if (null == instance || instance.isEmpty())

public Boolean startsWith(String prefix, int toffset);
public Boolean startsWith(String prefix) {
 starsWith(prefix, 0);
 // 调用的是双参的实现
}

char[] ta = value;
int pc = prefix.length;
if ((toffset < 0) || (toffset > value.length-pc))
{
 // 负数形式的偏移，
 // 偏移加前缀的长度大于此字符串的长度；
 return false;
}

public Boolean startsWith(String prefix, int toffset) {
 char ta[] = value;
 int to = toffset;
 char pa[] = prefix.value;
 int po = 0;
 int pc = prefix.value.length;
 // Note: toffset might be near -1>>>1.
 if ((toffset < 0) || (toffset > value.length - pc)) {
  return false;
 }
 while (--pc >= 0) {
  if (ta[to++] != pa[po++]) {
   return false;
  }
 }
 return true;
}