String concatenation vs StringBuilder

package com.company;

/**
 * Created by admin on 3/9/17.
 */
public class Stringbuilder2 {
    public static void main(String[] args) {
        
        int length = 100000;
        int c = 0;

        // 100,000 character length String generated with StringBuilder
        
        long tstart = System.currentTimeMillis();

        StringBuilder builder = new StringBuilder();

        while (c++ < length ){
            builder.append('-');
        }

        String line = builder.toString();

        long tdelta = System.currentTimeMillis() - tstart;

        System.out.println("Building a very long String with StringBuilder " + tdelta);
        // 6 - 7 milliseconds for StringBuilder 

        // Same length string, by concatenating "-" onto an existing String
        
        tstart = System.currentTimeMillis();

        String line2 = "";
        c = 0;

        while (c++ < length ){
            line2 += "-";
        }

        tdelta = System.currentTimeMillis() - tstart;

        System.out.println("Building a very long String with by concatenating Strings " + tdelta);
        // 2.5 - 3 seconds for String concatenation

    }
}