difference between String and StringBuilder and StringBuffer

StringTest.java

package ir.soheil_gh;

public class StringTest {

    public static void main(String[] args) {
        long t1 = System.currentTimeMillis();
        String initialString = "";
        for(int i=0; i<50000; i++ ){
            initialString += i;
        }
        long t2 = System.currentTimeMillis();

        System.out.println(t2 - t1  + " milliseconds");

        StringBuilder initialStringBuilder = new StringBuilder();
        long t3 = System.currentTimeMillis();
        for(int i=0; i<50000; i++ ){
            initialStringBuilder.append(i);
        }
        long t4 = System.currentTimeMillis();

        System.out.println(t4 - t3  + " milliseconds");

        StringBuffer initialStringBuffer = new StringBuffer();
        long t5 = System.currentTimeMillis();
        for(int i=0; i<50000; i++ ){
            initialStringBuffer.append(i);
        }
        long t6 = System.currentTimeMillis();

        System.out.println(t6 - t5  + " milliseconds");
    }
}

output :
7079 milliseconds
2 milliseconds
3 milliseconds