/**
  * 默认初始容量大小
  */
 private static final int DEFAULT_CAPACITY = 10;
 

 private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

 /**
  *默认构造函数，使用初始容量10构造一个空列表(无参数构造)
  */
 public ArrayList() {
     this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
 }
 
 /**
  * 带初始容量参数的构造函数。（用户自己指定容量）
  */
 public ArrayList(int initialCapacity) {
     if (initialCapacity > 0) {//初始容量大于0
         //创建initialCapacity大小的数组
         this.elementData = new Object[initialCapacity];
     } else if (initialCapacity == 0) {//初始容量等于0
         //创建空数组
         this.elementData = EMPTY_ELEMENTDATA;
     } else {//初始容量小于0，抛出异常
         throw new IllegalArgumentException("Illegal Capacity: "+
                                            initialCapacity);
     }
 }


/**
 *构造包含指定collection元素的列表，这些元素利用该集合的迭代器按顺序返回
 *如果指定的集合为null，throws NullPointerException。 
 */
  public ArrayList(Collection<? extends E> c) {
     elementData = c.toArray();
     if ((size = elementData.length) != 0) {
         // c.toArray might (incorrectly) not return Object[] (see 6260652)
         if (elementData.getClass() != Object[].class)
             elementData = Arrays.copyOf(elementData, size, Object[].class);
     } else {
         // replace with empty array.
         this.elementData = EMPTY_ELEMENTDATA;
     }
 }

String sql = "SELECT * FROM users WHERE username = '" + username + "' AND password = '" + password + "'";
Statement stmt = connection.createStatement();
ResultSet rs = stmt.executeQuery(sql);