java - 如何动态地将arraylist放入Hashmap中？

Question

public HashMap viewTableData(String tabName)
   {
       ArrayList<String> arrayList = new ArrayList<String>(); 
       ArrayList<String> arrayList2 = new ArrayList<String>(); 
       ArrayList<String> arrayList3 = new ArrayList<String>(); 
       ArrayList<String> arrayList4 = new ArrayList<String>(); 
       HashMap discoverMap = new HashMap();

        try {
            int i=1;
            con = DAOConnection.sqlconnection();
            stmt = con.createStatement();
            stmt = con.createStatement();
            query = "SELECT * FROM "+tabName;
            System.out.println("Qry executed");
            ps = con.prepareStatement(query);
            rs = ps.executeQuery(query);
           while(rs.next()){
              while(i<=4){ 
               arrayList.add(rs.getString(i));
               //arrayList2.add(rs.getString(2));
              discoverMap.put("qrycol"+i, arrayList);
              i++;
           }
           }
          // discoverMap.put("qrycol1", arrayList);
          // discoverMap.put("qrycol2",arrayList2);
           System.out.println("Map result:"+discoverMap);
        } catch (SQLException ex) {
            Logger.getLogger(Query.class.getName()).log(Level.SEVERE, null, ex);
            //return null;
        }
        return discoverMap;
   }

在这里您可以看到，我编写了一个方法，该方法将从数据库表中获取值并将其放入 Hashmap 中。

如果我初始化 4 个不同的数组列表，并添加 rs.getString(1),rs.getString(2),... 那么它就可以正常工作。但我想要一些动态功能，它将创建所需的数组列表并添加 rs.getString(i)。但它正在运行但没有得到期望的输出。 这里discoverMap显示结果: map 结果:{qrycol1=[1, Indranil, Banerjee, indranil1111@gmail.com], qrycol3=[1, Indranil, Banerjee,indranil1111@gmail.com], qrycol2=[1, Indranil, Banerjee, indranil1111@gmail.com], qrycol4=[1, Indranil, Banerjee, indranil1111@gmail.com]}

但是对于下面显示期望输出的代码:

public HashMap viewTableData(String tabName)
   {
       ArrayList<String> arrayList = new ArrayList<String>(); 
       ArrayList<String> arrayList2 = new ArrayList<String>(); 
       ArrayList<String> arrayList3 = new ArrayList<String>(); 
       ArrayList<String> arrayList4 = new ArrayList<String>(); 
       HashMap discoverMap = new HashMap();

        try {
            int i=1;
            con = DAOConnection.sqlconnection();
            stmt = con.createStatement();
            stmt = con.createStatement();
            query = "SELECT * FROM "+tabName;
            System.out.println("Qry executed");
            ps = con.prepareStatement(query);
            rs = ps.executeQuery(query);
           while(rs.next()){
              arrayList.add(rs.getString(1));
              arrayList2.add(rs.getString(2));
              arrayList3.add(rs.getString(3));
              arrayList4.add(rs.getString(4));
           }
            discoverMap.put("qrycol1", arrayList);
            discoverMap.put("qrycol2",arrayList2);
            discoverMap.put("qrycol3",arrayList3);
            discoverMap.put("qrycol4",arrayList4);
           System.out.println("Map result:"+discoverMap);
        } 

        catch (SQLException ex) {
            Logger.getLogger(Query.class.getName()).log(Level.SEVERE, null, ex);
            //return null;
        }
        return discoverMap;
   }

map 结果:{qrycol1=[1, 2, 3, 4, 6, 5, 7, 8], qrycol3=[Banerjee, JJJ, Saun, Gupta, efgh,aaaa, surname22, Sir], qrycol2=[ Indranil, pora, trao, Som, James, Anup, Abhi, Mia], qrycol4=[indranil111@gmail.com, pc@gmail.com, tit@gmail.com, so@gmail.com, ind@gmail.com, anu@gg.com、abhi@gg.com、mai@gg.com]}

这实际上是正确的。但我想要它是动态的。因为一直以来我们都无法假设需要多少个 arraylist。如果有人在这方面提供帮助，那将非常有帮助。提前致谢 。

数据库表结构:有4列，类型为varchar(但其他表可能有所不同，这就是我问这个问题的原因。)

Answer 1

需要知道从数据库返回的列数，如果知道可以按照:

        int numOfColumns = 6; // it can be any number you need

        // initialize the map
        for (int colIndex = 1; colIndex <= numOfColumns; ++colIndex) {
            String key = "qrycol" + colIndex;
            ArrayList<String> list = new ArrayList<>();
            discoverMap.put(key, list);
        }

        while (rs.next()) {
            // for each column got from DB, populate the corresponding array list
            for (int colIndex = 1; colIndex <= numOfColumns; ++colIndex) {
                String key = "qrycol" + colIndex;
                ArrayList<String> list = (ArrayList<String>) discoverMap.get(key);
                list.add(rs.getString(colIndex));
            }
        }