查看StringBuffer源码可知，其是线程安全的，初始值为一个长度为16的char[]，长度可变
StringBuilder与其的区别仅仅是去掉了线程安全方面的处理，单线程的情况下考虑用StringBuilder
/*
* %W% %E%
*
* Copyright (c) 2006, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*/
package java.lang;
/**
* A thread-safe, 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.
* <p>
* 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.
* <p>
* The principal operations on a <code>StringBuffer</code> are the
* <code>append</code> and <code>insert</code> 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
* <code>append</code> method always adds these characters at the end
* of the buffer; the <code>insert</code> method adds the characters at
* a specified point.
* <p>
* For example, if <code>z</code> refers to a string buffer object
* whose current contents are "<code>start</code>", then
* the method call <code>z.append("le")</code> would cause the string
* buffer to contain "<code>startle</code>", whereas
* <code>z.insert(4, "le")</code> would alter the string buffer to
* contain "<code>starlet</code>".
* <p>
* In general, if sb refers to an instance of a <code>StringBuffer</code>,
* then <code>sb.append(x)</code> has the same effect as
* <code>sb.insert(sb.length(), x)</code>.
* <p>
* Whenever an operation occurs involving a source sequence (such as
* appending or inserting from a source sequence) this class synchronizes
* only on the string buffer performing the operation, not on the source.
* <p>
* 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.
*
* As of release JDK 5, this class has been supplemented with an equivalent
* class designed for use by a single thread, {@link StringBuilder}. The
* <tt>StringBuilder</tt> class should generally be used in preference to
* this one, as it supports all of the same operations but it is faster, as
* it performs no synchronization.
*
* @authorArthur van Hoff
* @version %I%, %G%
* @see java.lang.StringBuilder
* @see java.lang.String
* @since JDK1.0
*/
public final class StringBuffer
extends AbstractStringBuilder
implements java.io.Serializable, CharSequence
{
/** use serialVersionUID from JDK 1.0.2 for interoperability */
static final long serialVersionUID = 3388685877147921107L;
/**
* Constructs a string buffer with no characters in it and an
* initial capacity of 16 characters.
*/
public StringBuffer() {
super(16);
}
/**
* Constructs a string buffer with no characters in it and
* the specified initial capacity.
*
* @param capacity the initial capacity.
* @exception NegativeArraySizeException if the <code>capacity</code>
* argument is less than <code>0</code>.
*/
public StringBuffer(int capacity) {
super(capacity);
}
/**
* Constructs a string buffer initialized to the contents of the
* specified string. The initial capacity of the string buffer is
* <code>16</code> plus the length of the string argument.
*
* @param str the initial contents of the buffer.
* @exception NullPointerException if <code>str</code> is <code>null</code>
*/
public StringBuffer(String str) {
super(str.length() + 16);
append(str);
}
/**
* Constructs a string buffer that contains the same characters
* as the specified <code>CharSequence</code>. The initial capacity of
* the string buffer is <code>16</code> plus the length of the
* <code>CharSequence</code> argument.
* <p>
* If the length of the specified <code>CharSequence</code> is
* less than or equal to zero, then an empty buffer of capacity
* <code>16</code> is returned.
*
* @param seq the sequence to copy.
* @exception NullPointerException if <code>seq</code> is <code>null</code>
* @since 1.5
*/
public StringBuffer(CharSequence seq) {
this(seq.length() + 16);
append(seq);
}
public synchronized int length() {
return count;
}
public synchronized int capacity() {
return value.length;
}
public synchronized void ensureCapacity(int minimumCapacity) {
if (minimumCapacity > value.length) {
expandCapacity(minimumCapacity);
}
}
/**
* @since 1.5
*/
public synchronized void trimToSize() {
super.trimToSize();
}
/**
* @throws IndexOutOfBoundsException {@inheritDoc}
* @see #length()
*/
public synchronized void setLength(int newLength) {
super.setLength(newLength);
}
/**
* @throws IndexOutOfBoundsException {@inheritDoc}
* @see #length()
*/
public synchronized char charAt(int index) {
if ((index < 0) || (index >= count))
throw new StringIndexOutOfBoundsException(index);
return value[index];
}
/**
* @since 1.5
*/
public synchronized int codePointAt(int index) {
return super.codePointAt(index);
}
/**
* @since 1.5
*/
public synchronized int codePointBefore(int index) {
return super.codePointBefore(index);
}
/**
* @since 1.5
*/
public synchronized int codePointCount(int beginIndex, int endIndex) {
return super.codePointCount(beginIndex, endIndex);
}
/**
* @since 1.5
*/
public synchronized int offsetByCodePoints(int index, int codePointOffset) {
return super.offsetByCodePoints(index, codePointOffset);
}
/**
* @throws NullPointerException {@inheritDoc}
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public synchronized void getChars(int srcBegin, int srcEnd, char dst[],
int dstBegin)
{
super.getChars(srcBegin, srcEnd, dst, dstBegin);
}
/**
* @throws IndexOutOfBoundsException {@inheritDoc}
* @see #length()
*/
public synchronized void setCharAt(int index, char ch) {
if ((index < 0) || (index >= count))
throw new StringIndexOutOfBoundsException(index);
value[index] = ch;
}
/**
* @see java.lang.String#valueOf(java.lang.Object)
* @see #append(java.lang.String)
*/
public synchronized StringBuffer append(Object obj) {
super.append(String.valueOf(obj));
return this;
}
public synchronized StringBuffer append(String str) {
super.append(str);
return this;
}
/**
* Appends the specified <tt>StringBuffer</tt> to this sequence.
