为什么会生成空的字符串呢

public class CollectionTest2 {

public String basic = "abcdefghijklmnopqrstuvwxyzABCDEFDGHIJKLMNOPQRSTUVWXYZ1234567890";

public void testString(){

List<String> strings = new ArrayList<String>();

Random random = new Random();

for(int i=0;i<10;i++){

StringBuffer k = new StringBuffer();

do {

for(int j=0;j<random.nextInt(10);j++){

int num = random.nextInt(62);

k.append(basic.charAt(num));

}

} while (strings.contains(k.toString()));

strings.add(k.toString());

}

System.out.println("-------排序前--------");

for (String string : strings) {

System.out.println("元素"+string);

}

Collections.sort(strings);

System.out.println("-------排序后--------");

for (String string : strings) {

System.out.println("元素"+string);

}

}

public static void main(String[] args){

CollectionTest2 ct2 = new CollectionTest2();

ct2.testString();

}

}