Sorting a text file in Java per line

Question

I'm a newbie in Java and I have a file where every first line is the title of every second. For example the text file looks like this:

John Holm

blue

Anna Karina

orange

etc...

where every first line is the name of a person and every second is their favourite colour. Now I would like to use the information to for example find what is for example John Holm's favourite colour or change his favourite colour to green instead. How do I best access the data?

I tried with scanner to see all the lines where blue reoccurs, but I cannot get the code to write the previous line.

Should I split the text file into a table with name and favColour as columns or is there a way to assign the previous line to the next line as a name in Java? I also thought of splitting the file every second line. However, I am unsure which solution would be most efficient. Would be very thankful for some insights!

Answer 1

I think a solution would be to create an object containing the properties of your characters (their name and their favourite colour). So, you can create a class named like how you want to name your object (let's call it Person), and set the name and the favourite colour as attribute of your Person object:

public class Person {

private String name;
private String favColour;

    public Person(String name, String favColour) { // Constructor of your object
        this.name = name; // Setting name
        this.favColour = favColour; // Setting favourite colour
    }

}

Now, create a main method to read your file, store the values into two lists, one containing the names, the other the colours, and then creating as many Person objects as you need:

public class Main {

public static void main(String[] args) throws IOException {
    
        File file = new File("src/wherever you stored it"); // Enter the file path as a String here
        Scanner scanner = new Scanner(file); // Create a scanner that will read your file
        List<String> names = new ArrayList<>(); // List that will contain the names read from the scanner 
        List<String> colours = new ArrayList<>(); // By the way don't forget to import *java.util.List* and *java.util.ArrayList* if your IDE doesn't do it automatically
        int index = 0; // The index that'll know whether you store a name or a colour
        try {
            while(scanner.hasNext()) { // While the scanner still reads something from the file
                if(index%2==0) { // If the index is even
                    names.add(scanner.next()); // Add the line to the names list
                    index++;
                } else {
                    colours.add(scanner.next()); // Else add it to the colours list
                    index++;
                }
            }
        } catch (Exception e) { // Don't forget to import *java.io.IOException*
            System.err.println("Error: " + e.getMessage()); // You can write a custom message that will appear in your console if there's an error, e.g. if the program can't find your file
        }
        scanner.close(); // Close your scanner
        // Now you can create a new Person object
        Person p1 = new Person(names.get(0), colours.get(0)); // Create a Person object called p1, with the first element (index 0) of names and colours lists as attributes
        System.out.println(p1); // Print your Person
    }

}

Almost done, the last line must print some weird string. If you want to print your Person properly, you have to override the toString() method of your object:

// In the Person class:
@Override
public String toString() {
    return "Name: " + this.name + "\nFavourite colour: " + this.favColour;
}

Now try again and you'll see the console prints properly what you want to see. In the case of the example you gave, your console should print:

Name: John Holm

Favourite colour: blue

You can create as many Person objects as you want, and maybe store them into a list as well.

To make this simpler, you can create a function in your Person class:

public static Person newPerson(int i, List<String> names, List<String> colours) {
        return new Person(names.get(i), colours.get(i));
    }

And you can call it in your main:

Person p2 = Person.newPerson(1, names, colours);

You can now create a method that will create new Person objects from this function, and store them into a list or whatever you want.

To set a value, you'll have to create setters in your Person class, in order to modify the private characteristics of your Person from outside the Person class:

public void setName(String name) {
    this.name = name;
}

You can do the same with the colour:

public void setFavouriteColour(String colour) {
    this.favColour = colour;
}

And now, you can call these methods from your main:

p1.setName("John Doe");
p1.setFavouriteColour("green");

You can also create a method to combine both setters:

public void setPerson(String name, String colour) {
    this.setName(name);
    this.setFavouriteColour(colour);
}

By the way if you need these values at some point, you can also create a getter along with your setter:

public String getName() {
    return this.name;
}

Feel free to ask anything you didn't understand or think is wrong :)