java.lang(StringBuffer)

public final class StringBuffer
    extends AbstractStringBuilder
    implements java.io.Serializable, CharSequence {
 /**
     * 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.
     */
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);
    }

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的内部实现方式和String不同，StringBuffer在进行字符串处理时，不生成新的对象，在内存使用上要优于String类。

所以在实际使用时，如果经常需要对一个字符串进行修改，例如插入、删除等操作，使用StringBuffer要更加适合一些。

在StringBuffer类中存在很多和String类一样的方法，这些方法在功能上和String类中的功能是完全一样的。

但是有一个最显著的区别在于，对于StringBuffer对象的每次修改都会改变对象自身，这点是和String类最大的区别。

对于StringBuffer的append操作基本是在AbstractStringBuilder继承实现，看源码：

abstract class AbstractStringBuilder implements Appendable, CharSequence {

    char[] value;   //The value is used for character storage.

    int count;      //The count is the number of characters used.

//Appends the specified string to this character sequence.
public AbstractStringBuilder append(String str) {
        if (str == null) str = "null";
        int len = str.length();
        ensureCapacityInternal(count + len);
        str.getChars(0, len, value, count);
        count += len;
        return this;
    }

// Documentation in subclasses because of synchro difference
    public AbstractStringBuilder append(StringBuffer sb) {
        if (sb == null)
            return append("null");
        int len = sb.length();
        ensureCapacityInternal(count + len);
        sb.getChars(0, len, value, count);
        count += len;
        return this;
    }

// Documentation in subclasses because of synchro difference
    public AbstractStringBuilder 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);
        return this.append(s, 0, s.length());
    }

/**
     * Appends the string representation of the {@code char} array
     * argument to this sequence.
 */
public AbstractStringBuilder append(char[] str) {
        int len = str.length;
        ensureCapacityInternal(count + len);
        System.arraycopy(str, 0, value, count, len);
        count += len;
        return this;
    }
}
/***************其他追加字符方法省略介绍了***********/

添加字符时的扩容方法介绍：

/**
     * This implements the expansion semantics of ensureCapacity with no
     * size check or synchronization.
     */
    void expandCapacity(int minimumCapacity) {
        int newCapacity = value.length * 2 + 2;
        if (newCapacity - minimumCapacity < 0)
            newCapacity = minimumCapacity;
        if (newCapacity < 0) {
            if (minimumCapacity < 0) // overflow
                throw new OutOfMemoryError();
            newCapacity = Integer.MAX_VALUE;
        }
        value = Arrays.copyOf(value, newCapacity);
    }

每次添加字符时扩容为：int newCapacity = value.length * 2 + 2;