R cannot be resolved to a variable怎么解决

Question

/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    mMsgRev = (TextView)findViewById(R.id.receive_msg);
    mMsgEdit = (EditText)findViewById(R.id.edit_msg);
    mMsgSendBtn = (Button)findViewById(R.id.send_msg);
            以上R都出现了R cannot be resolved to a variable的问题，可是R.java文件还可以自动生成，求大神指教

Answer 1

R是一个类型还是一个对象，如果是类型，那么id是静态成员么？
另外，R所在的命名空间导入了么？