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

Issue

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;

}

}

Solution

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 (i.e. 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 (i.e. i++) not decrement it (i.e. i--)

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

Answered By - Sidhant Jain
Answer Checked By - Katrina (JavaFixing Volunteer)