Trim a string based on the string length

StringUtils.abbreviate from Apache Commons Lang library could be your friend:

StringUtils.abbreviate("abcdefg", 6) = "abc..."
StringUtils.abbreviate("abcdefg", 7) = "abcdefg"
StringUtils.abbreviate("abcdefg", 8) = "abcdefg"
StringUtils.abbreviate("abcdefg", 4) = "a..."

Commons Lang3 even allow to set a custom String as replacement marker. With this you can for example set a single character ellipsis.

StringUtils.abbreviate("abcdefg", "\u2026", 6) = "abcde…"

As usual nobody cares about UTF-16 surrogate pairs. See about them: What are the most common non-BMP Unicode characters in actual use? Even authors of org.apache.commons/commons-lang3

You can see difference between correct code and usual code in this sample:

public static void main(String[] args) {
    //string with FACE WITH TEARS OF JOY symbol
    String s = "abcdafghi\uD83D\uDE02cdefg";
    int maxWidth = 10;
    System.out.println(s);
    //do not care about UTF-16 surrogate pairs
    System.out.println(s.substring(0, Math.min(s.length(), maxWidth)));
    //correctly process UTF-16 surrogate pairs
    if(s.length()>maxWidth){
        int correctedMaxWidth = (Character.isLowSurrogate(s.charAt(maxWidth)))&&maxWidth>0 ? maxWidth-1 : maxWidth;
        System.out.println(s.substring(0, Math.min(s.length(), correctedMaxWidth)));
    }
}

There is a Apache Commons StringUtils function which does this.

s = StringUtils.left(s, 10)

If len characters are not available, or the String is null, the String will be returned without an exception. An empty String is returned if len is negative.

StringUtils.left(null, ) = null
StringUtils.left(, -ve) = ""
StringUtils.left("", *) = ""
StringUtils.left("abc", 0) = ""
StringUtils.left("abc", 2) = "ab"
StringUtils.left("abc", 4) = "abc"

StringUtils.Left JavaDocs

Courtesy:Steeve McCauley

s = s.substring(0, Math.min(s.length(), 10));

Using Math.min like this avoids an exception in the case where the string is already shorter than 10.

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Notes:

1. The above does simple trimming. If you actually want to replace the last characters with three dots if the string is too long, use Apache Commons StringUtils.abbreviate; see @H6's solution. If you want to use the Unicode horizontal ellipsis character, see @Basil's solution.

2. For typical implementations of String, s.substring(0, s.length()) will return s rather than allocating a new String.

3. This may behave incorrectly¹ if your String contains Unicode codepoints outside of the BMP; e.g. Emojis. For a (more complicated) solution that works correctly for all Unicode code-points, see @sibnick's solution.

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^{1 - A Unicode codepoint that is not on plane 0 (the BMP) is represented as a "surrogate pair" (i.e. two char values) in the String. By ignoring this, we might trim the string to fewer than 10 code points, or (worse) truncate it in the middle of a surrogate pair. On the other hand, String.length() is not a good measure of Unicode text length, so trimming based on that property may be the wrong thing to do.}