String vs StringBuilder example

Test1.class

public class Test1 {
  public Test1();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: ldc           #2                  // String
       2: astore_1
       3: iconst_0
       4: istore_2
       5: iload_2
       6: bipush        10
       8: if_icmpge     36
      11: new           #3                  // class java/lang/StringBuilder
      14: dup
      15: invokespecial #4                  // Method java/lang/StringBuilder."<init>":()V
      18: aload_1
      19: invokevirtual #5                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      22: iload_2
      23: invokevirtual #6                  // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
      26: invokevirtual #7                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      29: astore_1
      30: iinc          2, 1
      33: goto          5
      36: getstatic     #8                  // Field java/lang/System.out:Ljava/io/PrintStream;
      39: aload_1
      40: invokevirtual #9                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      43: return
}

Test1.java

public class Test1 {
        public static void main(String[] args) {
                String aString = "";
                for (int i=0; i<10; i++) {
                        aString += i;
                }
                System.out.println(aString);
        }
}

Test2.class

public class Test2 {
  public Test2();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: new           #2                  // class java/lang/StringBuilder
       3: dup
       4: invokespecial #3                  // Method java/lang/StringBuilder."<init>":()V
       7: astore_1
       8: iconst_0
       9: istore_2
      10: iload_2
      11: bipush        10
      13: if_icmpge     28
      16: aload_1
      17: iload_2
      18: invokevirtual #4                  // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
      21: pop
      22: iinc          2, 1
      25: goto          10
      28: getstatic     #5                  // Field java/lang/System.out:Ljava/io/PrintStream;
      31: aload_1
      32: invokevirtual #6                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      35: invokevirtual #7                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      38: return
}

Test2.java

import java.lang.StringBuilder;

public class Test2 {
        public static void main(String[] args) {
                StringBuilder aString = new StringBuilder();
                for (int i=0; i<10; i++) {
                        aString.append(i);
                }
                System.out.println(aString.toString());
        }
}