How do you use methods from sub classes in the main class in Java?

Question

I am working on an assignment and I can not figure out what to do. I have three different Java classes. And I am trying to use the methods in one class to do something in a different class. I am making a very primitive playlist program. I have to check to see if the playlist is full, if its not i have to ask the title and artist. Then I have to call my method using the title and artist as parameters. I was wondering if anyone could point me in the right direction as to what I had to do to call the method? I still don't completely understand loops either but i know that I have to use a for loop in order to do this. Thankyou for your time.

Here is my code:

Main Class

import java.util.Scanner;
public class Main {

public static void main(String[] args) {
   PlayList p = new PlayList (5);
   Scanner sc = new Scanner(System.in);
   String command;
   String title;
   String artist;



  System.out.println("Enter a to add, r to remove, d to display,or q to 
  quit:");
  command = sc.nextLine();
  while (!command.equals("q")) {
  // Interpret command
  if (command.equals("a")) {
   //add song
 for (int i = 0; i <= PlayList.isFull(title, artist);i++) {
     if(songs[i])== null {
     songs[i] = filled;
  }


  }

  } else if (command.equals("r")) {
  // Remove a song
  System.out.print("Title: ");
  title = sc.nextLine();
  p.remove(title);
  } else if (command.equals("d")) {
  // Fill this in
  }
  // Get the next command
  System.out.println("Enter a to add, r to remove, d to display, or q to 
  quit:");
  command = sc.nextLine();
   }
  System.out.println("Program Ended");

   }
   }

PlayList Class

public class PlayList {
private Song [] songs;
private int filled;

public PlayList (int size){
    songs = new Song[size];
}
public boolean isFull() {
    return (filled >= songs.length);
}
public void add(String t, String a) {
    for (int i = 0; i < songs.length; i++){
        if (songs[i] == null){
            songs[i] = new Song(t,a);
            filled++;
        }
    }      
}
public void display() {
    for (int i = 0; i < songs.length; i++){
        if (songs[i] != null) {
            System.out.println(songs[i]);
        }
    }
}
public void remove(String t) {
    //return t?
    for (int i = 0; i < songs.length; i--){
        if (songs[i] == null){
            songs[i] = null;
            break;
}
}
}
}

Song Class

public class Song {
String title;
String artist; 

public Song (String t, String a) {
title = t;
artist = a; 
}
public String toString() {
return "Title: " + title + " " + "Artist: " + artist;

}

}

Answer 1

First of all you are using isFull function of class PlayList wrong.

for (int i = 0; i <= PlayList.isFull(title, artist);i++)

1. isFull is a no argument function, and you are using it with passing 2 arguments.

2. isFull function returns a boolean value (ie true/false), but you are comparing it with an int , which does not make any sense.

3. isFull is not a static function. Therefore you cannot use it directly with class name.

-either you will need to declare function isFull as static.

public static boolean isFull()

-or you will need to create an object of class PlayList in class Main and then call the java function using that java object .

Also, your Function remove is not performing any task

 if (songs[i] == null){
        songs[i] = null;
}

It is checking if songs[i] is already null and then it sets it back to null, which does not make any sense.

And you should increment i (ie i++) not decrement it (ie i--)

for (int i = 0; i < songs.length; i--)

Answer 2

If you want to call method from another class that method must be a static method. Then you can call it using Class name and Method name . For an example;

public class main(){
    A a = new A();
    a.x();
}
public class A{
    public static void x(){};
}

You called isFull method with two parameters but your PlayList class does not have any parameter for isFull method. That is an error . I re-write your assignment class set using ArrayList for PlayList class. Follow this codes. Hope you can understand it's concept of OOP(Follow this tutorials. https://www.javatpoint.com/java-oops-concepts ).

Main Class

import java.util.Scanner;
public class Main {

    public static void main(String[] args) {
        PlayList p = new PlayList (5);
        Scanner sc = new Scanner(System.in);
        String command;
        String title;
        String artist;


        System.out.println("Enter a to add, r to remove, d to display,or q to quit:");
        command = sc.nextLine();
        while (!command.equals("q")) {
            // Interpret command
            if (command.equals("a")) {
                //add song
                System.out.println("Enter Title:");
                title = sc.nextLine();
                System.out.println("Enter Artist:");
                artist = sc.nextLine();
                if(!p.isFull()) {
                    p.add(title, artist);
                    System.out.println("Added Success!");
                }
                else
                    System.out.println("Sorry,Playlist is full");

            } else if (command.equals("r")) {
                // Remove a song
                System.out.print("Title: ");
                title = sc.nextLine();
                p.remove(title);
            } else if (command.equals("d")) {
                // Fill this in
                p.display();
            }
            // Get the next command
            System.out.println("Enter a to add, r to remove, d to display, or q to quit:");
            command = sc.nextLine();
        }
        System.out.println("Program Ended");

    }
}

PlayList Class

import java.util.ArrayList;
import java.util.List;

public class PlayList {
    private static List<Song> songs;
    private static int filled;
    private  static int size = 0;

    public PlayList (int s){
        songs = new ArrayList<>();
        size = s;
    }
    public static boolean isFull() {
        return (filled == size);
    }
    public static void add(String t, String a) {
        songs.add(new Song(t,a));
                filled++;
    }
    public void display() {
        for (int i = 0; i < songs.size(); i++){
            if (songs.get(i) != null) {
                System.out.println(songs.get(i));
            }
        }
    }
    public void remove(String t) {
        //return t?
        for (int i = 0; i < songs.size(); i++){
            if (songs.get(i).title == t){
                songs.remove(i);
                break;
            }
        }
    }
    public static int getSize(){
        return songs.size();
    }
}

Song Class is same as you wrote.