//JDK 6
String(int offset, int count, char value[]) {
  this.value = value;
  this.offset = offset;
  this.count = count;
}

public String substring(int beginIndex, int endIndex) {
  //check boundary
  return new String(offset + beginIndex, endIndex - beginIndex, value);
}

  //JDK 7

  /**
   * Allocates a new {@code String} that contains characters from a subarray
   * of the character array argument. The {@code offset} argument is the
   * index of the first character of the subarray and the {@code count}
   * argument specifies the length of the subarray. The contents of the
   * subarray are copied; subsequent modification of the character array does
   * not affect the newly created string.
   *
   * @param value Array that is the source of characters
   * @param offset The initial offset
   * @param count The length
   * @throws IndexOutOfBoundsException If the {@code offset} and {@code count} arguments index
   *                  characters outside the bounds of the {@code value} array
   */
  public String(char value[], int offset, int count) {
    //check boundary
    this.value = Arrays.copyOfRange(value, offset, offset + count);
  }

  /**
   * Returns a string that is a substring of this string. The
   * substring begins at the specified {@code beginIndex} and
   * extends to the character at index {@code endIndex - 1}.
   * Thus the length of the substring is {@code endIndex-beginIndex}.
   * <p>
   * Examples:
   * <blockquote><pre>
   * "hamburger".substring(4, 8) returns "urge"
   * "smiles".substring(1, 5) returns "mile"
   * </pre></blockquote>
   *
   * @param beginIndex the beginning index, inclusive.
   * @param endIndex  the ending index, exclusive.
   * @return the specified substring.
   * @throws IndexOutOfBoundsException if the
   *                  {@code beginIndex} is negative, or
   *                  {@code endIndex} is larger than the length of
   *                  this {@code String} object, or
   *                  {@code beginIndex} is larger than
   *                  {@code endIndex}.
   */
  public String substring(int beginIndex, int endIndex) {
    //check boundary
    int subLen = endIndex - beginIndex;
    return ((beginIndex == 0) && (endIndex == value.length)) ?
        this :
        new String(value, beginIndex, subLen);
  }