the one returned by the Double.toString method of one argument.


valueOf
public static String valueOf( float f )

Returns the string representation of the float argument.

The representation is exactly the one returned by the Float.toString method of one argument.


valueOf
public static String valueOf( int i )

Returns the string representation of the int argument.

The representation is exactly the one returned by the Integer.toString method of one argument.


valueOf
public static String valueOf( long l )

Returns the string representation of the long argument.

The representation is exactly the one returned by the Long.toString method of one argument.


valueOf
public static String valueOf( Object obj )

Returns the string representation of the Object argument.

This is the definition of lexicographic ordering. If two strings are different, then either they have different characters at some index that is a valid index for both strings, or their lengths are different, or both. If they have different characters at one or more index positions, let k be the smallest such index; then the string whose character at position k has the smaller value, as determined by using the < operator, lexicographically precedes the other string. In this case, compareTo returns the difference of the two character values at position k in the two string -- that is, the value:

 
this.charAt(k)-anotherString.charAt(k) 

 
this.length()-anotherString.length() 

Note that this method does not take locale into account, and will result in an unsatisfactory ordering for certain locales. The java.text package provides collators to allow locale-sensitive ordering.


concat
public String concat( String str )

Concatenates the specified string to the end of this string.

If the length of the argument string is 0, then this String object is returned. Otherwise, a new String object is created, representing a character sequence that is the concatenation of the character sequence represented by this String object and the character sequence represented by the argument string.

Examples:

 
"cares".concat("s") returns "caress" 
"to".concat("get").concat("her") returns "together" 

Two characters c1 and c2 are considered the same, ignoring case if at least one of the following is true:

• The two characters are the same (as compared by the == operator).
• Applying the method toUpperCase(char) to each character produces the same result.
• Applying the method toLowerCase(char) to each character produces the same result.

The behavior of this method when this string cannot be encoded in the default charset is unspecified. The java.nio.charset.CharsetEncoder class should be used when more control over the encoding process is required.


getBytes
public void getBytes( int srcBegin, int srcEnd, byte[] dst, int dstBegin )

Copies characters from this string into the destination byte array. Each byte receives the 8 low-order bits of the corresponding character. The eight high-order bits of each character are not copied and do not participate in the transfer in any way.

The first character to be copied is at index srcBegin; the last character to be copied is at index srcEnd-1. The total number of characters to be copied is srcEnd-srcBegin. The characters, converted to bytes, are copied into the subarray of dst starting at index dstBegin and ending at index:

 
dstbegin + (srcEnd-srcBegin) - 1 

The behavior of this method when this string cannot be encoded in the given charset is unspecified. The java.nio.charset.CharsetEncoder class should be used when more control over the encoding process is required.


getChars
public void getChars( int srcBegin, int srcEnd, char[] dst, int dstBegin )

Copies characters from this string into the destination character array.

The first character to be copied is at index srcBegin; the last character to be copied is at index srcEnd-1 (thus the total number of characters to be copied is srcEnd-srcBegin). The characters are copied into the subarray of dst starting at index dstBegin and ending at index:

 
dstbegin + (srcEnd-srcBegin) - 1 

 
s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1] 

 
this.charAt(k) == ch 

If a character with value ch occurs in the character sequence represented by this String object at an index no smaller than fromIndex, then the index of the first such occurrence is returned--that is, the smallest value k such that:

 
(this.charAt(k) == ch) && (k >= fromIndex) 

There is no restriction on the value of fromIndex. If it is negative, it has the same effect as if it were zero: this entire string may be searched. If it is greater than the length of this string, it has the same effect as if it were equal to the length of this string: -1 is returned.


indexOf
public int indexOf( String str )

Returns the index within this string of the first occurrence of the specified substring. The integer returned is the smallest value k such that:

 
this.startsWith(str, k) 

 
k >= Math.min(fromIndex, str.length()) && this.startsWith(str, k) 

A pool of strings, initially empty, is maintained privately by the class String.

When the intern method is invoked, if the pool already contains a string equal to this String object as determined by the equals(Object) method, then the string from the pool is returned. Otherwise, this String object is added to the pool and a reference to this String object is returned.

It follows that for any two strings s and t, s.intern() == t.intern() is true if and only if s.equals(t) is true.

All literal strings and string-valued constant expressions are interned. String literals are defined in §3.10.5 of the Java Language Specification


lastIndexOf
public int lastIndexOf( int ch )

Returns the index within this string of the last occurrence of the specified character. That is, the index returned is the largest value k such that:

 
this.charAt(k) == ch 

 
this.charAt(k) == ch) && (k <= fromIndex) 

 
this.startsWith(str, k) 

 
k <= Math.min(fromIndex, str.length()) && this.startsWith(str, k) 

An invocation of this method of the form str.matches(regex) yields exactly the same result as the expression

java.util.regex.Pattern.matches(regex, str)

A substring of this String object is compared to a substring of the argument other. The result is true if these substrings represent character sequences that are the same, ignoring case if and only if ignoreCase is true. The substring of this String object to be compared begins at index toffset and has length len. The substring of other to be compared begins at index ooffset and has length len. The result is false if and only if at least one of the following is true:

• toffset is negative.
• ooffset is negative.
• toffset+len is greater than the length of this String object.
• ooffset+len is greater than the length of the other argument.
• ignoreCase is false and there is some nonnegative integer k less than len such that:

   
  this.charAt(toffset+k) != other.charAt(ooffset+k) 
  

