java - Java 2d String 空指针异常

Question

所以我有一个字符串[][]，我想在其中保存数据库中的一些信息。 Eclipse 告诉我需要初始化 string[][]，所以我说 string[]][] = null。现在我得到一个空指针异常，但我不确定我做错了什么。

try {
            Statement st = con.createStatement();
            String systems[][] = null;
            int firstcounter = 0;

            String item = "SELECT ram, cpu, mainboard, cases, gfx FROM computer_system";

            ResultSet rs = st.executeQuery(item);
            while (rs.next()){
                systems[firstcounter][0] = rs.getString("ram");
                systems[firstcounter][1] = rs.getString("cpu");
                systems[firstcounter][2] = rs.getString("mainboard");
                systems[firstcounter][3] = rs.getString("cases");
                systems[firstcounter][4] = rs.getString("gfx");
                firstcounter++;
            }
            System.out.println(systems[0][0]);

Answer 1

您应该将数组初始化为非空值才能使用它:

String systems[][] = new String[n][5];

哪里n是数组的最大容量。

如果您事先不知道数组应包含多少行，则可以使用 ArrayList<String[]>相反，因为 ArrayList 的容量可以成长。

例如:

        Statement st = con.createStatement();
        List<String[]> systems = new ArrayList<String[]>();
        String item = "SELECT ram, cpu, mainboard, cases, gfx FROM computer_system";
        ResultSet rs = st.executeQuery(item);
        while (rs.next()){
            String[] row = new String[5];
            row[0] = rs.getString("ram");
            row[1] = rs.getString("cpu");
            row[2] = rs.getString("mainboard");
            row[3] = rs.getString("cases");
            row[4] = rs.getString("gfx");
            systems.add(row);
        }