setContentView如何在Android中工作？

Question

I am trying to display a button on my android app but everytime i run the app it crashes. i realise this is because i use setContentView multiple times? I dont understand how it works, and dont understand how i can fix this problem so my button will display. my code is below.

public class MainActivity extends Activity {
    Draw draw;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        draw = new Draw(this);
        draw.setBackgroundColor(Color.BLUE);
        setContentView(draw);

        LinearLayout l = new LinearLayout(this);
        l.setOrientation(LinearLayout.VERTICAL);
        setContentView(l);
        l.addView(new Draw(this));
        //setContentView(R.layout.activity_main);



        setUpBlockBtn();
    }

    private void setUpBlockBtn(){
        setContentView(R.layout.activity_main);
        Button addBlockButton = (Button)findViewById(R.id.btnBlock);

        addBlockButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Log.i("DemoButtonApp", "you clicked the button");
                //finish();
            }
        });
    }

Answer 1

You try to access Button from android xml layout but you do not set this layout in Activity.

Put you button activity_main.xml and use this button in your activity.

Thanks

Answer 2

You can create one more layout and add Draw and Linear layout to that layout.

Something like this.

LinearLayout l1=new LinearLayout(this);
l1.setOrientation(LinearLayout.VERTICAL);

l1.addView(draw);
l1.addView(l2)   // your linearLayout.

setContentView(l1)

Remember you can't use setContentView more than one time. There should be top layout which includes subview and other layouts and then you can add that layout to your activity.