求问android TextView控件 在onCreate 函数中操作

Question

//MyActivity.javapublic class MainActivity extends ActionBarActivity { protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); TextView tv = (TextView) findViewById(R.id.tv); //tv.setText("hello world!"); tv.setTextSize(20); if (savedInstanceState == null) { getSupportFragmentManager().beginTransaction() .add(R.id.container, new PlaceholderFragment()).commit(); } }}//fragment_main.xml
对TextView进行任何操作都会导致崩溃 求解答!

Answer 1

findViewById找的是当前activity(即setContentView(R.layout.activity_main);)而你的TextView tv = (TextView) findViewById(R.id.tv); 是定义在fragment_main.xml上你执行后 tv是null,所以调用什么都是NUllPointException