Android: why setVisibility(View.GONE); or setVisibility(View.INVISIBLE); do not work

I see quite a few things wrong. For starters, you don’t have your magic button defined and there is no event handler for it.

Also you shouldn’t use:

dp2.setVisibility(View.GONE);
dp2.setVisibility(View.INVISIBLE); 

Use only one of the two. From Android documentation:

View.GONE This view is invisible, and it doesn’t take any space for
layout purposes.

View.INVISIBLE This view is invisible, but it still
takes up space for layout purposes.

In your example, you are overriding the View.GONE assignment with the View.INVISIBLE one.

---

Try replacing:

final DatePicker dp2 = new DatePicker(this)

with:

DatePicker dp2 = (DatePicker) findViewById(R.id.datePick2);  

Similarly for other widgets:

    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        LinearLayout ll = new LinearLayout(this);
        ll.setOrientation(LinearLayout.VERTICAL);

        final DatePicker dp2 = new DatePicker(this);
        final Button btn2 = new Button(this);
        final Button magicButton = new Button(this);
        final TextView txt2 = new TextView(TestActivity.this);

        dp2.setVisibility(View.GONE);
        btn2.setVisibility(View.GONE);
        btn2.setText("set Date");

        btn2.setOnClickListener(new View.OnClickListener() {
            public void onClick(View arg0) {
                txt2.setText("You selected "
                    + dp2.getDayOfMonth() + "https://stackoverflow.com/" + (dp2.getMonth() + 1) 
                    + "https://stackoverflow.com/" + dp2.getYear());
            }
        });

        magicButton.setText("Magic Button");
        magicButton.setOnClickListener(new View.OnClickListener()    
            public void onClick(View arg0) {
                dp2.setVisibility(View.VISIBLE);
                btn2.setVisibility(View.VISIBLE);
            }
        });

    ll.addView(dp2);
    ll.addView(btn2);
    ll.addView(magicButton);
    ll.addView(txt2);

    setContentView(ll);
}