一个android 用java设置linearlayout的问题 求人解答

之前看别人设置 把onCreate方法重写了
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
LayoutInflater layoutInflater = (LayoutInflater) getSystemService(Activity.LAYOUT_INFLATER_SERVICE);
LinearLayout ll = (LinearLayout) layoutInflater.inflate(R.layout.activity_main, null);

ll.setBackgroundColor(color.black);
setContentView(R.layout.activity_main);
}
想把背景颜色设为黑色，但还是白色没变的
希望好心的大神解答一下 谢谢啦~~~

 protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
LayoutInflater layoutInflater = (LayoutInflater) getSystemService(Activity.LAYOUT_INFLATER_SERVICE);
LinearLayout ll = (LinearLayout) layoutInflater.inflate(R.layout.activity_main, null);

ll.setBackgroundColor(color.black);
setContentView(R.layout.activity_main);//这里初始化的ll不是你上面设置的ll，里面会重新根据xml的内容生成布局的。
}
改成
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
LinearLayout ll = (LinearLayout) findViewById(R.layout.activity_main);
ll.setBackgroundColor(color.black);
}

protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
LayoutInflater layoutInflater = (LayoutInflater) getSystemService(Activity.LAYOUT_INFLATER_SERVICE);
LinearLayout ll = (LinearLayout) layoutInflater.inflate(R.layout.activity_main, null);

ll.setBackgroundColor(color.black);
setContentView(R.layout.activity_main);//这里初始化的ll不是你上面设置的ll，里面会重新根据xml的内容生成布局的。
}
改成
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
LinearLayout ll = (LinearLayout) findViewById(R.layout.activity_main);
ll.setBackgroundColor(color.black);
}