在 Java 中删除字符串中的最后 n 行（句子）

Question

I am looking for an efficient way to remove last n lines from a String. Efficient as in- fast performing as well as something that does not create too may objects. Therefore would like to stay away from split(). Especially because, at times, my strings could be a few hundred or even thousand lines.

For instance, I am getting a string as such:

This is a sample code line 1.
This is a sample code line 2.

Warm Regards,
SomeUser.

The last 3 lines (an empty line, "Warm Regards,", and "SomeUser.") is what I am trying to get rid of. Note that the content (including the last 3 lines) isn't fixed.

I am thinking of counting the lines first using this solution here: https://stackoverflow.com/a/18816371/1353174 and then again, use another similar loop to reach to a position that is lines - n and do a substring till that position.

However, just posting this problem here to know if there are any other and perhaps more efficient ways to achieve this. External library-based solutions (like Apache Commons StringUtils) are also welcome.

Answer 1

You can use String.lastIndexOf to find last third occurrence of '\\n' symbol and then do String.substring to get the result.

     public static void main(String[] args) {
        String s = "This is a sample code line 1.\n" +
                "This is a sample code line 2.\n" +
                "\n" +
                "Warm Regards,\n" +
                "SomeUser.";

        int truncateIndex = s.length();

        for (int i = 0; i < 3; i++) {
            System.out.println(truncateIndex);
            truncateIndex = s.lastIndexOf('\n', truncateIndex - 1);
        }

        System.out.println(s.substring(0, truncateIndex));
        System.out.println("--");
    }

This code snippet intentionally doesn't care for corner cases, such as when there is less than three lines in input string, to make code simple and readable.

Answer 2

public static final String SAMPLE_TEXT = "This is a sample code line 1.\nThis is a sample code line 2.\r\n\nWarm Regards,\r\nSomeUser.";

public static void main (String[] args) throws java.lang.Exception {
    String[] lines = SAMPLE_TEXT.split("\\r?\\n"); // catches Windows newlines (\r) as well)
    for (int i = 0; i < lines.length - 3; i++) {   // lines.length - 3 to discard the last 3 lines
        System.out.println(lines[i]);
    }
}

Here's a runnable example:

http://ideone.com/nwaMcD

Answer 3

  @scala.annotation.tailrec
  def rmLines(in: String, nlines: Int): String =
    if (nlines == 0) {
      in
    } else {
      val lastBreakIndex = in.lastIndexOf('\n')
      if (lastBreakIndex == -1) {
        in
      } else {
        rmLines(in.substring(0, lastBreakIndex), nlines - 1)
      }
    }

Answer 4

使用正则表达式来做到这一点： http : //docs.oracle.com/javase/tutorial/essential/regex/