/**
     * The value is used for character storage.
     */
    char value[];
    /**
     * The count is the number of characters used.
     */
    int count;

AbstractStringBuilder() {
  }

 AbstractStringBuilder(int capacity) {
    value = new char[capacity];
  }

 void expandCapacity(int minimumCapacity) {
    int newCapacity = (value.length + 1) * 2;
    if (newCapacity < 0) {
      newCapacity = Integer.MAX_VALUE;
    } else if (minimumCapacity > newCapacity) {
      newCapacity = minimumCapacity;
    }
    value = Arrays.copyOf(value, newCapacity);
  }

public void ensureCapacity(int minimumCapacity)

 public void ensureCapacity(int minimumCapacity) {
    if (minimumCapacity > value.length) {
      expandCapacity(minimumCapacity);
    }
  }

    StringBuffer s = new StringBuffer();
    System.out.println("容量：" + s.capacity());// 容量：16
    s.ensureCapacity(10);
    System.out.println("容量：" + s.capacity());// 容量：16
    s.ensureCapacity(30);
    System.out.println("容量：" + s.capacity());// 容量：34
    s.ensureCapacity(80);
    System.out.println("容量：" + s.capacity());// 容量：80

public int codePointAt(int index) {
    if ((index < 0) || (index >= count)) {
      throw new StringIndexOutOfBoundsException(index);
    }
    return Character.codePointAtImpl(value, index, count);
  }

public void getChars(int srcBegin, int srcEnd, char[] dst, int dstBegin)
  {
    if (srcBegin < 0)
      throw new StringIndexOutOfBoundsException(srcBegin);
    if ((srcEnd < 0) || (srcEnd > count))
      throw new StringIndexOutOfBoundsException(srcEnd);
    if (srcBegin > srcEnd)
      throw new StringIndexOutOfBoundsException("srcBegin > srcEnd");
    System.arraycopy(value, srcBegin, dst, dstBegin, srcEnd - srcBegin);
  }

public AbstractStringBuilder append(String str) {
    if (str == null)
      return appendNull();
    int len = str.length();
    ensureCapacityInternal(count + len);
    str.getChars(0, len, value, count);
    count += len;
    return this;
  }

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);
  }

public void setLength(int newLength) {
    if (newLength < 0)
      throw new StringIndexOutOfBoundsException(newLength);
    ensureCapacityInternal(newLength);

    if (count < newLength) {
      Arrays.fill(value, count, newLength, '\0');
    }

    count = newLength;
  }

public AbstractStringBuilder delete(int start, int end) {
    if (start < 0)
      throw new StringIndexOutOfBoundsException(start);
    if (end > count)
      end = count;
    if (start > end)
      throw new StringIndexOutOfBoundsException();
    int len = end - start;
    if (len > 0) {
      System.arraycopy(value, start+len, value, start, count-end);
      count -= len;
    }
    return this;
  }

 public AbstractStringBuilder insert(int index, char[] str, int offset,
                    int len)
  {
    if ((index < 0) || (index > length()))
      throw new StringIndexOutOfBoundsException(index);
    if ((offset < 0) || (len < 0) || (offset > str.length - len))
      throw new StringIndexOutOfBoundsException(
        "offset " + offset + ", len " + len + ", str.length "
        + str.length);
    ensureCapacityInternal(count + len);
    System.arraycopy(value, index, value, index + len, count - index);
    System.arraycopy(str, offset, value, index, len);
    count += len;
    return this;
  }