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Java(TM) 2 Platform, Standard Edition, v1.2.2 API Specification: Class StringBuffer - JDK 5 Documentation v1.2.2, Java 2 SDK 英文文档

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java.lang
Class StringBuffer

java.lang.Object
  |
  +--java.lang.StringBuffer
public final class StringBuffer
extends Object
implements Serializable

A string buffer implements a mutable sequence of characters. A string buffer is like a String, but can be modified. At any point in time it contains some particular sequence of characters, but the length and content of the sequence can be changed through certain method calls.

String buffers are safe for use by multiple threads. The methods are synchronized where necessary so that all the operations on any particular instance behave as if they occur in some serial order that is consistent with the order of the method calls made by each of the individual threads involved.

String buffers are used by the compiler to implement the binary string concatenation operator +. For example, the code: 

     x = "a" + 4 + "c"

is compiled to the equivalent of: 

     x = new StringBuffer().append("a").append(4).append("c")
                           .toString()
which creates a new string buffer (initially empty), appends the string representation of each operand to the string buffer in turn, and then converts the contents of the string buffer to a string. Overall, this avoids creating many temporary strings.

The principal operations on a StringBuffer are the append and insert methods, which are overloaded so as to accept data of any type. Each effectively converts a given datum to a string and then appends or inserts the characters of that string to the string buffer. The append method always adds these characters at the end of the buffer; the insert method adds the characters at a specified point.

For example, if z refers to a string buffer object whose current contents are "start", then the method call z.append("le") would cause the string buffer to contain "startle", whereas z.insert(4, "le") would alter the string buffer to contain "starlet".

In general, if sb refers to an instance of a StringBuffer, then sb.append(x) has the same effect as sb.insert(sb.length(), x).

Every string buffer has a capacity. As long as the length of the character sequence contained in the string buffer does not exceed the capacity, it is not necessary to allocate a new internal buffer array. If the internal buffer overflows, it is automatically made larger.

Since:
JDK1.0
See Also:
ByteArrayOutputStream, String, Serialized Form

Constructor Summary
StringBuffer()
          Constructs a string buffer with no characters in it and an initial capacity of 16 characters.
StringBuffer(int length)
          Constructs a string buffer with no characters in it and an initial capacity specified by the length argument.
StringBuffer(String str)
          Constructs a string buffer so that it represents the same sequence of characters as the string argument; in other words, the initial contents of the string buffer is a copy of the argument string.
 
