StringBuffer 详解
StringBuffer 简介
StringBuffer 是一个线程安全的可变的字符序列。它继承于AbstractStringBuilder，实现了CharSequence接口。
StringBuilder 也是继承于AbstractStringBuilder的子类；但是，StringBuilder和StringBuffer不同，前者是非线程安全的，后者是线程安全的。
StringBuffer 和 CharSequence之间的关系图如下：
StringBuffer
StringBuffer源码(基于jdk1.7.40)
package java.lang;
public final class StringBuffer
extends AbstractStringBuilder
implements java.io.Serializable, CharSequence
{
static final long serialVersionUID = 3388685877147921107L;
public StringBuffer() {
super(16);
}
public StringBuffer(int capacity) {
super(capacity);
}
public StringBuffer(String str) {
super(str.length() + 16);
append(str);
}
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);
}
}
public synchronized void trimToSize() {
super.trimToSize();
}
public synchronized void setLength(int newLength) {
super.setLength(newLength);
}
public synchronized char charAt(int index) {
if ((index < 0) || (index >= count))
throw new StringIndexOutOfBoundsException(index);
return value[index];
}
public synchronized int codePointAt(int index) {
return super.codePointAt(index);
}
public synchronized int codePointBefore(int index) {
return super.codePointBefore(index);
}
public synchronized int codePointCount(int beginIndex, int endIndex) {
return super.codePointCount(beginIndex, endIndex);
}
public synchronized int offsetByCodePoints(int index, int codePointOffset) {
return super.offsetByCodePoints(index, codePointOffset);
}
public synchronized void getChars(int srcBegin, int srcEnd, char[] dst,
int dstBegin)
{
super.getChars(srcBegin, srcEnd, dst, dstBegin);
}
public synchronized void setCharAt(int index, char ch) {
if ((index < 0) || (index >= count))
throw new StringIndexOutOfBoundsException(index);
value[index] = ch;
}
public synchronized StringBuffer append(Object obj) {
super.append(String.valueOf(obj));
return this;
}
public synchronized StringBuffer append(String str) {
super.append(str);
return this;
}
public synchronized StringBuffer append(StringBuffer sb) {
super.append(sb);
return this;
}
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());
}
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;
}
public synchronized StringBuffer append(boolean b) {
super.append(b);
return this;
}
public synchronized StringBuffer append(char c) {
super.append(c);
return this;
}
public synchronized StringBuffer append(int i) {
super.append(i);
return this;
}
public synchronized StringBuffer appendCodePoint(int codePoint) {
super.appendCodePoint(codePoint);
return this;
}
public synchronized StringBuffer append(long lng) {
super.append(lng);
return this;
}
public synchronized StringBuffer append(float f) {
super.append(f);
return this;
}
public synchronized StringBuffer append(double d) {
super.append(d);
return this;
}
public synchronized StringBuffer delete(int start, int end) {
super.delete(start, end);
return this;
}
public synchronized StringBuffer deleteCharAt(int index) {
super.deleteCharAt(index);
return this;
}
public synchronized StringBuffer replace(int start, int end, String str) {
super.replace(start, end, str);
return this;
}
public synchronized String substring(int start) {
return substring(start, count);
}
public synchronized CharSequence subSequence(int start, int end) {
return super.substring(start, end);
}
public synchronized String substring(int start, int end) {
return super.substring(start, end);
}
public synchronized StringBuffer insert(int index, char[] str, int offset,
int len)
{
super.insert(index, str, offset, len);
return this;
}
public synchronized StringBuffer insert(int offset, Object obj) {
super.insert(offset, String.valueOf(obj));
return this;
}
public synchronized StringBuffer insert(int offset, String str) {
super.insert(offset, str);
return this;
}
public synchronized StringBuffer insert(int offset, char[] str) {
super.insert(offset, str);
return this;
}
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());
}
public synchronized StringBuffer insert(int dstOffset, CharSequence s,
int start, int end)
{
super.insert(dstOffset, s, start, end);
return this;
}
public StringBuffer insert(int offset, boolean b) {
return insert(offset, String.valueOf(b));
}
public synchronized StringBuffer insert(int offset, char c) {
super.insert(offset, c);
return this;
}
public StringBuffer insert(int offset, int i) {
return insert(offset, String.valueOf(i));
}
public StringBuffer insert(int offset, long l) {
return insert(offset, String.valueOf(l));
}
public StringBuffer insert(int offset, float f) {
return insert(offset, String.valueOf(f));
}
public StringBuffer insert(int offset, double d) {
return insert(offset, String.valueOf(d));
}
public int indexOf(String str) {
return indexOf(str, 0);
}
public synchronized int indexOf(String str, int fromIndex) {
return String.indexOf(value, 0, count,
str.toCharArray(), 0, str.length(), fromIndex);
}
public int lastIndexOf(String str) {
// Note, synchronization achieved via other invocations
return lastIndexOf(str, count);
}
public synchronized int lastIndexOf(String str, int fromIndex) {
return String.lastIndexOf(value, 0, count,
str.toCharArray(), 0, str.length(), fromIndex);
}
public synchronized StringBuffer reverse() {
super.reverse();
return this;
}
public synchronized String toString() {
return new String(value, 0, count);
}
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),
};
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();
}
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 = fields.get("count", 0);
}
}StringBuffer 示例
源码如下(StringBufferTest.java):
/**
* StringBuffer 演示程序
*
* @author skywang
*/
import java.util.HashMap;
public class StringBufferTest {
public static void main(String[] args) {
testInsertAPIs() ;
testAppendAPIs() ;
testReplaceAPIs() ;
testDeleteAPIs() ;
testIndexAPIs() ;
testOtherAPIs() ;
}
/**
* StringBuffer 的其它API示例
*/
private static void testOtherAPIs() {
System.out.println("-------------------------------- testOtherAPIs --------------------------------");
StringBuffer sbuilder = new StringBuffer("0123456789");
int cap = sbuilder.capacity();
System.out.printf("cap=%d\n", cap);
char c = sbuilder.charAt(6);
System.out.printf("c=%c\n", c);
char[] carr = new char[4];
sbuilder.getChars(3, 7, carr, 0);
for (int i=0; i运行结果 ：
-------------------------------- testInsertAPIs -------------------------------
{3=three, 2=two, 1=one}
12345StringBUFFERStringBufferBUILDERStringBuilder12345100
true3.141591.414ABCabcde
-------------------------------- testAppendAPIs -------------------------------
abcdeABC1.4143.14159true
10012345StringBuilderBUILDERStringBufferBUFFERString12345
{3=three, 2=two, 1=one}
字符编码
-------------------------------- testReplaceAPIs ------------------------------
sbuilder=ABCDE3456789
sbuilder=9876543210
sbuilder=M123456789
-------------------------------- testDeleteAPIs -------------------------------
sbuilder=123789
str1=23789
str2=78
str3=78
-------------------------------- testIndexAPIs --------------------------------
sbuilder=abcAbcABCabCaBcAbCaBCabc
sbuilder.indexOf("bc") = 1
sbuilder.indexOf("bc", 5) = 22
sbuilder.lastIndexOf("bc") = 22
sbuilder.lastIndexOf("bc", 4) = 4
-------------------------------- testOtherAPIs --------------------------------
cap=26
c=6
carr[0]=3 carr[1]=4 carr[2]=5 carr[3]=6