Get input from file and put it in set, Java

Question

In my task I need to store class Car into setOfCars. Reading must be done from file. I have id, nameOfCar, noOfSeats.

Input:

1
Mercedes
GTR 2

In main I have this:

Set<Car> listOfCars= eneterCars();

But intellij cast it to:

 Set<Cars> listOfCars= (Set<Car>) eneterCars();

And this doesnt work.

Method I call is also in main:

  File cars= new File("dat/cars.txt");

try(FileReader fileReader = new FileReader(cars);
    BufferedReader reader = new BufferedReader(fileReader)){
    String readLine;
    List<Car> listOfCars= new ArrayList<>();
    while((readLine= reader.readLine()) != null){
        Long id = Long.parseLong(readLine);
        String name= reader.readLine();
        Integer noOfSeats=Integer.parseInt(reader.readLine());
        listOfCars.add(new Car(id, name, noOfSeats));
        //System.out.println(id + " name" + name+ " " + noOfSeats);
    }

return cars;

I want to store information from file to set so that I could use it later. But I get that cast error when I call this function.

return cars actually returns the file not the list.

Runcorn
– Runcorn

2020-12-01 19:50:06 +00:00
Commented Dec 1, 2020 at 19:50 — Runcorn
– Runcorn, Commented Dec 1, 2020 at 19:50

Arvind Kumar Avinash · Accepted Answer · 2020-12-01 19:57:29Z

2

You have written

return cars;

which is File cars= new File("dat/cars.txt");. You need to return the correct object as

return listOfCars;

Also, instead of declaring List<Car> listOfCars= new ArrayList<>(); in the method, you could have declared it as Set<Car> listOfCars= new LinkedHashSet<>(); if you needed a Set instead of a List.

If you still want to keep the declaration as it is, an idiomatic way to convert it to Set is as follows:

Set<Cars> listOfCars= new LinkedHashSet<>(eneterCars());

Note: LinkedHashSet retains the order to insertion. If the order to insertion does not matter for you, you can use HashSet.

edited Dec 1, 2020 at 19:57

answered Dec 1, 2020 at 19:51

Arvind Kumar Avinash

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2 Comments

sdavb Over a year ago

Thanks it helped !

sdavb Over a year ago

Thanks I did that

Oleg Cherednik · Accepted Answer · 2020-12-01 20:10:45Z

public final class Car {

    private final int id;
    private final String manufacture;
    private final String model;

    public Car(int id, String manufacture, String model) {
        this.id = id;
        this.manufacture = manufacture;
        this.model = model;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (!(obj instanceof Car))
            return false;
        return id == ((Car)obj).id;
    }

    @Override
    public int hashCode() {
        return Objects.hash(id);
    }
}

public static Set<Car> readCarsFromFile(File file) throws IOException {
    try (Scanner scan = new Scanner(file)) {
        Set<Car> cars = new LinkedHashSet<>();

        while (scan.hasNext()) {
            int id = scan.nextInt();
            scan.nextLine();
            String manufacture = scan.nextLine();
            String model = scan.nextLine();
            cars.add(new Car(id, manufacture, model));
        }

        return cars;
    }
}

cars.txt

1
Mercedes
GTR 2
2
BMW
X5

Demo:

Set<Car> cars = readCarsFromFile(new File("dat/cars.txt"));
// 2 cars: Mersedes, BMW

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