为什么我们将className作为参数传递，特别是这个。 ArrayList <ClassName>

Question

好吧，所以我看到一个示例，其中有一个名为Song的类，具有两个标题和持续时间，然后他又创建了一个类似ArrayList<Song> songs的ArrayList，但是我的问题是为什么我们需要这样做，这真的让我感到困惑。

package linkedList.practic.task;

public class Song {
    private String title;
    private double duration;

    public Song(String title, double duration) {
        this.title = title;
        this.duration = duration;
    }

    public String getTitle() {
        return this.title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public double getDuration() {
        return duration;
    }

    public void setDuration(double duration) {
        this.duration = duration;
    }

    @Override
    public String toString() {
        return
                "title='" + title + '\'' +
                ", duration='" + duration;
    }
}


package linkedList.practic.task;

import java.util.*;

public class Album {
    String name;
    String artist;
    private ArrayList<Song> songs;

    public Album(String name, String artist) {
        this.name = name;
        this.artist=artist;
        songs = new ArrayList<>();
    }

    public boolean addSong(String title, double duration){
        if(findSong(title) == null){
            this.songs.add(new Song(title, duration));
            return true;
        }
        return false;
    }

    private Song findSong(String name){
        for(Song checkSong : this.songs){
            if(checkSong.getTitle().equals(name)){
                return checkSong;
            }
        }
        return null;
    }


    public boolean addToPlaylist(int trackNumber , LinkedList<Song> playlist){
        int index=trackNumber-1;
        if((index>=0)&&(index<this.songs.size())){
            playlist.add(this.songs.get(index));
                    return true;
        }
        System.out.println("This album doesnot hava a track " + trackNumber);
        return false;
    }

    public boolean addToPlaylist(String title, LinkedList<Song> playlist){
        Song checkSong = findSong(title);
        if(checkSong!=null){
            playlist.add(checkSong);
            return true;
        }
        System.out.println("The song " + title + " is not in this album");
        return false;
    }
    public void printSongs(){
        for(int i = 0; i<songs.size(); i++){

            System.out.println(i+1+"."+ songs.get(i));
        }
    }
}