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() 

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.

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.


append
public synchronized 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 valueOf(char) and the character in that string were then appended to this StringBuffer object.


append
public synchronized 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 valueOf(char[]) and the characters of that string were then appended to this StringBuffer object.


append
public synchronized 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 valueOf(char[],int,int) and the characters of that string were then appended to this StringBuffer object.


append
public synchronized 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.


append
public synchronized 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.


append
public synchronized 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.


append
public synchronized 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.


append
public synchronized 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.


append
public synchronized 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.


append
public synchronized StringBuffer append( StringBuffer sb )

Appends the specified StringBuffer to this StringBuffer.

The characters of the StringBuffer argument are appended, in order, to the contents of this StringBuffer, increasing the length of this StringBuffer by the length of the argument. If sb is null, then the four characters "null" are appended to this StringBuffer.

Let n be the length of the old character sequence, the one contained in the StringBuffer 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 sb.

The method ensureCapacity is first called on this StringBuffer with the new buffer length as its argument. (This ensures that the storage of this StringBuffer is adequate to contain the additional characters being appended.)


capacity
public synchronized 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.


charAt
public synchronized 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.


delete
public synchronized 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.


deleteCharAt
public synchronized StringBuffer deleteCharAt( int index )

Removes the character at the specified position in this StringBuffer (shortening the StringBuffer by one character).


ensureCapacity
public synchronized 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.

 
dstbegin + (srcEnd-srcBegin) - 1 

 
this.toString().startsWith(str, k) 

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

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.


insert
public synchronized 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 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.


insert
public synchronized 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 valueOf(char[]) and the characters of that string were then inserted into this StringBuffer object at the position indicated by offset.


insert
public synchronized 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.


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.


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.


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.


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.


insert
public synchronized 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.


insert
public synchronized 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.


lastIndexOf
public synchronized int lastIndexOf( String str )

Returns the index within this string of the rightmost occurrence of the specified substring. The rightmost empty string "" is considered to occur at the index value this.length(). The returned index is the largest value k such that

 
this.toString().startsWith(str, k) 

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

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.


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


setLength
public synchronized 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 '\u0000'. 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.


subSequence
public CharSequence subSequence( int start, int end )

Returns a new character sequence that is a subsequence of this sequence.

An invocation of this method of the form

 
sb.subSequence(begin, end)

 
sb.substring(begin, end)

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.