Method Summary
 StringBuffer append(boolean b)
          Appends the string representation of the boolean argument to the string buffer.
 StringBuffer append(char c)
          Appends the string representation of the char argument to this string buffer.
 StringBuffer append(char[] str)
          Appends the string representation of the char array argument to this string buffer.
 StringBuffer append(char[] str, int offset, int len)
          Appends the string representation of a subarray of the char array argument to this string buffer.
 StringBuffer append(double d)
          Appends the string representation of the double argument to this string buffer.
 StringBuffer append(float f)
          Appends the string representation of the float argument to this string buffer.
 StringBuffer append(int i)
          Appends the string representation of the int argument to this string buffer.
 StringBuffer append(long l)
          Appends the string representation of the long argument to this string buffer.
 StringBuffer append(Object obj)
          Appends the string representation of the Object argument to this string buffer.
 StringBuffer append(String str)
          Appends the string to this string buffer.
 int capacity()
          Returns the current capacity of the String buffer.
 char charAt(int index)
          The specified character of the sequence currently represented by the string buffer, as indicated by the index argument, is returned.
 StringBuffer delete(int start, int end)
          Removes the characters in a substring of this StringBuffer.
 StringBuffer deleteCharAt(int index)
          Removes the character at the specified position in this StringBuffer (shortening the StringBuffer by one character).
 void ensureCapacity(int minimumCapacity)
          Ensures that the capacity of the buffer is at least equal to the specified minimum.
 void getChars(int srcBegin, int srcEnd, char[] dst, int dstBegin)
          Characters are copied from this string buffer into the destination character array dst.
 StringBuffer insert(int offset, boolean b)
          Inserts the string representation of the boolean argument into this string buffer.
 StringBuffer insert(int offset, char c)
          Inserts the string representation of the char argument into this string buffer.
 StringBuffer insert(int offset, char[] str)
          Inserts the string representation of the char array argument into this string buffer.
 StringBuffer insert(int index, char[] str, int offset, int len)
          Inserts the string representation of a subarray of the str array argument into this string buffer.
 StringBuffer insert(int offset, double d)
          Inserts the string representation of the double argument into this string buffer.
 StringBuffer insert(int offset, float f)
          Inserts the string representation of the float argument into this string buffer.
 StringBuffer insert(int offset, int i)
          Inserts the string representation of the second int argument into this string buffer.
 StringBuffer insert(int offset, long l)
          Inserts the string representation of the long argument into this string buffer.
 StringBuffer insert(int offset, Object obj)
          Inserts the string representation of the Object argument into this string buffer.
 StringBuffer insert(int offset, String str)
          Inserts the string into this string buffer.
 int length()
          Returns the length (character count) of this string buffer.
 StringBuffer replace(int start, int end, String str)
          Replaces the characters in a substring of this StringBuffer with characters in the specified String.
 StringBuffer reverse()
          The character sequence contained in this string buffer is replaced by the reverse of the sequence.
 void setCharAt(int index, char ch)
          The character at the specified index of this string buffer is set to ch.
 void setLength(int newLength)
          Sets the length of this String buffer.
 String substring(int start)
          Returns a new String that contains a subsequence of characters currently contained in this StringBuffer.The substring begins at the specified index and extends to the end of the StringBuffer.
 String substring(int start, int end)
          Returns a new String that contains a subsequence of characters currently contained in this StringBuffer.
 String toString()
          Converts to a string representing the data in this string buffer.
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait
 

Constructor Detail

StringBuffer

public StringBuffer()
Constructs a string buffer with no characters in it and an initial capacity of 16 characters.

StringBuffer

public StringBuffer(int length)
Constructs a string buffer with no characters in it and an initial capacity specified by the length argument.
Parameters:
length - the initial capacity.
Throws:
NegativeArraySizeException - if the length argument is less than 0.

StringBuffer

public StringBuffer(String str)
Constructs a string buffer so that it represents the same sequence of characters as the string argument; in other words, the initial contents of the string buffer is a copy of the argument string. The initial capacity of the string buffer is 16 plus the length of the string argument.
Parameters:
str - the initial contents of the buffer.
Method Detail

length

public int length()
Returns the length (character count) of this string buffer.
Returns:
the length of the sequence of characters currently represented by this string buffer.

capacity

public int capacity()
Returns the current capacity of the String buffer. The capacity is the amount of storage available for newly inserted characters; beyond which an allocation will occur.
Returns:
the current capacity of this string buffer.

ensureCapacity

public void ensureCapacity(int minimumCapacity)
Ensures that the capacity of the buffer is at least equal to the specified minimum. If the current capacity of this string buffer is less than the argument, then a new internal buffer is allocated with greater capacity. The new capacity is the larger of:
  • The minimumCapacity argument.
  • Twice the old capacity, plus 2.
If the minimumCapacity argument is nonpositive, this method takes no action and simply returns.
Parameters:
minimumCapacity - the minimum desired capacity.

setLength

public void setLength(int newLength)
Sets the length of this String buffer. This string buffer is altered to represent a new character sequence whose length is specified by the argument. For every nonnegative index k less than newLength, the character at index k in the new character sequence is the same as the character at index k in the old sequence if k is less than the length of the old character sequence; otherwise, it is the null character ''. In other words, if the newLength argument is less than the current length of the string buffer, the string buffer is truncated to contain exactly the number of characters given by the newLength argument.

