在主程序中调用方法

Question

I am trying to access some information that my program isnt letting me. in my method "addTrack" it doesnt recognise the myTracklist object. how do i call myTracklist.count by using a method in the main class?

public class CD {
    String art;
    String tit;
    public CD(String artist, String title){
        art = artist;
        tit = title;
        tracklist myTracklist = new tracklist(100);
    }

    public static void main(String[] args) {
        String mainArtist;
        String mainTitle;
        CD myCD = new CD("Awesomeguy", "AwesomeCDName");
        mainArtist = myCD.getArtist();
        System.out.println(mainArtist);
        mainTitle = myCD.getTitle();
        System.out.println(mainTitle);
        myCD.display();
    }

    public String getArtist(){
        String person;
        person = art;
        return person;
    }

    public String getTitle(){
        String name;
        name = tit;
        return name;
    }

    public boolean addTrack(String trackinfo){
        boolean result = false;
        if (myTracklist.count < 100){
            myTracklist.add(trackinfo);
            result = true;
        }
        return result;
    }

    public int numTracks(){
        int amount;
        amount = myTracklist.count();
        return amount;
    }

    public void display(){
        System.out.println("Artist = "+ art);
        System.out.println("Title = "+ tit);
        tracklist.display();
    }
}

here is my tracklist class

public class tracklist {
    int length;
    int numUsed;
    String[] storage;

    public tracklist(int size){
    length = size;
    numUsed = 0;
    storage = new String[length];
    }

    public int count(){
        return numUsed;
    }
}

Answer 1

You've got scope problems in that you're declaring tracklist inside the CD constructor so that it only exists inside of the constructor and nowhere else. You must make it a field that is declared in the class for it to be usable at all the methods of the class.

So instead of

public class CD {
    String art;
    String tit;
    public CD(String artist, String title){
        art = artist;
        tit = title;
        tracklist myTracklist = new tracklist(100);
    }

do

public class CD {
    private String art;
    private String tit;
    private tracklist myTracklist; // declared

    public CD(String artist, String title){
        art = artist;
        tit = title;
        myTracklist = new tracklist(100); // initialized
    }

    // getter and setter methods of course.

It's a subtle but important distinction.

As an aside: you'll want to learn Java naming conventions so that others can more readily understand your code and your questions. Class names begin with an upper case letter.

As a second aside: don't have outside classes directly manipulate class fields. Use private fields and public getters and setters to allow the class to have more control over just what can be seen and what can be done.

Answer 2

Keep the declaration of myTrackList outside the constructor . Like this:-

String tit;
tracklist myTracklist;

public CD(String artist, String title){
    art = artist;
    tit = title;
    myTracklist = new tracklist(100);
}

Answer 3

it should be myTracklist.count() here:

if (myTracklist.count < 100){

Also:

String tit; // oh I love this object name!
tracklist myTracklist;
public CD(String artist, String title){
    art = artist;
    tit = title;
    myTracklist = new tracklist(100);
}

Answer 4

Since you have declared it in your customer the "myTracklist" variable will only be visible in your constructor. Hence "addTrack" doesnt recognise the myTracklist object.

Declare it globally,

public class CD {
    String art;
    String tit;
    tracklist myTracklist;

And initialize in the constructor as below.

myTracklist = new tracklist(100);