如何为每个Java运行从CSV文件解析一行，为下一次运行解析下一行？

Question

I'm a beginner to java and I have a java class that reads in data from a CSV file which looks like this:

BUS|ID|Load|Max_Power

1 | 2 | 1 | 10.9

2 | 3 | 2 | 8.0

My problem is this: I have to consider for each java run (program execution), only 1 row at a time. For example for my first run I need to read in only the first row and then for my second run I need to read in the data from the second row. Would using Hashmaps be the right way to search for the keys for each run?

Answer 1

when you read the 1st line (the header), you split by | got string array ( headerArray ). Then init a hashmap <String, List<String>> (or Multimap if you use guava or other api) with elements in your string array as key.

Then you read each data row, split by | , again you got string array( dataArray ), you just get the map value by: map.get(headerArray[index of dataArray]) . Once you locate the map entry/value, you can do following logic (add to the list).

You can also design a ValueObjectType type with those attributes, and a special setter accepting int index, String value , there you check which attribute the value should go. In this way, you don't need map any longer, you need a List<ValueObjectType>

Answer 2

When your program has to "remember" something beyond it's termination, you need to store this information somewhere (file, registry, ...).

Step 1 Figure out how to do file I/O (read/write files) with java. You need this to store your information, the line number in this case).

Step 2 Implement the logic:

• read lineToRead from memory file (eg 1 )
• read line lineToRead ( 1 ) from data file and parse the data (take a look at @Kent s answer for a nice explanation how to do so)
• increment lineToRead ( 1 -> 2 ) and save it in to the memory file.

Hint: When mulitple instances of your program are going to run in parallel, you have to ensure the mutual exclusion / make the whole process (read, increment, write) atomic to prevent the lost update effect .

Answer 3

You can use com.csvreader.CsvReader class (available in javacsv.jar) This class provides functionality to read CSV file row by row . This will serve your purpose .

here is the sample code :-

    CsvReader csv = new CsvReader(FileName);
      csv.readHeaders(); // headers
        while (products.readRecord()) {

            String cal = csv.get(0);
        }