If the newLength argument is greater than or equal to the current length, sufficient null characters ('\u0000') are appended to the string buffer so that length becomes the newLength argument.

The newLength argument must be greater than or equal to 0.

Parameters:
newLength - the new length of the buffer.
Throws:
IndexOutOfBoundsException - if the newLength argument is negative.
See Also:
length()

charAt

public char charAt(int index)
The specified character of the sequence currently represented by the string buffer, as indicated by the index argument, is returned. The first character of a string buffer is at index 0, the next at index 1, and so on, for array indexing.

The index argument must be greater than or equal to 0, and less than the length of this string buffer.

Parameters:
index - the index of the desired character.
Returns:
the character at the specified index of this string buffer.
Throws:
IndexOutOfBoundsException - if index is negative or greater than or equal to length().
See Also:
length()

getChars

public void getChars(int srcBegin,
                     int srcEnd,
                     char[] dst,
                     int dstBegin)
Characters are copied from this string buffer into the destination character array dst. 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 are copied into the subarray of dst starting at index dstBegin and ending at index: 

 dstbegin + (srcEnd-srcBegin) - 1
Parameters:
srcBegin - start copying at this offset in the string buffer.
srcEnd - stop copying at this offset in the string buffer.
dst - the array to copy the data into.
dstBegin - offset into dst.
Throws:
NullPointerException - if dst is null.
IndexOutOfBoundsException - if any of the following is true:
  • srcBegin is negative
  • the srcBeing argument is greater than the srcEnd argument.
  • srcEnd is greater than this.length(), the current length of this string buffer.
  • dstBegin+srcEnd-srcBegin is greater than dst.length

setCharAt

public void setCharAt(int index,
                      char ch)
The character at the specified index of this string buffer is set to ch. The string buffer is altered to represent a new character sequence that is identical to the old character sequence, except that it contains the character ch at position index.

The offset argument must be greater than or equal to 0, and less than the length of this string buffer.

Parameters:
index - the index of the character to modify.
ch - the new character.
Throws:
IndexOutOfBoundsException - if index is negative or greater than or equal to length().
See Also:
length()

append

public StringBuffer append(Object obj)
Appends the string representation of the Object argument to this string buffer.

The argument is converted to a string as if by the method String.valueOf, and the characters of that string are then appended to this string buffer.

Parameters:
obj - an Object.
Returns:
a reference to this StringBuffer object.
See Also:
String.valueOf(java.lang.Object), append(java.lang.String)

append

public StringBuffer append(String str)
Appends the string to this string buffer.

The characters of the String argument are appended, in order, to the contents of this string buffer, increasing the length of this string buffer by the length of the argument. If str is null, then the four characters "null" are appended to this string buffer.

Let n be the length of the old character sequence, the one contained in the string buffer just prior to execution of the append method. Then the character at index k in the new character sequence is equal to the character at index k in the old character sequence, if k is less than n; otherwise, it is equal to the character at index k-n in the argument str.

Parameters:
str - a string.
Returns:
a reference to this StringBuffer.

append

public StringBuffer append(char[] str)
Appends the string representation of the char array argument to this string buffer.

The characters of the array argument are appended, in order, to the contents of this string buffer. The length of this string buffer increases by the length of the argument.

The overall effect is exactly as if the argument were converted to a string by the method String.valueOf(char[]) and the characters of that string were then appended to this StringBuffer object.

Parameters:
str - the characters to be appended.
Returns:
a reference to this StringBuffer object.

append

public StringBuffer append(char[] str,
                           int offset,
                           int len)
Appends the string representation of a subarray of the char array argument to this string buffer.

Characters of the character array str, starting at index offset, are appended, in order, to the contents of this string buffer. The length of this string buffer increases by the value of len.

The overall effect is exactly as if the arguments were converted to a string by the method String.valueOf(char[],int,int) and the characters of that string were then appended to this StringBuffer object.