* <p>
* The characters of the <tt>StringBuffer</tt> argument are appended,
* in order, to the contents of this <tt>StringBuffer</tt>, increasing the
* length of this <tt>StringBuffer</tt> by the length of the argument.
* If <tt>sb</tt> is <tt>null</tt>, then the four characters
* <tt>"null"</tt> are appended to this <tt>StringBuffer</tt>.
* <p>
* Let <i>n</i> be the length of the old character sequence, the one
* contained in the <tt>StringBuffer</tt> just prior to execution of the
* <tt>append</tt> method. Then the character at index <i>k</i> in
* the new character sequence is equal to the character at index <i>k</i>
* in the old character sequence, if <i>k</i> is less than <i>n</i>;
* otherwise, it is equal to the character at index <i>k-n</i> in the
* argument <code>sb</code>.
* <p>
* This method synchronizes on <code>this</code> (the destination)
* object but does not synchronize on the source (<code>sb</code>).
*
* @param sb the <tt>StringBuffer</tt> to append.
* @return a reference to this object.
* @since 1.4
*/
public synchronized StringBuffer append(StringBuffer sb) {
super.append(sb);
return this;
}
/**
* Appends the specified <code>CharSequence</code> to this
* sequence.
* <p>
* The characters of the <code>CharSequence</code> argument are appended,
* in order, increasing the length of this sequence by the length of the
* argument.
*
* <p>The result of this method is exactly the same as if it were an
* invocation of this.append(s, 0, s.length());
*
* <p>This method synchronizes on this (the destination)
* object but does not synchronize on the source (<code>s</code>).
*
* <p>If <code>s</code> is <code>null</code>, then the four characters
* <code>"null"</code> are appended.
*
* @param s the <code>CharSequence</code> to append.
* @return a reference to this object.
* @since 1.5
*/
public StringBuffer append(CharSequence s) {
// Note, synchronization achieved via other invocations
if (s == null)
s = "null";
if (s instanceof String)
return this.append((String)s);
if (s instanceof StringBuffer)
return this.append((StringBuffer)s);
return this.append(s, 0, s.length());
}
/**
* @throws IndexOutOfBoundsException {@inheritDoc}
* @since 1.5
*/
public synchronized StringBuffer append(CharSequence s, int start, int end)
{
super.append(s, start, end);
return this;
}
public synchronized StringBuffer append(char str[]) {
super.append(str);
return this;
}
public synchronized StringBuffer append(char str[], int offset, int len) {
super.append(str, offset, len);
return this;
}
/**
* @see java.lang.String#valueOf(boolean)
* @see #append(java.lang.String)
*/
public synchronized StringBuffer append(boolean b) {
super.append(b);
return this;
}
public synchronized StringBuffer append(char c) {
super.append(c);
return this;
}
/**
* @see java.lang.String#valueOf(int)
* @see #append(java.lang.String)
*/
public synchronized StringBuffer append(int i) {
super.append(i);
return this;
}
/**
* @since 1.5
*/
public synchronized StringBuffer appendCodePoint(int codePoint) {
super.appendCodePoint(codePoint);
return this;
}
/**
* @see java.lang.String#valueOf(long)
* @see #append(java.lang.String)
*/
public synchronized StringBuffer append(long lng) {
super.append(lng);
return this;
}
/**
* @see java.lang.String#valueOf(float)
* @see #append(java.lang.String)
*/
public synchronized StringBuffer append(float f) {
super.append(f);
return this;
}
/**
* @see java.lang.String#valueOf(double)
* @see #append(java.lang.String)
*/
public synchronized StringBuffer append(double d) {
super.append(d);
return this;
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
* @since 1.2
*/
public synchronized StringBuffer delete(int start, int end) {
super.delete(start, end);
return this;
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
* @since 1.2
*/
public synchronized StringBuffer deleteCharAt(int index) {
super.deleteCharAt(index);
return this;
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
* @since 1.2
*/
public synchronized StringBuffer replace(int start, int end, String str) {
super.replace(start, end, str);
return this;
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
* @since 1.2
*/
public synchronized String substring(int start) {
return substring(start, count);
}
/**
* @throws IndexOutOfBoundsException {@inheritDoc}
* @since 1.4
*/
public synchronized CharSequence subSequence(int start, int end) {
return super.substring(start, end);
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
* @since 1.2
*/
public synchronized String substring(int start, int end) {
return super.substring(start, end);
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
* @since 1.2
*/
public synchronized StringBuffer insert(int index, char str[], int offset,
int len)
{
super.insert(index, str, offset, len);
return this;
