java第三季老师布置的作业:随机字符串

public class CollectionsTest {  
  /**
   * 生成一定长度的随机字符串, 并返回.
   *
   * @param length 字符串长度
   * @return
   */
  public String generatorRandomString(int length) {
    String chars = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";

    Random random = new Random();
    StringBuilder result = new StringBuilder();

    // 生成
    for (int i = 0; i < length; i++) {
      int l = random.nextInt(chars.length());
      result.append(chars.charAt(l));
    }

    return result.toString();
  }

  /**
   * 对随机字符串进行排序
   */
  public void testSortRandomString() {
    List<String> strings = new ArrayList<String>();

    String str = null;
    for (int i = 0; i < 20; i++) {
      do {
        str = generatorRandomString(5);
      }
      // 如果序列中含有该字符串, 那就再生成一次, 直到序列中没有该字符串为止.
      while (strings.contains(str));
      strings.add(str);
      System.out.println("添加元素: " + str);
    }

    System.out.println("------排序前------");
    for (String s : strings) {
      System.out.print(s + ", ");
    }

    System.out.println();
    System.out.println("------排序后------");
    Collections.sort(strings);
    for (String s : strings) {
      System.out.print(s + ", ");
    }
  }

  public static void main(String[] args) {
    CollectionsTest test = new CollectionsTest();
    test.testSortRandomString();
  }
}

打印结果:
添加元素: HJ9FC
添加元素: aLEIv
添加元素: Dlxzm
添加元素: NrOh4
添加元素: GGoHJ
添加元素: 6Yhk5
添加元素: jYPgs
添加元素: 2kMvH
添加元素: ngoEN
添加元素: fRCTO
添加元素: 8l5Zx
添加元素: VYyl4
添加元素: lSb52
添加元素: qd3f8
添加元素: RgPGn
添加元素: NOuFK
添加元素: Y1Dfb
添加元素: cayRB
添加元素: WuQT0
添加元素: w2MAO
------排序前------
HJ9FC, aLEIv, Dlxzm, NrOh4, GGoHJ, 6Yhk5, jYPgs, 2kMvH, ngoEN, fRCTO, 8l5Zx, VYyl4, lSb52, qd3f8, RgPGn, NOuFK, Y1Dfb, cayRB, WuQT0, w2MAO,
------排序后------
2kMvH, 6Yhk5, 8l5Zx, Dlxzm, GGoHJ, HJ9FC, NOuFK, NrOh4, RgPGn, VYyl4, WuQT0, Y1Dfb, aLEIv, cayRB, fRCTO, jYPgs, lSb52, ngoEN, qd3f8, w2MAO,
Process finished with exit code 0