You need to pass the context to the CounteDown class
new CountDown(MyActivityName.this,otherparams);
Now use the context passed to display the toast
Context context;
public CountDown(Context context,long millisInFuture, long countDownInterval) {
super(millisInFuture, countDownInterval);
this.context = context;
}
Then in onFinish()
Toast.makeText(context," Message", Toast.LENGTH_SHORT).show();