Java method to read text file and return ArrayList type object

    public static void main(String[] args)
{
ArrayList <Locations>       LocationsList       = readFile("Locations.csv", "Locations");
//ArrayList <Movies>          MoviesList          = readFile("Movies.csv", "Movies");
//ArrayList <Operators>       OperatorsList       = readFile("Operators.csv", "Operators");
//ArrayList <PersonCategory>  PersonCategoryList  = readFile("PersonCategory.csv", "PersonCategory");
}
public static ArrayList readFile(String fileName, String whichFile)
{
ArrayList list = new ArrayList();
try
{
BufferedReader br = new BufferedReader(new FileReader(fileName));
String indata;
int line = 0;
while((indata=br.readLine())!=null)
{
StringTokenizer st = new StringTokenizer(indata,",");
if(line != 0)
{
if(whichFile.equals("Locations"))
{
int id = Integer.parseInt(st.nextToken());
String city = st.nextToken();
if(city.charAt(0) == '"')
{
String c = st.nextToken();
city = city.substring(1,city.length()) +"," +c.substring(0,c.length()-1);
}
int stateId = Integer.parseInt(st.nextToken());
Locations x = new Locations(id, city, stateId);
list.add(x);
}
else if(whichFile.equals("Movies"))
{
int id = Integer.parseInt(st.nextToken());
String name = st.nextToken();
int ratingId = Integer.parseInt(st.nextToken());
Movies x = new Movies(id, name, ratingId);
list.add(x);
}                              
}
line++;
}
br.close();
}
catch (FileNotFoundException fnfe){System.out.println(fnfe.getMessage());}
catch (IOException io){System.out.println(io.getMessage());}
catch (Exception e){System.out.println(e.getMessage());}
return list;
}

I'm trying to create a method that will read a text file and can return an ArrayList type object for the usage of multiple Class. With my code above, it can run successfully.

But, there are lines of warning like:
"The expression of type ArrayList needs unchecked conversion to conform to ArrayList<Locations>"

How do I fix this?

Try this.

public static <T> ArrayList<T> readFile(String fileName, Function<String[], T> converter) throws IOException {
ArrayList<T> result = new ArrayList<>();
try (BufferedReader reader = Files.newBufferedReader(Paths.get(fileName))) {
String line = reader.readLine();
String[] fields = line.split(",");
T object = converter.apply(fields);
result.add(object);
}
return result;
}

and define the converters which convert a CSV line to an object.

static Locations convertLocations(String[] fields) {
int id = Integer.parseInt(fields[0]);
String city = fields[1];
if (city.charAt(0) == '"') {
String c = fields[2];
city = city.substring(1, city.length()) + "," + c.substring(0, c.length() - 1);
}
int stateId = Integer.parseInt(fields[3]);
Locations x = new Locations(id, city, stateId);
return x;
}
static Movies convertMovies(String[] fields) {
/* Make Movies object from fields */
}

and combine them.

ArrayList<Locations> LocationsList = readFile("Locations.csv", fields -> convertLocations(fields));
ArrayList<Movies> MoviesList = readFile("Movies.csv", fields -> convertMovies(fields));

You need to create the proper generics-based ArrayList using for example: new ArrayList<Location>()

You could solve this by passing a class to readFile like this:

public static <T> ArrayList<T> readFile(....., Class<T> clazz)
{
ArrayList<T> list = new ArrayList<T>();
...
}

Essentially, you need to specify the type parameter for generic class ArrayList.

Since you are adding objects created from different classes to the same list, you could create an interface, say, MyInterface

public interface MyInterface {
    ....
}

All classes you return from readFile must implement this interface. For eg.

public class Movies implements MyInterface {
    ....
}

Now, you can add type parameter MyInterface at appropriate places:

public static void main(String[] args) {
        ArrayList<MyInterface> LocationsList = readFile("Locations.csv", "Locations");
        ....
    }

public static ArrayList<MyInterface> readFile(String fileName, String whichFile) {
 
  ArrayList<MyInterface> list = new ArrayList<>();
            ....
        }

Added below info based on reply

You may in fact choose to leave the interface blank, but then you will have to explicitly cast objects to concrete classes to do anything useful.

1. You could cast each object when needed

	    MyInterface myInterfaceObject = locationsList.get(0)
Locations locations = Locations.class.cast(myInterfaceObject);

OR

	    MyInterface myInterfaceObject = locationsList.get(0)
Locations locations = (Locations) myInterfaceObject;

2. OR You could write a list converter function for each concrete type

public class ListConverter {
public ArrayList<Locations> toLocationsArraylist(ArrayList<MyInterface> inList) {
ArrayList<Locations> outList = new ArrayList<>();
for (MyInterface listItem : inList) {
outList.add((Locations) listItem);
}
return outList;
}
}

and then

public static void main(String[] args) {
ArrayList<MyInterface> myInterfaceList = readFile("Locations.csv", "Locations");
ArrayList<Locations> locationList = ListConverter.toLocationsArraylist(myInterfaceList);
}

If you do consider using this solution, then consider renaming MyInterface more appropriately, say, to CsvRecord, or anything domain-specific.

This is my final code that I took from @saka1029 and made some adjustments so that it will read every line in the file except the first one.

    public static <T> ArrayList<T> readFile(String fileName, Function<String[], T> converter)
{
ArrayList <T> list = new ArrayList<>();
try
{
BufferedReader br = new BufferedReader(new FileReader(fileName));
br.readLine();
String inData;
while((inData=br.readLine()) != null)
{
String[] fields = inData.split(",");
T object = converter.apply(fields);
list.add(object);
}
br.close();
}
catch (FileNotFoundException fnfe){System.out.println(fnfe.getMessage());}
catch (IOException io){System.out.println(io.getMessage());}
catch (Exception e){System.out.println(e.getMessage());}
return list;
}

And this is my version of the correction of the method convertLocations from @saka1029 answer.

    static Locations convertLocations(String[] fields)
{
int id = Integer.parseInt(fields[0]);
String city = fields[1];
int stateId;
if (city.charAt(0) == '"')
{
String c = fields[2];
city = city.substring(1, city.length()) + "," + c.substring(0, c.length() - 1);
stateId = Integer.parseInt(fields[3]);
}
else
stateId = Integer.parseInt(fields[2]);
Locations x = new Locations(id, city, stateId);
return x;
}