Question

我正在使用 jdbc sql 从 XLS 表中检索数据。以下是我的计划。

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

案例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]

请有人解释为什么arraylist表现得很苛刻。

更新：首先输出预期结果。

Answer 1

此

System.out.println(baseColumnList);

打印列表的实际内容¹。结果并不奇怪。每次迭代时列表都会变大。

重复第一个例子的输出：

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

但那是胡说八道。以下代码段更优雅：

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

¹实际上会调用baseColumnList.toString()。

Answer 2

我不太确定你认为'奇怪'。在第二种情况下，您将为结果集中的每个结果打印整个baseColumnList。

System.out.println(baseColumnList);

正如你所看到的那样，它会不断地将这些项目添加到你的baseColumnlist中，所以这一点都不奇怪，事实上它就是你期望发生的事情

祝你好运！

Answer 3

这似乎是您的代码所期望的。让我解释一下：

//this line adds a value to the list
baseColumnList.add(rs.getString(1));

//this line prints the whole list
System.out.println(baseColumnList);

在第一行中，您将值添加到列表
在第二行中打印整个列表。

在每次迭代中，列表增长一个元素并在其后打印。为防止这种情况，请不要打印整个列表，而是打印一些中间值，例如：

//intermediate value
String temp= rs.getString(1);
//add it to the list
baseColumnList.add(temp);
//print only the value, and not the whole list
System.out.println(temp);

Answer 4

    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));
        }
       for(Object o: baseColumnList)
       {
         System.out.println((String)o);

       }

ArrayList表现得非常奇怪

4 个答案: