Java button help

[RoastBeefer]

Hello, we are brand new to programming in Java this year. We are following the cookbook from firstforge. We were able to move the robot using a joystick but cannot figure out how to use a button to enable a motor. We created a subsystem called shooter then a command called ShootBall. We get an error in out code and can't figure out why. Our code is in the attachment.

Here is shooter:

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */
package edu.wpi.first.wpilibj.templates.subsystems;

import edu.wpi.first.wpilibj.Jaguar;
import edu.wpi.first.wpilibj.buttons.Button;
import edu.wpi.first.wpilibj.command.Subsystem;
import edu.wpi.first.wpilibj.templates.commands.ShootBall;


/**
 *
 * @author Developer
 */
public class Shooter extends Subsystem {
    Jaguar leftMotor, rightMotor;
    // Put methods for controlling this subsystem
    // here. Call these from Commands.

    public void initDefaultCommand() {
        setDefaultCommand(new ShootBall());
        // Set the default command for a subsystem here.
        //setDefaultCommand(new MySpecialCommand());
    }
    
    public Shooter(){
        leftMotor = new Jaguar(3);
        rightMotor = new Jaguar(4);
        leftMotor.setSafetyEnabled(false);
        rightMotor.setSafetyEnabled(false);
    }

    public void ShootBall(Button a){
        a.whileHeld(new ShootBall());
    }
}

And here is ShootBall:

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */
package edu.wpi.first.wpilibj.templates.commands;


/**
 *
 * @author Developer
 */
public class ShootBall extends CommandBase {
    
    public ShootBall() {
        requires(shooter);
        // Use requires() here to declare subsystem dependencies
        // eg. requires(chassis);
    }

    // Called just before this Command runs the first time
    protected void initialize() {
    }

    // Called repeatedly when this Command is scheduled to run
    protected void execute() {
        shooter.ShootBall(oi.getButton());
        
    }

    // Make this return true when this Command no longer needs to run execute()
    protected boolean isFinished() {
        return false;
    }

    // Called once after isFinished returns true
    protected void end() {
    }

    // Called when another command which requires one or more of the same
    // subsystems is scheduled to run
    protected void interrupted() {
    }
}

Here is OI in case you need it:

package edu.wpi.first.wpilibj.templates;

import edu.wpi.first.wpilibj.Joystick;
import edu.wpi.first.wpilibj.buttons.Button;
import edu.wpi.first.wpilibj.buttons.JoystickButton;

/**
 * This class is the glue that binds the controls on the physical operator
 * interface to the commands and command groups that allow control of the robot.
 */
public class OI {
    public static final int JOYSTICK_PORT = 1;
    private Joystick stick;
    private Button a;
    
    public OI(){
        stick = new Joystick(JOYSTICK_PORT);
        a = new JoystickButton(stick, 1);
    }
    
    public Joystick getJoystick(){
        return stick;
        
    }
    
    public Button getButton(){
        return a;
    } 
    //// CREATING BUTTONS
    // One type of button is a joystick button which is any button on a joystick.
    // You create one by telling it which joystick it's on and which button
    // number it is.
    // Joystick stick = new Joystick(port);
    // Button button = new JoystickButton(stick, buttonNumber);
    
    // Another type of button you can create is a DigitalIOButton, which is
    // a button or switch hooked up to the cypress module. These are useful if
    // you want to build a customized operator interface.
    // Button button = new DigitalIOButton(1);
    
    // There are a few additional built in buttons you can use. Additionally,
    // by subclassing Button you can create custom triggers and bind those to
    // commands the same as any other Button.
    
    //// TRIGGERING COMMANDS WITH BUTTONS
    // Once you have a button, it's trivial to bind it to a button in one of
    // three ways:
    
    // Start the command when the button is pressed and let it run the command
    // until it is finished as determined by it's isFinished method.
    // button.whenPressed(new ExampleCommand());
    
    // Run the command while the button is being held down and interrupt it once
    // the button is released.
    // button.whileHeld(new ExampleCommand());
    
    // Start the command when the button is released  and let it run the command
    // until it is finished as determined by it's isFinished method.
    // button.whenReleased(new ExampleCommand());
}

Sorry if we posted things wrong. We didn't know how to attach code