[java] Trim a string based on the string length

tl;dr

You seem to be asking for an ellipsis (…) character in the last place, when truncating. Here is a one-liner to manipulate your input string.

String input = "abcdefghijkl";
String output = ( input.length () > 10 ) ? input.substring ( 0 , 10 - 1 ).concat ( "…" ) : input;

See this code run live at IdeOne.com.

abcdefghi…

Ternary operator

We can make a one-liner by using the ternary operator.

String input = "abcdefghijkl" ;

String output = 
    ( input.length() > 10 )          // If too long…
    ?                                
    input     
    .substring( 0 , 10 - 1 )         // Take just the first part, adjusting by 1 to replace that last character with an ellipsis.
    .concat( "…" )                   // Add the ellipsis character.
    :                                // Or, if not too long…
    input                            // Just return original string.
;

See this code run live at IdeOne.com.

abcdefghi…

Java streams

The Java Streams facility makes this interesting, as of Java 9 and later. Interesting, but maybe not the best approach.

We use code points rather than char values. The char type is legacy, and is limited to the a subset of all possible Unicode characters.

String input = "abcdefghijkl" ;
int limit = 10 ;
String output =
        input
                .codePoints()
                .limit( limit )
                .collect(                                    // Collect the results of processing each code point.
                        StringBuilder::new,                  // Supplier<R> supplier
                        StringBuilder::appendCodePoint,      // ObjIntConsumer<R> accumulator
                        StringBuilder::append                // BiConsumer<R,?R> combiner
                )
                .toString()
        ;

If we had excess characters truncated, replace the last character with an ellipsis.

if ( input.length () > limit )
{
    output = output.substring ( 0 , output.length () - 1 ) + "…";
}

If only I could think of a way to put together the stream line with the "if over limit, do ellipsis" part.