StringBufferpublic final class StringBuffer extends Object | A string buffer implements a mutable sequence of characters.
A string buffer is like a {@link 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. |
| Fields Summary |
|---|
| private char[] | valueThe value is used for character storage. | | private int | countThe count is the number of characters in the buffer. | | private boolean | sharedA flag indicating whether the buffer is shared |
| Constructors Summary |
|---|
public StringBuffer()Constructs a string buffer with no characters in it and an
initial capacity of 16 characters.
value = new char[16];
| public StringBuffer(int length)Constructs a string buffer with no characters in it and an
initial capacity specified by the length argument.
value = new char[length];
| 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.
this(str.length() + 16);
append(str);
|
| Methods Summary |
|---|
| public synchronized java.lang.StringBuffer | append(java.lang.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.
return append(String.valueOf(obj));
| | public synchronized java.lang.StringBuffer | append(java.lang.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.
if (str == null) {
str = String.valueOf(str);
}
int len = str.length();
int newcount = count + len;
if (newcount > value.length)
expandCapacity(newcount);
str.getChars(0, len, value, count);
count = newcount;
return this;
| | public synchronized java.lang.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 {@link String#valueOf(char[])} and the
characters of that string were then {@link #append(String) appended}
to this StringBuffer object.
int len = str.length;
int newcount = count + len;
if (newcount > value.length)
expandCapacity(newcount);
JVM.unchecked_char_arraycopy(str, 0, value, count, len);
count = newcount;
return this;
| | public synchronized java.lang.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 {@link String#valueOf(char[],int,int)} and the
characters of that string were then {@link #append(String) appended}
to this StringBuffer object.
int newcount = count + len;
if (newcount > value.length)
expandCapacity(newcount);
// NOTE: str and offset not checked, cannot use unchecked arraycopy
System.arraycopy(str, offset, value, count, len);
count = newcount;
return this;
| | public java.lang.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.
return append(String.valueOf(b));
| | public synchronized java.lang.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 {@link String#valueOf(char)} and the character
in that string were then {@link #append(String) appended} to this
StringBuffer object.
int newcount = count + 1;
if (newcount > value.length)
expandCapacity(newcount);
value[count++] = c;
return this;
| | public java.lang.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.
return append(String.valueOf(i));
| | public java.lang.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.
return append(String.valueOf(l));
| | 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.
return value.length;
| | 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.
if ((index < 0) || (index >= count)) {
throw new StringIndexOutOfBoundsException(
/* #ifdef VERBOSE_EXCEPTIONS */
/// skipped index
/* #endif */
);
}
return value[index];
| | private final void | copy()Copies the buffer value. This is normally only called when shared
is true. It should only be called from a synchronized method.
char newValue[] = new char[value.length];
JVM.unchecked_char_arraycopy(value, 0, newValue, 0, count);
value = newValue;
shared = false;
| | public synchronized java.lang.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.
if (start < 0)
throw new StringIndexOutOfBoundsException(
/* #ifdef VERBOSE_EXCEPTIONS */
/// skipped start
/* #endif */
);
if (end > count)
end = count;
if (start > end)
throw new StringIndexOutOfBoundsException();
int len = end - start;
if (len > 0) {
if (shared)
copy();
JVM.unchecked_char_arraycopy(value, start+len,
value, start, count-end);
count -= len;
}
return this;
| | public synchronized java.lang.StringBuffer | deleteCharAt(int index)Removes the character at the specified position in this
StringBuffer (shortening the StringBuffer
by one character).
if ((index < 0) || (index >= count))
throw new StringIndexOutOfBoundsException();
if (shared)
copy();
JVM.unchecked_char_arraycopy(value, index+1,
value, index, count-index-1);
count--;
return this;
| | 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.
