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Copyright © 2006 Sun Microsystems, Inc. and Motorola, Inc. All rights reserved.

java.lang
Class StringBuffer

java.lang.Object
  java.lang.StringBuffer

public final class StringBuffer

extends Object

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, CLDC 1.0

See Also:

ByteArrayOutputStream, String

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 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	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	toString() Converts to a string representing the data in this string buffer.

Methods inherited from class java.lang.Object

equals, 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 '