https://developer.salesforce.com/docs/atlas.ja-jp.200.0.apexcode.meta/apexcode/apex_methods_system_string.htm

StringAbbreviateDemo.cls

String target = 'Helloooooooooooo!!';
String result = '';

// subString
result = target.substring(0, 5) + '...';
System.debug('subString : ' + result + ' (' + result.length() + ')');

// abbreviate
result = target.abbreviate(8);
System.debug('abbreviate : ' + result + ' (' + result.length() + ')');

// abbreviate
target = 'Helloooo';
result = target.abbreviate(8);
System.debug('abbreviate : ' + result + ' (' + result.length() + ')');

// abbreviate
target = 'Hi!';
result = target.abbreviate(8);
System.debug('abbreviate : ' + result);

// Null
try {
    target = null;
    result = target.substring(0, 5) + '...';
} catch (Exception e) {
    System.debug('subString : ' + e.getMessage());
}

// Null
try {
    target = null;
    result = target.abbreviate(8);
} catch (Exception e) {
    System.debug('abbreviate : ' + e.getMessage());
}