Parameters:
str - the characters to be appended.
offset - the index of the first character to append.
len - the number of characters to append.
Returns:
a reference to this StringBuffer object.

append

public StringBuffer append(boolean b)
Appends the string representation of the boolean argument to the string buffer.

The argument is converted to a string as if by the method String.valueOf, and the characters of that string are then appended to this string buffer.

Parameters:
b - a boolean.
Returns:
a reference to this StringBuffer.
See Also:
String.valueOf(boolean), append(java.lang.String)

append

public StringBuffer append(char c)
Appends the string representation of the char argument to this string buffer.

The argument is appended to the contents of this string buffer. The length of this string buffer increases by 1.

The overall effect is exactly as if the argument were converted to a string by the method String.valueOf(char) and the character in that string were then appended to this StringBuffer object.

Parameters:
ch - a char.
Returns:
a reference to this StringBuffer object.

append

public StringBuffer append(int i)
Appends the string representation of the int argument to this string buffer.

The argument is converted to a string as if by the method String.valueOf, and the characters of that string are then appended to this string buffer.

Parameters:
i - an int.
Returns:
a reference to this StringBuffer object.
See Also:
String.valueOf(int), append(java.lang.String)

append

public StringBuffer append(long l)
Appends the string representation of the long argument to this string buffer.

The argument is converted to a string as if by the method String.valueOf, and the characters of that string are then appended to this string buffer.

Parameters:
l - a long.
Returns:
a referenct to this StringBuffer object.
See Also:
String.valueOf(long), append(java.lang.String)

append

public StringBuffer append(float f)
Appends the string representation of the float argument to this string buffer.

The argument is converted to a string as if by the method String.valueOf, and the characters of that string are then appended to this string buffer.

Parameters:
f - a float.
Returns:
a reference to this StringBuffer object.
See Also:
String.valueOf(float), append(java.lang.String)

append

public StringBuffer append(double d)
Appends the string representation of the double argument to this string buffer.

The argument is converted to a string as if by the method String.valueOf, and the characters of that string are then appended to this string buffer.

Parameters:
d - a double.
Returns:
a reference to this StringBuffer object.
See Also:
String.valueOf(double), append(java.lang.String)

delete

public StringBuffer delete(int start,
                           int end)
Removes the characters in a substring of this StringBuffer. The substring begins at the specified start and extends to the character at index end - 1 or to the end of the StringBuffer if no such character exists. If start is equal to end, no changes are made.
Parameters:
start - The beginning index, inclusive.
end - The ending index, exclusive.
Returns:
This string buffer.
Throws:
StringIndexOutOfBoundsException - if start is negative, greater than length(), or greater than end.
Since:
JDK1.2

deleteCharAt

public StringBuffer deleteCharAt(int index)
Removes the character at the specified position in this StringBuffer (shortening the StringBuffer by one character).
Parameters:
index - Index of character to remove
Returns:
This string buffer.
Throws:
StringIndexOutOfBoundsException - if the index is negative or greater than or equal to length().
Since:
JDK1.2

replace

public StringBuffer replace(int start,
                            int end,
                            String str)
Replaces the characters in a substring of this StringBuffer with characters in the specified String. The substring begins at the specified start and extends to the character at index end - 1 or to the end of the StringBuffer if no such character exists. First the characters in the substring are removed and then the specified String is inserted at start. (The StringBuffer will be lengthened to accommodate the specified String if necessary.)
Parameters:
start - The beginning index, inclusive.
end - The ending index, exclusive.
str - String that will replace previous contents.
Returns:
This string buffer.
Throws:
StringIndexOutOfBoundsException - if start is negative, greater than length(), or greater than end.
Since:
JDK1.2

substring

public String substring(int start)
Returns a new String that contains a subsequence of characters currently contained in this StringBuffer.The substring begins at the specified index and extends to the end of the StringBuffer.
Parameters:
start - The beginning index, inclusive.
Returns:
The new string.
Throws:
StringIndexOutOfBoundsException - if start is less than zero, or greater than the length of this StringBuffer.
Since:
JDK1.2

