Android context空指针异常

Fragment中的代码 context 是null

public class PlaceholderFragment extends Fragment{
	
	Context context = getActivity();
	
}

Activity中的代码  context不是null

public class MainActivity extends FragmentActivity {

	Context context = this;
	
}

以下这样的方式获取context 会抛异常

public class MainActivity extends FragmentActivity {

	Context context = getApplicationContext();;
	
}