StringBuffer扩容

1：默认初始长度　16

public StringBuffer() {
    super(16);
}

2：append追加

    @Override
    public synchronized StringBuffer append(String str) {
        toStringCache = null;
        super.append(str);
        return this;
    }

    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;
    }

    private void ensureCapacityInternal(int minimumCapacity) {
        // overflow-conscious code
        if (minimumCapacity - value.length > 0) {
            value = Arrays.copyOf(value,
                    newCapacity(minimumCapacity));
        }
    }

每次长度补够时，都会调用Arrays.copy() ,在此之前，还要计算新的char[]长度   newCapacity(minimumCapacity)

    private int newCapacity(int minCapacity) {
        // overflow-conscious code
        int newCapacity = (value.length << 1) + 2;
        if (newCapacity - minCapacity < 0) {
            newCapacity = minCapacity;
        }
        return (newCapacity <= 0 || MAX_ARRAY_SIZE - newCapacity < 0)
            ? hugeCapacity(minCapacity)
            : newCapacity;
    }

新的长度=最小长度*2+2  （最小长度：追加字符串后需要占用的长度）