substring

public String substring(int start,
                        int end)
Returns a new String that contains a subsequence of characters currently contained in this StringBuffer. The substring begins at the specified start and extends to the character at index end - 1. An exception is thrown if
Parameters:
start - The beginning index, inclusive.
end - The ending index, exclusive.
Returns:
The new string.
Throws:
StringIndexOutOfBoundsException - if start or end are negative or greater than length(), or start is greater than end.
Since:
JDK1.2

insert

public StringBuffer insert(int index,
                           char[] str,
                           int offset,
                           int len)
Inserts the string representation of a subarray of the str array argument into this string buffer. The subarray begins at the specified offset and extends len characters. The characters of the subarray are inserted into this string buffer at the position indicated by index. The length of this StringBuffer increases by len characters.
Parameters:
index - position at which to insert subarray.
str - A character array.
offset - the index of the first character in subarray to to be inserted.
len - the number of characters in the subarray to to be inserted.
Returns:
This string buffer.
Throws:
StringIndexOutOfBoundsException - if index is negative or greater than length(), or offset or len are negative, or (offset+len) is greater than str.length.
Since:
JDK1.2

insert

public StringBuffer insert(int offset,
                           Object obj)
Inserts the string representation of the Object argument into this string buffer.

The second argument is converted to a string as if by the method String.valueOf, and the characters of that string are then inserted into this string buffer at the indicated offset.

The offset argument must be greater than or equal to 0, and less than or equal to the length of this string buffer.

Parameters:
offset - the offset.
b - an Object.
Returns:
a reference to this StringBuffer object.
Throws:
StringIndexOutOfBoundsException - if the offset is invalid.
See Also:
String.valueOf(java.lang.Object), insert(int, java.lang.String), length()

insert

public StringBuffer insert(int offset,
                           String str)
Inserts the string into this string buffer.

The characters of the String argument are inserted, in order, into this string buffer at the indicated offset, moving up any characters originally above that position and increasing the length of this string buffer by the length of the argument. If str is null, then the four characters "null" are inserted into this string buffer.

The character at index k in the new character sequence is equal to:

  • the character at index k in the old character sequence, if k is less than offset
  • the character at index k-offset in the argument str, if k is not less than offset but is less than offset+str.length()
  • the character at index k-str.length() in the old character sequence, if k is not less than offset+str.length()

The offset argument must be greater than or equal to 0, and less than or equal to the length of this string buffer.

Parameters:
offset - the offset.
str - a string.
Returns:
a reference to this StringBuffer object.
Throws:
StringIndexOutOfBoundsException - if the offset is invalid.
See Also:
length()

insert

public StringBuffer insert(int offset,
                           char[] str)
Inserts the string representation of the char array argument into this string buffer.

The characters of the array argument are inserted into the contents of this string buffer at the position indicated by offset. The length of this string buffer increases by the length of the argument.

The overall effect is exactly as if the argument were converted to a string by the method String.valueOf(char[]) and the characters of that string were then inserted into this StringBuffer object at the position indicated by offset.

Parameters:
offset - the offset.
ch - a character array.
Returns:
a reference to this StringBuffer object.
Throws:
StringIndexOutOfBoundsException - if the offset is invalid.

insert

public StringBuffer insert(int offset,
                           boolean b)
Inserts the string representation of the boolean argument into this string buffer.

The second argument is converted to a string as if by the method String.valueOf, and the characters of that string are then inserted into this string buffer at the indicated offset.

The offset argument must be greater than or equal to 0, and less than or equal to the length of this string buffer.

Parameters:
offset - the offset.
b - a boolean.
Returns:
a reference to this StringBuffer object.
Throws:
StringIndexOutOfBoundsException - if the offset is invalid.
See Also:
String.valueOf(boolean), insert(int, java.lang.String), length()

insert

public StringBuffer insert(int offset,
                           char c)
Inserts the string representation of the char argument into this string buffer.