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
* @see java.lang.String#valueOf(java.lang.Object)
* @see #insert(int, java.lang.String)
* @see #length()
*/
public synchronized StringBuffer insert(int offset, Object obj) {
super.insert(offset, String.valueOf(obj));
return this;
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
* @see #length()
*/
public synchronized StringBuffer insert(int offset, String str) {
super.insert(offset, str);
return this;
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
*/
public synchronized StringBuffer insert(int offset, char str[]) {
super.insert(offset, str);
return this;
}
/**
* @throws IndexOutOfBoundsException {@inheritDoc}
* @since 1.5
*/
public StringBuffer insert(int dstOffset, CharSequence s) {
// Note, synchronization achieved via other invocations
if (s == null)
s = "null";
if (s instanceof String)
return this.insert(dstOffset, (String)s);
return this.insert(dstOffset, s, 0, s.length());
}
/**
* @throws IndexOutOfBoundsException {@inheritDoc}
* @since 1.5
*/
public synchronized StringBuffer insert(int dstOffset, CharSequence s,
int start, int end)
{
super.insert(dstOffset, s, start, end);
return this;
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
* @see java.lang.String#valueOf(boolean)
* @see #insert(int, java.lang.String)
* @see #length()
*/
public StringBuffer insert(int offset, boolean b) {
return insert(offset, String.valueOf(b));
}
/**
* @throws IndexOutOfBoundsException {@inheritDoc}
* @see #length()
*/
public synchronized StringBuffer insert(int offset, char c) {
super.insert(offset, c);
return this;
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
* @see java.lang.String#valueOf(int)
* @see #insert(int, java.lang.String)
* @see #length()
*/
public StringBuffer insert(int offset, int i) {
return insert(offset, String.valueOf(i));
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
* @see java.lang.String#valueOf(long)
* @see #insert(int, java.lang.String)
* @see #length()
*/
public StringBuffer insert(int offset, long l) {
return insert(offset, String.valueOf(l));
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
* @see java.lang.String#valueOf(float)
* @see #insert(int, java.lang.String)
* @see #length()
*/
public StringBuffer insert(int offset, float f) {
return insert(offset, String.valueOf(f));
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
* @see java.lang.String#valueOf(double)
* @see #insert(int, java.lang.String)
* @see #length()
*/
public StringBuffer insert(int offset, double d) {
return insert(offset, String.valueOf(d));
}
/**
* @throws NullPointerException {@inheritDoc}
* @since 1.4
*/
public int indexOf(String str) {
return indexOf(str, 0);
}
/**
* @throws NullPointerException {@inheritDoc}
* @since 1.4
*/
public synchronized int indexOf(String str, int fromIndex) {
return String.indexOf(value, 0, count,
str.toCharArray(), 0, str.length(), fromIndex);
}
/**
* @throws NullPointerException {@inheritDoc}
* @since 1.4
*/
public int lastIndexOf(String str) {
// Note, synchronization achieved via other invocations
return lastIndexOf(str, count);
}
/**
* @throws NullPointerException {@inheritDoc}
* @since 1.4
*/
public synchronized int lastIndexOf(String str, int fromIndex) {
return String.lastIndexOf(value, 0, count,
str.toCharArray(), 0, str.length(), fromIndex);
}
/**
* @since JDK1.0.2
*/
public synchronized StringBuffer reverse() {
super.reverse();
return this;
}
public synchronized String toString() {
return new String(value, 0, count);
}
/**
* Serializable fields for StringBuffer.
*
* @serialField value char[]
* The backing character array of this StringBuffer.
* @serialField count int
* The number of characters in this StringBuffer.
* @serialField shared boolean
* A flag indicating whether the backing array is shared.
* The value is ignored upon deserialization.
*/
private static final java.io.ObjectStreamField[] serialPersistentFields =
{
new java.io.ObjectStreamField("value", char[].class),
new java.io.ObjectStreamField("count", Integer.TYPE),
new java.io.ObjectStreamField("shared", Boolean.TYPE),
};
/**
* readObject is called to restore the state of the StringBuffer from
* a stream.
*/
private synchronized void writeObject(java.io.ObjectOutputStream s)
throws java.io.IOException {
java.io.ObjectOutputStream.PutField fields = s.putFields();
fields.put("value", value);
fields.put("count", count);
fields.put("shared", false);
s.writeFields();
}
/**
* readObject is called to restore the state of the StringBuffer from
* a stream.
*/
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
java.io.ObjectInputStream.GetField fields = s.readFields();
value = (char[])fields.get("value", null);
count = (int)fields.get("count", 0);
}
}
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