How to spawn tubes in random heights like flappy bird? [duplicate]

this code work very well but without random heights, i have 2 tubes roof tube and floor tube i am tried to change random heights of tubes but top of roof tube and botom of floor tube genrating in center of screen

import android.graphics.Point;
import android.os.Bundle;
import android.os.Handler;
import android.view.Display;
import android.view.WindowManager;
import android.widget.ImageView;
import android.widget.LinearLayout;
import java.util.Timer;
import java.util.TimerTask;

public class MainActivity extends AppCompatActivity {


    ImageView tube1,tube2;

    private int screenW,screenH;

    //tube1 x,y
    private float tube1X;
    private float tube1Y;

    //tube2 x,y
    private float tube2X;
    private float tube2Y;

    private Handler handler = new Handler();
    private Timer timer = new Timer();

    int w = 1200;
    int h = 1000;
    LinearLayout.LayoutParams layout = new LinearLayout.LayoutParams(w,h);




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



        tube1 = (ImageView)findViewById(R.id.tube1);

        tube2 = (ImageView)findViewById(R.id.tube2);






        //get Screen Size:

        WindowManager wn = getWindowManager();
        Display disply = wn.getDefaultDisplay();
        Point size = new Point();
        disply.getSize(size);

        screenW = size.x;
        screenH = size.y;

        //move object out of screen

        tube1.setX(-80.0f);
        tube1.setY(-80.0f);

        tube2.setX(-80.0f);
        tube2.setY(-80.0f);





        //stary the timer

        timer.schedule(new TimerTask() {
            @Override
            public void run() {
                handler.post(new Runnable() {
                    @Override
                    public void run() {
                        changepostub1();
                        changepostub2();


                    }
                });
            }
        },0,20);

    }// end onCreate

    public void changepostub1(){

        //tube1

        tube1X -=10;
        if (tube1.getX()+ tube1.getHeight()<0){

            tube1Y = (int)Math.floor(Math.random()*(screenW- tube1.getWidth()));

            //tube1.setLayoutParams(layout);

            tube1.setRotation(-360);

            tube1Y = -10;

            tube1X = screenH +100.0f;


        }//end if tube1



        tube1.setX(tube1X);
        tube1.setY(tube1Y);





    } //end changepostub1 function

    public void changepostub2(){

        //tube2

        tube2X -=10;
        if (tube2.getX()+ tube2.getHeight()< 0){

            tube2Y = (int)Math.floor(Math.random()*screenW- tube2.getWidth());

            //tube2.setLayoutParams(layout);



            tube2Y = +1200;

            tube2X = screenH +100.0f;


        }//end if tube2



        tube2.setX(tube2X);
        tube2.setY(tube2Y);





    } //end changepostub2 function




    }//end class MainActivity

I want hide the top of roof tube out of screen and hide the botom of floor tube out of screen