The second argument is inserted into the contents of this string buffer at the position indicated by offset. The length of this string buffer increases by one.

The overall effect is exactly as if the argument were converted to a string by the method String.valueOf(char) and the character in that string were then inserted into this StringBuffer object at the position indicated by offset.

The offset argument must be greater than or equal to 0, and less than or equal to the length of this string buffer.

Parameters:
offset - the offset.
ch - a char.
Returns:
a reference to this StringBuffer object.
Throws:
StringIndexOutOfBoundsException - if the offset is invalid.
See Also:
length()

insert

public StringBuffer insert(int offset,
                           int i)
Inserts the string representation of the second int argument into this string buffer.

The second argument is converted to a string as if by the method String.valueOf, and the characters of that string are then inserted into this string buffer at the indicated offset.

The offset argument must be greater than or equal to 0, and less than or equal to the length of this string buffer.

Parameters:
offset - the offset.
b - an int.
Returns:
a reference to this StringBuffer object.
Throws:
StringIndexOutOfBoundsException - if the offset is invalid.
See Also:
String.valueOf(int), insert(int, java.lang.String), length()

insert

public StringBuffer insert(int offset,
                           long l)
Inserts the string representation of the long argument into this string buffer.

The second argument is converted to a string as if by the method String.valueOf, and the characters of that string are then inserted into this string buffer at the position indicated by offset.

The offset argument must be greater than or equal to 0, and less than or equal to the length of this string buffer.

Parameters:
offset - the offset.
b - a long.
Returns:
a reference to this StringBuffer object.
Throws:
StringIndexOutOfBoundsException - if the offset is invalid.
See Also:
String.valueOf(long), insert(int, java.lang.String), length()

insert

public StringBuffer insert(int offset,
                           float f)
Inserts the string representation of the float argument into this string buffer.

The second argument is converted to a string as if by the method String.valueOf, and the characters of that string are then inserted into this string buffer at the indicated offset.

The offset argument must be greater than or equal to 0, and less than or equal to the length of this string buffer.

Parameters:
offset - the offset.
b - a float.
Returns:
a reference to this StringBuffer object.
Throws:
StringIndexOutOfBoundsException - if the offset is invalid.
See Also:
String.valueOf(float), insert(int, java.lang.String), length()

insert

public StringBuffer insert(int offset,
                           double d)
Inserts the string representation of the double argument into this string buffer.

The second argument is converted to a string as if by the method String.valueOf, and the characters of that string are then inserted into this string buffer at the indicated offset.

The offset argument must be greater than or equal to 0, and less than or equal to the length of this string buffer.

Parameters:
offset - the offset.
b - a double.
Returns:
a reference to this StringBuffer object.
Throws:
StringIndexOutOfBoundsException - if the offset is invalid.
See Also:
String.valueOf(double), insert(int, java.lang.String), length()

reverse

public StringBuffer reverse()
The character sequence contained in this string buffer is replaced by the reverse of the sequence.

Let n be the length of the old character sequence, the one contained in the string buffer just prior to execution of the reverse method. Then the character at index k in the new character sequence is equal to the character at index n-k-1 in the old character sequence.

Returns:
a reference to this object..
Since:
JDK1.0.2

toString

public String toString()
Converts to a string representing the data in this string buffer. A new String object is allocated and initialized to contain the character sequence currently represented by this string buffer. This String is then returned. Subsequent changes to the string buffer do not affect the contents of the String.

Implementation advice: This method can be coded so as to create a new String object without allocating new memory to hold a copy of the character sequence. Instead, the string can share the memory used by the string buffer. Any subsequent operation that alters the content or capacity of the string buffer must then make a copy of the internal buffer at that time. This strategy is effective for reducing the amount of memory allocated by a string concatenation operation when it is implemented using a string buffer.

Overrides:
toString in class Object
Returns:
a string representation of the string buffer.
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