HashMap overridden by last value

问题

Statement selectStatement = connection.createStatement();
        ResultSet selectResultSet = selectStatement.executeQuery(query);
        logger.info("sql query " + selectStatement.toString());
        ResultSetMetaData rsmd = selectResultSet.getMetaData();
        int columnsNumber = rsmd.getColumnCount();
        String[] row = new String[columnsNumber];

        int ctr = 0;
        logger.info("Reading from database...");
        while(selectResultSet.next())
        {
            for(int i=0;i<columnsNumber;i++){
                row[i] = selectResultSet.getString(i+1);
            }
            System.out.println();
            dataFromQuery.put(ctr++, row);
        }
        selectResultSet.close();

On reading back from HashMap, it prints only the last row 'n' number of times.

回答1:

What you are missing is - row = new String[columnsNumber]; in while loop, which actually cause over writing array object value and you are adding same object to HashMap.

...
while(selectResultSet.next()){
     row = new String[columnsNumber];
     for(int i=0;i<columnsNumber;i++){
         row[i] = selectResultSet.getString(i+1);
     }
     System.out.println();
     dataFromQuery.put(ctr++, row);
}
...

回答2:

Declare row array inside the while loop, so that each row gets its own copy, currently each row in map references the same row array which is overwritten by the last loop.

来源：https://stackoverflow.com/questions/19047809/hashmap-overridden-by-last-value