Strings

@GwtCompatible
public final class Strings {
  private Strings() {}

  /**
   *若string为null，则返回空串；否则，返回自身
   */
  public static String nullToEmpty(@Nullable String string) {
    return (string == null) ? "" : string;
  }

  /**
   * 若string为null或者empty，都返回null；否则，返回自身
   */
  @Nullable
  public static String emptyToNull(@Nullable String string) {
    return isNullOrEmpty(string) ? null : string;
  }

  /**
   *当string为null或者为空时，返回true；否则，返回false
   *这里判断空串是通过length()==0，而java6中是isEmpty()
   */
  public static boolean isNullOrEmpty(@Nullable String string) {
    return string == null || string.length() == 0; // string.isEmpty() in Java 6
  }

  /**
   * 字符串填补
   * @param string 放在结果字符串的最后一部分
   * @param minLength 返回结果串应该具有的最小长度。可以为0或者负数，这样返回的是string本身
   * @param padChar 用于填补的字符，插入在string的前面，插入次数直到返回串的总长度达到minLength
   * @return the padded string
   */
  public static String padStart(String string, int minLength, char padChar) {
    checkNotNull(string);  
    if (string.length() >= minLength) {
      return string;
    }
    StringBuilder sb = new StringBuilder(minLength);
    for (int i = string.length(); i < minLength; i++) {
      sb.append(padChar);
    }
    sb.append(string);
    return sb.toString();
  }

  /**
   * 与padStart类似，只不过padChar放到了string的后面
   */
  public static String padEnd(String string, int minLength, char padChar) {
    checkNotNull(string);
    if (string.length() >= minLength) {
      return string;
    }
    StringBuilder sb = new StringBuilder(minLength);
    sb.append(string);
    for (int i = string.length(); i < minLength; i++) {
      sb.append(padChar);
    }
    return sb.toString();
  }

  /**
   *字符串拷贝
   * @param string 非空字符串
   * @param count 重复次数，非负的
   * @return a string containing {@code string} repeated {@code count} times
   *     (the empty string if {@code count} is zero)
   * @throws IllegalArgumentException if {@code count} is negative
   */
  public static String repeat(String string, int count) {
    checkNotNull(string);  // eager for GWT.

    if (count <= 1) {
      checkArgument(count >= 0, "invalid count: %s", count);
      return (count == 0) ? "" : string;
    }

    final int len = string.length();
    final long longSize = (long) len * (long) count;
    final int size = (int) longSize;
　　//如果最后得到的字符串过长，直接抛出异常
    if (size != longSize) {
      throw new ArrayIndexOutOfBoundsException(
          "Required array size too large: " + longSize);
    }

    final char[] array = new char[size];
　　//直接调用系统的字符串拷贝函数：System.arraycopy(value, srcBegin, dst, dstBegin, srcEnd - srcBegin);

 　　string.getChars(0, len, array, 0);
 　　int n;
    for (n = len; n < size - n; n <<= 1) {
      System.arraycopy(array, 0, array, n, n);
    }
　　//最后一次拷贝，拷贝长度是size-n
    System.arraycopy(array, 0, array, n, size - n);
    return new String(array);
  }

  /**
   *计算两个字符串的公共前缀
   */
  public static String commonPrefix(CharSequence a, CharSequence b) {
    checkNotNull(a);
    checkNotNull(b);
    int maxPrefixLength = Math.min(a.length(), b.length());
    int p = 0;
    while (p < maxPrefixLength && a.charAt(p) == b.charAt(p)) {
      p++;
    }
    if (validSurrogatePairAt(a, p - 1) || validSurrogatePairAt(b, p - 1)) {
      p--;
    }
    return a.subSequence(0, p).toString();
  }

  /**
   * 同commonPrefix，只不过求的是公共后缀
   */
  public static String commonSuffix(CharSequence a, CharSequence b) {
    checkNotNull(a);
    checkNotNull(b);

    int maxSuffixLength = Math.min(a.length(), b.length());
    int s = 0;
    while (s < maxSuffixLength
        && a.charAt(a.length() - s - 1) == b.charAt(b.length() - s - 1)) {
      s++;
    }
    if (validSurrogatePairAt(a, a.length() - s - 1)
        || validSurrogatePairAt(b, b.length() - s - 1)) {
      s--;
    }
    return a.subSequence(a.length() - s, a.length()).toString();
  }

  @VisibleForTesting
  static boolean validSurrogatePairAt(CharSequence string, int index) {
    return index >= 0 && index <= (string.length() - 2)
        && Character.isHighSurrogate(string.charAt(index))
        && Character.isLowSurrogate(string.charAt(index + 1));
  }
}