java - ArrayList 的行为非常奇怪

Question

我正在使用jdbc sql从XLS表中检索数据。下面是我的程序。

案例1:

Class.forName( "sun.jdbc.odbc.JdbcOdbcDriver" );
c = DriverManager.getConnection( "jdbc:odbc:Excel Files", "", "" );
stmnt = c.createStatement();
String query = "select * from [Proficiency$];";
ResultSet rs = stmnt.executeQuery( query );
baseColumnList = new ArrayList();
while( rs.next()) {
    //baseColumnList.add(rs.getString(1));
    System.out.println(rs.getString(1));
    //System.out.println(baseColumnList);
}

输出:

Aadmin
Badmin
Badminimage
batch
Cadmin
Dadmin
Eadmin
rulesengine
Fadmin
genadmin
Gadmin
Hdomain-EU
Padmin
IAA

情况2:

Class.forName( "sun.jdbc.odbc.JdbcOdbcDriver" );
c = DriverManager.getConnection( "jdbc:odbc:Excel Files", "", "" );
stmnt = c.createStatement();
String query = "select * from [Proficiency$];";
ResultSet rs = stmnt.executeQuery( query );
baseColumnList = new ArrayList();
while( rs.next()) {
    baseColumnList.add(rs.getString(1));
    System.out.println(baseColumnList);
}

输出:

[Aadmin]
[Aadmin, Badmin]
[Aadmin, Badmin, Badminimage]
[Aadmin, Badmin, Badminimage, batch]
[Aadmin, Badmin, Badminimage, batch, Cadmin]
[Aadmin, Badmin, Badminimage, batch, Cadmin, Dadmin]
[Aadmin, Badmin, Badminimage, batch, Cadmin, Dadmin, Eadmin]
[Aadmin, Badmin, Badminimage, batch, Cadmin, Dadmin, Eadmin, rulesengine]
[Aadmin, Badmin, Badminimage, batch, Cadmin, Dadmin, Eadmin, rulesengine, Fadmin]
[Aadmin, Badmin, Badminimage, batch, Cadmin, Dadmin, Eadmin, rulesengine, Fadmin, genadmin]
[Aadmin, Badmin, Badminimage, batch, Cadmin, Dadmin, Eadmin, rulesengine, Fadmin, genadmin, Gadmin]
[Aadmin, Badmin, Badminimage, batch, Cadmin, Dadmin, Eadmin, rulesengine, Fadmin, genadmin, Gadmin, Hadmin-EU]
[Aadmin, Badmin, Badminimage, batch, Cadmin, Dadmin, Eadmin, rulesengine, Fadmin, genadmin, Gadmin, Hadmin-EU, Padmin]
[Aadmin, Badmin, Badminimage, batch, Cadmin, Dadmin, Eadmin, rulesengine, Fadmin, genadmin, Gadmin, Hadmin-EU, Padmin, IAA]

请谁能解释一下为什么 arraylist 的行为很奇怪。

更新:预期结果是第一个输出。

Answer 1

这个

System.out.println(baseColumnList);

打印列表的实际内容¹。结果一点也不奇怪。每次迭代该列表都会变得更大。

<小时/>

要重复第一个示例的输出:

while( rs.next()) {
  baseColumnList.add(rs.getString(1));
  System.out.println(baseColumnList.get(baseColumnList.size()-1));
}

但那是无稽之谈。下面的代码片段更加优雅:

while( rs.next()) {
  String columnValue = rs.getString(1);
  baseColumnList.add(columnValue);
  System.out.println(columnValue);
}

<小时/>

¹ 实际上调用了baseColumnList.toString()。