How to scroll HorizontalScrollView when a button is clicked in android?

I have a horizontal scrollview in my Android application with Next and Previous buttons. I want to show these buttons only when scollview needs scrolling.ie, the scrollview content is wider than the display width. You also need to hide the previous and next buttons when reaching the first and last elements, respectively. How to the next / previous paragraphs when you click on these buttons?

main.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
        android:id="@+id/mainLayout"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:background="#ffffff" >

        <Button
            android:id="@+id/btnPrevoius"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignParentLeft="true"
            android:layout_centerVertical="true"
            android:text="Previous"
            android:visibility="gone" />

        <HorizontalScrollView
            android:id="@+id/horizontalScrollView1"
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:layout_centerVertical="true"
            android:layout_marginLeft="5dip"
            android:layout_marginRight="5dip"
            android:layout_toLeftOf="@+id/btnNext"
            android:layout_toRightOf="@+id/btnPrevoius"
            android:fillViewport="true" >

            <LinearLayout
                android:id="@+id/linearLayout1"
                android:layout_width="fill_parent"
                android:layout_height="wrap_content"
                android:orientation="horizontal" >
            </LinearLayout>
        </HorizontalScrollView>

        <Button
            android:id="@+id/btnNext"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignParentRight="true"
            android:layout_centerVertical="true"
            android:text="Next"
            android:visibility="gone" />

    </RelativeLayout>

activity

 public class SampleActivity extends Activity {
            private static LinearLayout linearLayout;
            private static HorizontalScrollView horizontalScrollView;
            private static Button btnPrevious;
            private static Button btnNext;
            private static int displayWidth = 0;
            private static int arrowWidth = 0;
            @Override
            public void onCreate(Bundle savedInstanceState) {
                super.onCreate(savedInstanceState);
                setContentView(R.layout.main);
                horizontalScrollView = (HorizontalScrollView) findViewById(R.id.horizontalScrollView1);
                linearLayout = (LinearLayout) findViewById(R.id.linearLayout1);
                btnPrevious = (Button) findViewById(R.id.btnPrevoius);
                btnNext = (Button) findViewById(R.id.btnNext);
                for (int i = 0; i < 15; i++) {
                    Button button = new Button(this);
                    button.setTag(i);
                    button.setText("---");
                    linearLayout.addView(button);
                }
                ViewTreeObserver vto = linearLayout.getViewTreeObserver();
                vto.addOnGlobalLayoutListener(new OnGlobalLayoutListener() {

                    @Override
                    public void onGlobalLayout() {
                        ViewTreeObserver obs = linearLayout.getViewTreeObserver();
                        obs.removeGlobalOnLayoutListener(this);
                        Display display = getWindowManager().getDefaultDisplay();
                        displayWidth = display.getWidth();
                        if (linearLayout.getMeasuredWidth() > (displayWidth - 40)) {
                            btnPrevious.setVisibility(View.VISIBLE);
                            btnNext.setVisibility(View.VISIBLE);
                        }
                    }

                });
                btnPrevious.setOnClickListener(listnerLeftArrowButton);
                horizontalScrollView.setOnTouchListener(listenerScrollViewTouch);
            }

            private OnTouchListener listenerScrollViewTouch = new OnTouchListener() {

                @Override
                public boolean onTouch(View v, MotionEvent event) {
                    showHideViews();
                    return false;
                }
            };

            private OnClickListener listnerLeftArrowButton = new OnClickListener() {

                @Override
                public void onClick(View v) {
                    horizontalScrollView.onKeyDown(KeyEvent.KEYCODE_DPAD_RIGHT, new KeyEvent(0, 0));
                }
            };


        public static void showHideViews() {
            int maxScrollX = horizontalScrollView.getChildAt(0).getMeasuredWidth()- displayWidth;
            Log.e("TestProjectActivity", "scroll X = " +horizontalScrollView.getScrollX() );
            Log.i("TestProjectActivity", "scroll Width = " +horizontalScrollView.getMeasuredWidth() );
            Log.d("TestProjectActivity", "Max scroll X = " + maxScrollX);

            if (horizontalScrollView.getScrollX() == 0) {
                hideLeftArrow();
            } else {
                showLeftArrow();
            }
            if (horizontalScrollView.getScrollX() == maxScrollX) {
                showRightArrow();
            } else {
                //hideRightArrow();
            }
        }

        private static void hideLeftArrow() {
            btnPrevious.setVisibility(View.GONE);
        }

        private static void showLeftArrow() {
            btnPrevious.setVisibility(View.VISIBLE);
        }

        private static void hideRightArrow() {
            btnNext.setVisibility(View.GONE);
        }

        private static void showRightArrow() {
            btnNext.setVisibility(View.VISIBLE);
        }
    }

The value 'maxScrollX'is wrong for me. How to find the maximum scroll value for this? Thanks at Advance