Сравнение скорости работы StringBuffer_StringBuilder

import java.io.*;
import java.util.Arrays;

public class SpeedTest {
static final String FILE_NAME = "test.txt";
    static final int STRINGS = 10000;
    static long st;
    static long en;

    public static void main(String[] args) throws IOException{
        BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(new FileOutputStream(FILE_NAME, false)));
        for (int i = 0; i < STRINGS; i++) {
            bw.append(i+"Some short string...");
            bw.newLine();
        }
        bw.close();
            //С использованием String
        st = System.currentTimeMillis();
        String str1 = "";
        BufferedReader bf = new BufferedReader(new InputStreamReader(new FileInputStream(FILE_NAME)));
        while (bf.ready()){
            str1 = str1 + bf.readLine();
        }
        bf.close();
        en = System.currentTimeMillis();
        System.out.println("String read elapsed " + (en - st) + "ms");
            //С использованием StringBuilder
        st = System.currentTimeMillis();
        StringBuilder sb = new StringBuilder();
        bf = new BufferedReader(new InputStreamReader(new FileInputStream(FILE_NAME)));
        while (bf.ready()){
            sb.append(bf.readLine());
        }
        bf.close();
        String str2 = sb.toString();
        en = System.currentTimeMillis();
        System.out.println("StringBuilder read elapsed " + (en-st)+ "ms");

        //С использованием StringBuffer
        st = System.currentTimeMillis();
        FileInputStream stream = new FileInputStream(FILE_NAME);
        StringBuffer sb1 = new StringBuffer(stream.available());
        bf = new BufferedReader(new InputStreamReader(new FileInputStream(FILE_NAME)));
        while (bf.ready()){
            sb.append(bf.readLine());
        }
        bf.close();
        String str3 = sb1.toString();
        en = System.currentTimeMillis();
        System.out.println("StringBuffer read elapsed " + (en-st)+ "ms");
    }
}