java - 使用索引从 Map 中检索值

Question

我有一张类似这样的 map -

ConcurrentHashMap<String, ConcurrentHashMap<String, String>> tableList

其中会有这样的数据-

示例-

{table1={DRIVER=oracle.jdbc.driver.OracleDriver, PASSWORD=stage_cs_user, URL=jdbc_url, SUFFIX=xt1, SQL=sql, USER=user}, table2={DRIVER=driver_name, PASSWORD=pass, URL=jdbc_url2, SUFFIX=xt2, SQL=sql2, USER=user}}

我正在尝试迭代上面的 map -

class Task implements Runnable {

private Connection[] dbConnection = null;
private final ConcurrentHashMap<String, ConcurrentHashMap<String, String>> tableLists;

public Task(ConcurrentHashMap<String, ConcurrentHashMap<String, String>> tableNames) {
    this.tableLists = tableNames;
}

@Override
public void run() {

for (int i = 0; i < tableLists.size(); i++) {

dbConnection[i] = getDBConnection(tableLists.get(i).get("URL"), tableLists.get(i).get("USERNAME"), tableLists.get(i).get("PASSWORD"), tableLists.get(i).get("DRIVER"));
    }
  }
}

它给了我 Null Pointer Exception在 get call 。我在这里缺少什么吗？如果是的话我该如何修复它？因为我需要分配 dbConnection 0 和 1 取决于 tableLists尺寸。

更新的代码:-

进行这样的更改后-

private Connection[] dbConnection = null;


        int j=0;
        for (Map<String, String> map : tableLists.values()) {

            dbConnection[j] = getDBConnection(map.get("URL"), map.get("USER"), map.get("PASSWORD"), map.get("DRIVER"));
            callableStatement[j] = dbConnection[j].prepareCall(map.get("SQL"));

            methods[j] = getRequiredMethods(map.get("SUFFIX"));
            j++;
        }

private Connection getDBConnection(String url, String username, String password, String driver) {

    Connection dbConnection = null;

    try {
        Class.forName(driver);
        dbConnection = DriverManager.getConnection(url, username, password);
    }

    return dbConnection;
}

它再次抛出 NPE，但这一次它在建立连接并返回后立即抛出。我在这里做错了什么？

Answer 1

Map 接口(interface)有一个Key，而不是索引。要获取所有条目，请尝试

int i=0;
dbConnection = new DbConnection[tableLists.size()];
for (Map<String, String> map : tableLists.values()) {
  dbConnection[i] = getDBConnection(
    map.get("URL"), 
    map.get("USER"), 
    map.get("PASSWORD"), 
    map.get("DRIVER"));
  i++;
}