If the minimumCapacity argument is nonpositive, this
method takes no action and simply returns.
if (minimumCapacity > value.length) {
expandCapacity(minimumCapacity);
}
| | private void | expandCapacity(int minimumCapacity)This implements the expansion semantics of ensureCapacity but is
unsynchronized for use internally by methods which are already
synchronized.
int newCapacity = (value.length + 1) * 2;
if (newCapacity < 0) {
newCapacity = Integer.MAX_VALUE;
} else if (minimumCapacity > newCapacity) {
newCapacity = minimumCapacity;
}
char newValue[] = new char[newCapacity];
JVM.unchecked_char_arraycopy(value, 0, newValue, 0, count);
value = newValue;
shared = false;
| | public synchronized 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
if (srcBegin < 0) {
throw new StringIndexOutOfBoundsException(
/* #ifdef VERBOSE_EXCEPTIONS */
/// skipped srcBegin
/* #endif */
);
}
if ((srcEnd < 0) || (srcEnd > count)) {
throw new StringIndexOutOfBoundsException(
/* #ifdef VERBOSE_EXCEPTIONS */
/// skipped srcEnd
/* #endif */
);
}
if (srcBegin > srcEnd) {
throw new StringIndexOutOfBoundsException(
/* #ifdef VERBOSE_EXCEPTIONS */
/// skipped "srcBegin > srcEnd"
/* #endif */
);
}
// NOTE: dst not checked, cannot use unchecked arraycopy
System.arraycopy(value, srcBegin, dst, dstBegin, srcEnd - srcBegin);
| | final char[] | getValue() return value;
| | public synchronized java.lang.StringBuffer | insert(int offset, java.lang.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.
return insert(offset, String.valueOf(obj));
| | public synchronized java.lang.StringBuffer | insert(int offset, java.lang.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.
if ((offset < 0) || (offset > count)) {
throw new StringIndexOutOfBoundsException();
}
if (str == null) {
str = String.valueOf(str);
}
int len = str.length();
int newcount = count + len;
if (newcount > value.length)
expandCapacity(newcount);
else if (shared)
copy();
JVM.unchecked_char_arraycopy(value, offset,
value, offset + len, count - offset);
str.getChars(0, len, value, offset);
count = newcount;
return this;
| | public synchronized java.lang.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 {@link String#valueOf(char[])} and the
characters of that string were then
{@link #insert(int,String) inserted} into this
StringBuffer object at the position indicated by
offset.
if ((offset < 0) || (offset > count)) {
throw new StringIndexOutOfBoundsException();
}
int len = str.length;
int newcount = count + len;
if (newcount > value.length)
expandCapacity(newcount);
else if (shared)
copy();
JVM.unchecked_char_arraycopy(value, offset,
value, offset + len, count - offset);
JVM.unchecked_char_arraycopy(str, 0, value, offset, len);
count = newcount;
return this;
| | public java.lang.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.
return insert(offset, String.valueOf(b));
| | public synchronized java.lang.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 {@link String#valueOf(char)} and the character
in that string were then {@link #insert(int, String) 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.
int newcount = count + 1;
if (newcount > value.length)
expandCapacity(newcount);
else if (shared)
copy();
// NOTE: offset not checked, cannot use unchecked arraycopy
System.arraycopy(value, offset, value, offset + 1, count - offset);
value[offset] = c;
count = newcount;
return this;
| | public java.lang.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.
return insert(offset, String.valueOf(i));
| | public java.lang.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.
return insert(offset, String.valueOf(l));
| | public int | length()Returns the length (character count) of this string buffer.
return count;
| | public synchronized java.lang.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.
if (shared) copy();
int n = count - 1;
for (int j = (n-1) >> 1; j >= 0; --j) {
char temp = value[j];
value[j] = value[n - j];
value[n - j] = temp;
}
return this;
| | 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 offset argument must be greater than or equal to
0, and less than the length of this string buffer.
if ((index < 0) || (index >= count)) {
throw new StringIndexOutOfBoundsException(
/* #ifdef VERBOSE_EXCEPTIONS */
/// skipped index
/* #endif */
);
}
if (shared) copy();
value[index] = ch;
| | 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.
if (newLength < 0) {
throw new StringIndexOutOfBoundsException(
/* #ifdef VERBOSE_EXCEPTIONS */
/// skipped newLength
/* #endif */
);
}
if (newLength > value.length) {
expandCapacity(newLength);
}
if (count < newLength) {
if (shared) copy();
for (; count < newLength; count++) {
value[count] = '\0";
}
} else {
count = newLength;
if (shared) {
if (newLength > 0) {
copy();
} else {
// If newLength is zero, assume the StringBuffer is being
// stripped for reuse; Make new buffer of default size
value = new char[16];
shared = false;
}
}
}
| | final void | setShared() shared = true;
| | public java.lang.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.
return new String(this);
|
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