由于前面学习了StringBuffer和StringBuilder的父类AbstractStringBuilder,他们俩的很多方法都是直接super了父类的，也为了较好的比较StringBuffer和StringBuilder，所以把二者放在同一博文中。
一.StringBuffer
1.静态类，不能被继承，实现了Serializable和CharSequence接口。
public final class StringBuffer extends AbstractStringBuilder implements java.io.Serializable, CharSequence2.构造函数
public StringBuffer() {//StringBuffer的默认 容量 是16，注意是容量不是长度
super(16);
}
public StringBuffer(int capacity) {//也可以在初始化StringBuffer时为它指定容量，为了减少不必要的扩容，尽量在初始化StringBuffer时指定 容量
super(capacity);
}
public StringBuffer(String str) {//若初始化StringBuffer时传入字符串，则 容量 为字符串长度+默认容量16
super(str.length() + 16);
append(str);
}
public StringBuffer(CharSequence seq) {
this(seq.length() + 16);
append(seq);
}3.成员函数
方法大部分都定义为synchronized，这是StringBuffer和StringBuilder的最大区别，也就是StringBuffer是同步的，线程安全的，而StringBuilder是非线程安全的。当然嘛，上帝给你关了一扇门总是会为你开一扇窗的,StringBuffer是线程安全的但是效率上去略逊与StringBuilder.
由于几乎都是super的父类的，故不再累述，见我的前一篇JDK源码学习----AbstractStringBuilder http://blog.csdn.net/sheepmu/article/details/26095203
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 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 delete(int start, int end) {
super.delete(start, end);
return this;
}
public synchronized StringBuffer deleteCharAt(int index) {
super.deleteCharAt(index);
return this;
}
public synchronized String substring(int start) {
return substring(start, count);
}
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 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 = (int)fields.get("count", 0);
}
}
二.StringBuilder
1.静态类，不能被继承，实现了Serializable和CharSequence接口。 (同StringBuffer)
public final class StringBuilder extends AbstractStringBuilder implements java.io.Serializable, CharSequence2.构造函数 (同StringBuffer)
public StringBuilder() {
super(16);
}
public StringBuilder(int capacity) {
super(capacity);
}
public StringBuilder(String str) {
super(str.length() + 16);
append(str);
}
public StringBuilder(CharSequence seq) {
this(seq.length() + 16);
append(seq);
}3.成员函数 （与StringBuffer最大的区别就是非同步的）public StringBuilder append(Object obj) {
return append(String.valueOf(obj));
}
public StringBuilder append(String str) {
super.append(str);
return this;
}
private StringBuilder append(StringBuilder sb) {
if (sb == null)
return append("null");
int len = sb.length();
int newcount = count + len;
if (newcount > value.length)
expandCapacity(newcount);
sb.getChars(0, len, value, count);
count = newcount;
return this;
}
public StringBuilder append(StringBuffer sb) {
super.append(sb);
return this;
}
public StringBuilder append(CharSequence s) {
if (s == null)
s = "null";
if (s instanceof String)
return this.append((String)s);
if (s instanceof StringBuffer)
return this.append((StringBuffer)s);
if (s instanceof StringBuilder)
return this.append((StringBuilder)s);
return this.append(s, 0, s.length());
}
public StringBuilder delete(int start, int end) {
super.delete(start, end);
return this;
}
public StringBuilder deleteCharAt(int index) {
super.deleteCharAt(index);
return this;
}
public StringBuilder insert(int index, char str[], int offset,
int len)
{
super.insert(index, str, offset, len);
return this;
}
public int indexOf(String str) {
return indexOf(str, 0);
}
public int indexOf(String str, int fromIndex) {
return String.indexOf(value, 0, count,
str.toCharArray(), 0, str.length(), fromIndex);
}
public int lastIndexOf(String str) {
return lastIndexOf(str, count);
}
public StringBuilder reverse() {
super.reverse();
return this;
}
public String toString() {
// Create a copy, don't share the array
return new String(value, 0, count);
}
JDK源码学习----StringBuffer+StringBuilder,布布扣,bubuko.com
JDK源码学习----StringBuffer+StringBuilder
原文：http://blog.csdn.net/sheepmu/article/details/27123541