• ignoreCase is true and there is some nonnegative integer k less than len such that:

   
  Character.toLowerCase(this.charAt(toffset+k)) != 
  Character.toLowerCase(other.charAt(ooffset+k)) 
  

  and:

   
  Character.toUpperCase(this.charAt(toffset+k)) != 
  Character.toUpperCase(other.charAt(ooffset+k)) 
  

A substring of this String object is compared to a substring of the argument other. The result is true if these substrings represent identical character sequences. The substring of this String object to be compared begins at index toffset and has length len. The substring of other to be compared begins at index ooffset and has length len. The result is false if and only if at least one of the following is true:

• toffset is negative.
• ooffset is negative.
• toffset+len is greater than the length of this String object.
• ooffset+len is greater than the length of the other argument.
• There is some nonnegative integer k less than len such that: this.charAt(toffset+k) != other.charAt(ooffset+k)

If the character oldChar does not occur in the character sequence represented by this String object, then a reference to this String object is returned. Otherwise, a new String object is created that represents a character sequence identical to the character sequence represented by this String object, except that every occurrence of oldChar is replaced by an occurrence of newChar.

Examples:

 
"mesquite in your cellar".replace('e', 'o') 
returns "mosquito in your collar" 
"the war of baronets".replace('r', 'y') 
returns "the way of bayonets" 
"sparring with a purple porpoise".replace('p', 't') 
returns "starring with a turtle tortoise" 
"JonL".replace('q', 'x') returns "JonL" (no change) 

An invocation of this method of the form str.replaceAll(regex, repl) yields exactly the same result as the expression

java.util.regex.Pattern.compile(regex).matcher(str).replaceAll(repl)

An invocation of this method of the form str.replaceFirst(regex, repl) yields exactly the same result as the expression

java.util.regex.Pattern.compile(regex).matcher(str).replaceFirst(repl)

This method works as if by invoking the two-argument split method with the given expression and a limit argument of zero. Trailing empty strings are therefore not included in the resulting array.

The string "boo:and:foo", for example, yields the following results with these expressions:

Regex	Result
:	{ "boo", "and", "foo" }
o	{ "b", "", ":and:f" }

The array returned by this method contains each substring of this string that is terminated by another substring that matches the given expression or is terminated by the end of the string. The substrings in the array are in the order in which they occur in this string. If the expression does not match any part of the input then the resulting array has just one element, namely this string.

The limit parameter controls the number of times the pattern is applied and therefore affects the length of the resulting array. If the limit n is greater than zero then the pattern will be applied at most n - 1 times, the array's length will be no greater than n, and the array's last entry will contain all input beyond the last matched delimiter. If n is non-positive then the pattern will be applied as many times as possible and the array can have any length. If n is zero then the pattern will be applied as many times as possible, the array can have any length, and trailing empty strings will be discarded.

The string "boo:and:foo", for example, yields the following results with these parameters:

Regex	Limit	Result
:	2	{ "boo", "and:foo" }
:	5	{ "boo", "and", "foo" }
:	-2	{ "boo", "and", "foo" }
o	5	{ "b", "", ":and:f", "", "" }
o	-2	{ "b", "", ":and:f", "", "" }
o	0	{ "b", "", ":and:f" }

An invocation of this method of the form str.split(regex, n) yields the same result as the expression

java.util.regex.Pattern.compile(regex).split(str, n)

An invocation of this method of the form

 
str.subSequence(begin, end)

 
str.substring(begin, end)

Examples:

 
"unhappy".substring(2) returns "happy" 
"Harbison".substring(3) returns "bison" 
"emptiness".substring(9) returns "" (an empty string) 

Examples:

 
"hamburger".substring(4, 8) returns "urge" 
"smiles".substring(1, 5) returns "mile" 

toLowerCase
public String toLowerCase( Locale locale )

Converts all of the characters in this String to lower case using the rules of the given Locale. Case mappings rely heavily on the Unicode specification's character data. Since case mappings are not always 1:1 char mappings, the resulting String may be a different length than the original String.

Examples of lowercase mappings are in the following table:

Language Code of Locale	Upper Case	Lower Case	Description
tr (Turkish)	\u0130	\u0069	capital letter I with dot above -> small letter i
tr (Turkish)	\u0049	\u0131	capital letter I -> small letter dotless i
(all)	French Fries	french fries	lowercased all chars in String
(all)			lowercased all chars in String

toUpperCase
public String toUpperCase( Locale locale )

Converts all of the characters in this String to upper case using the rules of the given Locale. Case mappings rely heavily on the Unicode specification's character data. Since case mappings are not always 1:1 char mappings, the resulting String may be a different length than the original String.

Examples of locale-sensitive and 1:M case mappings are in the following table.

If this String object represents an empty character sequence, or the first and last characters of character sequence represented by this String object both have codes greater than '\u0020' (the space character), then a reference to this String object is returned.

Otherwise, if there is no character with a code greater than '\u0020' in the string, then a new String object representing an empty string is created and returned.

Otherwise, let k be the index of the first character in the string whose code is greater than '\u0020', and let m be the index of the last character in the string whose code is greater than '\u0020'. A new String object is created, representing the substring of this string that begins with the character at index k and ends with the character at index m-that is, the result of this.substring(k, m+1).

This method may be used to trim whitespace from the beginning and end of a string; in fact, it trims all ASCII control characters as well.