Simple interface & constructor question

[Ravaa Bal]

Hi all,

Interfaces still get me! I have a class ClassOne...

public class ClassOne

{ 
public ClassOne()
{

try
{
...
Sequencer sequencer = MidiSystem.getSequencer(false);
...
sequencer.setSequence(MidiSystem.getSequence(myMidiFile));
...
}
        catch (...){}
}
}

And then I have another class, ClassTwo.

public class ClassTwo
{
    public static void main(String args[])
    {
       private Sequencer classone = new ClassOne();
            
       ...

I'd like to obtain a Sequencer instance of ClassOne, but does ClassOne need to implement the entire Sequencer interface (i.e. every single method) for this to be made (http://java.sun.com/j2se/1.5.0/docs/api/javax/sound/midi/Sequencer.html)? I hoped that by calling MidiSystem.getSequencer() in ClassOne this would be suffice for ClassTwo's code to work, but I get the error "cannot find symbol" for 'new ClassOne()'.

What ought I be doing to get the Sequencer instance of ClassOne?

Thank you for any insight on the matter!

[Christophe Verré]

but I get the error "cannot find symbol" for 'new ClassOne()'.

The problem is that the compiler does not find ClassOne in your classpath. In which package is ClassOne.java ? Did you import it in ClassTwo ? How did you compile ClassTwo.java ?

[Rob Spoor]

private Sequencer classone = new ClassOne();

ClassOne does not implement Sequencer.

If you want to retrieve the Sequencer through ClassOne, I see two possibilities:
1) give ClassOne a Sequencer field and a getter for it:

public class ClassOne   
{    
    private Sequencer sequencer;

    public ClassOne()   
    {   
        try   
        {   
            ...   
            sequencer = MidiSystem.getSequencer(false);   
            ...   
            sequencer.setSequence(MidiSystem.getSequence(myMidiFile));   
        ...   
        }   
        catch (...){ e.printStackTrace(); }   // NEVER EVER EVER eat exceptions
    }   

    public Sequencer getSequencer()
    {
        return sequencer;
    }
} 

2) Make ClassOne implement Sequencer, and delegate control to the backing Sequencer:

public class ClassOne implements Sequencer
{    
    private Sequencer sequencer;

    public ClassOne()   
    {   
        try   
        {   
            ...   
            sequencer = MidiSystem.getSequencer(false);   
            ...   
            sequencer.setSequence(MidiSystem.getSequence(myMidiFile));   
            ...   
        }   
        catch (...){ e.printStackTrace(); }   // NEVER EVER EVER eat exceptions
    }   

    public int getLoopCount()
    {
        return sequencer.getLoopCount();
    }

    public long getLoopEndPoint()
    {
        return sequencer.getLoopEndPoint();
    }
}

I'd go for solution one, mostly because it's much easier and I see no benefit in making ClassOne a Sequencer itself.

One other thing: if you always use ClassOne like that (i.e. you use it only for retrieving a Sequencer) you may consider making it a utility class with a static method:

public final class ClassOne
{
    // private constructor since instantiating is never needed
    private ClassOne() {}

    public static Sequencer getSequencer()
    {
        try   
        {   
            ...   
            sequencer = MidiSystem.getSequencer(false);   
            ...   
            sequencer.setSequence(MidiSystem.getSequence(myMidiFile));   
            ...   
            return sequencer;
        }   
        catch (...){ e.printStackTrace(); }   // NEVER EVER EVER eat exceptions
        return null;
    }
}

You can then simply use ClassOne.getSequencer() to retrieve the Sequencer object.

[Ravaa Bal]

Thank you both.

Rob I followed your advice and it seemed to do the trick - now there's only a problem with static methods

[Ravaa Bal]

Hey guys, I have an issue with interfaces in general (didn't want to create a new topic for it).

If say I wanted to implement Sequencer after all like so:

public class ClassOne implements Sequencer
        
{

 public int getLoopCount()
    {return getLoopCount();}
    
    public int setLoopCount()
    {return 1;}
...
}

Why do I get an error highlighting the implement line saying something like this?
"ClassOne is not abstract and does not override abstract method setLoopCount(int) in ...etc etc... Sequencer"

Cheers.

[Steve Luke]

Ravaa Bal wrote:Hey guys, I have an issue with interfaces in general (didn't want to create a new topic for it).

If say I wanted to implement Sequencer after all like so:

public class ClassOne implements Sequencer
        
{

 public int getLoopCount()
    {return getLoopCount();}

First, don't do this. Your getLoopCount() will call getLoopCount() which will call getLoopCount() which will... you get the idea. An infinite recursive loop. What do you really want getLoopCount() to return?

    
    public int setLoopCount()
    {return 1;}
...
}

Why do I get an error highlighting the implement line saying something like this?
"ClassOne is not abstract and does not override abstract method setLoopCount(int) in ...etc etc... Sequencer"

Cheers.

Looks like the method defined in the interface is:

public void setLoopCount(int count);

And you are implementing a method like:

public int setLoopCount();

These aren't the same method. What is the difference? How do you fix it?

[Ravaa Bal]

lol oh dear, thanks for pointing that out!