Java non static method playCompletely cannot be referenced from a static context

I've been trying to create a method (in a separate class) that takes a String as a parameter and uses a premade SoundEngine (a different class) object to play that file inputted as a string. This is the code I have so far. The problem comes up with

SoundEngine.playCompletely(file);

import java.util.ArrayList;

public class AudioFileList
{
   // Field for a quantity of notes.
   private ArrayList<String> files;

   // Constructor that initializes the array field.
   public AudioFileList()
   {
        files = new ArrayList<String>();
   }

   // Method to add file names to the collection.
   public void addFile(String file)
   {
        files.add(file);
   }

   // Method to return number of files in the collection.
   public int getNumberOfFiles()
   {
        return files.size();
   }

   // Method to print the strings in the collection.
   public void listFiles()
   {
        int index = 0;
        while(index < files.size())
        {
            System.out.println( index + ":" + files.get(index));
            index++;
            }
   }

   // Method that removes an item from the collection.
   public void removeFile(int fileNumber)
   {
        if(fileNumber < 0) 
            { 
                System.out.println ("This is not a valid file number");
            }
        else if(fileNumber < getNumberOfFiles())
            { 
                files.remove(fileNumber);
            }
        else 
            { 
                System.out.println ("This is not a valid file number");
     开发者_如何学C       }
   }    

   // Method that causes the files in the collection to be played.

   public void playFile(String file)
   {
   SoundEngine.playCompletely(file);
   }

}

Any help is very much appreciated.

SoundEngine's playCompletely function is an instance function, not a class function. So instead of:

SoundEngine.playCompletely(file); // Compilation error

you want:

// First, create an instance of SoundEngine
SoundEngine se = new SoundEngine();

// Then use that instance's `playCompletely` function
se.playCompletely(file);