我看到你在这里做的错误

I see the error u r doing here

LinearLayout mainLayout = (LinearLayout) findViewById(R.layout.main);

您将布局作为 Linearlayout 对象，您应该将 LinearLayout id 设为

You r taking the layout as Linearlayout object, you should take the LinearLayout id

试试这个

LinearLayout lnr = (LinearLayout) findViewById(R.id.LinearLayout01);

Button b1 = new Button(this);

b1.setText("Btn");

lnr.addView(b1);