Android : Get selected checkbox in listview

I'm trying to get the selected checkbox in a listview. I've been searching in the web, but I always found examples using the "Holder", but I'm not using it. I asked my teacher too, but also didn't know why isn't this working.

    public class MyActivity extends Activity {

        String s;

        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_my);

            ListView list=(ListView)findViewById(R.id.listView);
            Button checkBtn=(Button)findViewById(R.id.button);
            final ArrayList<SportItem> sportArray=new ArrayList<SportItem>();

            SportItem sp;

            sp=new SportItem(getResources().getDrawable(R.drawable.baloncesto),"Basketball",false);
            sportArray.add(sp);
            sp=new SportItem(getResources().getDrawable(R.drawable.futbol),"Football",false);
            sportArray.add(sp);
            sp=new SportItem(getResources().getDrawable(R.drawable.judo),"Judo",false);
            sportArray.add(sp);
            sp=new SportItem(getResources().getDrawable(R.drawable.atletismo),"Athletism",false);
            sportArray.add(sp);
            sp=new SportItem(getResources().getDrawable(R.drawable.tenis),"Tennis",false);
            sportArray.add(sp);

            SportAdapter spadapter=new SportAdapter(this, sportArray);

            list.setAdapter(spadapter);

            checkBtn.setOnClickListener(new View.OnClickListener() {

                @Override
                public void onClick(View view) {
                    s="";
                    for (SportItem ar:sportArray){

                        if(ar.isCheck()){
                            s+=ar.getName()+", ";
                        }
                    }

                    Toast.makeText(getApplicationContext(), s,
                            Toast.LENGTH_SHORT).show();        
                }
            });
        }

    }

    public class SportAdapter extends BaseAdapter {

        protected Activity activity;
        protected ArrayList<SportItem> items;
        CheckBox chk;
        SportItem sp;

        public SportAdapter(Activity activity, ArrayList<SportItem> items) {
            this.activity = activity;
            this.items = items;
        }

        @Override
        public int getCount(){
            return items.size();
        }

        @Override
        public Object getItem(int i) {
            return items.get(i);
        }

        @Override
        public long getItemId(int i) {
            return items.get(i).getId();
        }

        @Override
        public View getView(int i, View view, ViewGroup viewGroup) {
            View v=view;

            if(view==null){
                LayoutInflater linf=(LayoutInflater)activity.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
                v=linf.inflate(R.layout.item_lista,null);
            }

            sp=items.get(i);
            ImageView img=(ImageView)v.findViewById(R.id.imageView);
            img.setImageDrawable(sp.getImage());

            TextView name=(TextView)v.findViewById(R.id.textView);
            name.setText(sp.getName());

            chk=(CheckBox)v.findViewById(R.id.checkBox);
            chk.setChecked(sp.isCheck());

            chk.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() {
                @Override
                public void onCheckedChanged(CompoundButton compoundButton, boolean b) {
                    if(!chk.isChecked()) {
                        sp.setCheck(true);
                        chk.setChecked(sp.isCheck());
                    }
                    else {
                        sp.setCheck(false);
                        chk.setChecked(sp.isCheck());
                    }
                }
            });

            return v;
        }

    }


public class SportItem {

    protected Drawable image;
    protected String name;
    protected boolean check;
    protected long id;

    public SportItem(Drawable image, String name, boolean check) {
        super();
        this.image = image;
        this.name = name;
        this.check = check;
    }

    public SportItem(Drawable image, String name, boolean check, long id) {
        super();
        this.image = image;
        this.name = name;
        this.check = check;
        this.id = id;
    }

    public Drawable getImage() {
        return image;
    }

    public void setImage(Drawable image) {
        this.image = image;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public boolean isCheck() {
        return check;
    }

    public void setCheck(boolean check) {
        this.check = check;
    }

    public long getId() {
        return id;
    }

    public void setId(long id) {
        this.id = id;
    }
}