为列命名。

String query = "SELECT COUNT(*) as count FROM

引用该列从ResultSet对象到int，并从那里做你的逻辑。

PreparedStatement statement = connection.prepareStatement(query);
statement.setString(1, item.getProductId());
ResultSet resultSet = statement.executeQuery();
while (resultSet.next()) {
    int count = resultSet.getInt("count");
    if (count >= 1) {
        System.out.println("Product ID already exists.");
    } else {
        System.out.println("New Product ID.");
    }
}

ResultSet rs = ps.executeQuery();
int rowcount = 0;
if (rs.last()) {
  rowcount = rs.getRow();
  rs.beforeFirst(); // not rs.first() because the rs.next() below will move on, missing the first element
}
while (rs.next()) {
  // do your standard per row stuff
}

int i = 0;
while(rs.next()) {
    i++;
}

(速度考虑)

很多人在这里建议ResultSet.last()，但你需要打开连接作为一个ResultSet。TYPE_SCROLL_INSENSITIVE，对于Derby嵌入式数据库，它比ResultSet.TYPE_FORWARD_ONLY慢10倍。

根据我对嵌入式Derby和H2数据库的微型测试，在SELECT之前调用SELECT COUNT(*)要快得多。

下面是我的代码和基准测试的更详细信息

当使用rs.last()时，我得到了一个异常

if(rs.last()){
    rowCount = rs.getRow(); 
    rs.beforeFirst();
}

:

java.sql.SQLException: Invalid operation for forward only resultset

因为默认情况下它是ResultSet。TYPE_FORWARD_ONLY，这意味着只能使用rs.next()

解决方案是:

stmt=conn.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE,
    ResultSet.CONCUR_READ_ONLY); 

为列命名。

String query = "SELECT COUNT(*) as count FROM

引用该列从ResultSet对象到int，并从那里做你的逻辑。

PreparedStatement statement = connection.prepareStatement(query);
statement.setString(1, item.getProductId());
ResultSet resultSet = statement.executeQuery();
while (resultSet.next()) {
    int count = resultSet.getInt("count");
    if (count >= 1) {
        System.out.println("Product ID already exists.");
    } else {
        System.out.println("New Product ID.");
    }
}