为特定的布局调用findViewById()？findViewById()上的NullPointerException

Question

我有第二个类，它从主类调用一个方法，这个方法将设置一个带有数字的TextView，但是应用程序会因为findViewById()的nullPointerException而崩溃。我认为问题在于，当从另一个类调用findViewById时，它将在调用类的setContentView()中查找视图。

那么，如何指定findViewById()正在查看的布局呢？为什么会有“nullPointerException”？

*************************************************
                  MAIN CLASS
*************************************************

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    if (!NUMBER.exists()) {
        new File("/data/data/com.app.clicker/files").mkdir();
        writeInt(NUMBER, total = 0);
        writeInt(CLICK, perClick = 1);
        writeInt(CASH, $ = 500);
        setView(R.id.numberView, "", readInt(NUMBER));
        setView(R.id.perClickView, "Per Click ", readInt(CLICK));
        setView(R.id.cash, "Cash ", readInt(CASH));
    } else {
        setView(R.id.perClickView, "Per Click ", readInt(CLICK));
        setView(R.id.cash, "Cash ", readInt(CASH));
        setView(R.id.numberView, "", readInt(NUMBER));
    }
}

   public void setTotal(int total) {
    writeInt(NUMBER, total);
    setView(R.id.numberView, "", this.total);
   }
   public void setView(int view, String writeBefore, int num) {
    TextView textView = (TextView) findViewById(view); >> NullPointerException
    textView.setText(writeBefore + Integer.toString(num));
   }

************************************************
                  Second Class 
************************************************    

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_dev);
    Main main = new Main();
    total = main.getTotal();
    perClick = main.getPerClick();
    $ = main.get$();
    devListView();
}

final EditText editTotal = new EditText(Dev.this);
            final EditText editPerClick = new EditText(Dev.this);
            final EditText edit$ = new EditText(Dev.this);
            editTotal.setInputType(InputType.TYPE_CLASS_NUMBER);
            editPerClick.setInputType(InputType.TYPE_CLASS_NUMBER);
            edit$.setInputType(InputType.TYPE_CLASS_NUMBER);
            switch (position) {
            case 0:
                Builder devTotal = new AlertDialog.Builder(Dev.this);
                devTotal.setTitle("Set Total").setView(editTotal).setNeutralButton("Set", new DialogInterface.OnClickListener() {
                    @Override
                    public void onClick(DialogInterface dialog, int which) {
                        finish();
                        main.setTotal(Integer.parseInt(editTotal.getText().toString())); ************ Call to Main to SetTotal
                    }
                });
                devTotal.create().show();
                break;

Answer 1

从你的代码中：

Main main = new Main();

看起来你是在通过调用new Main()来创建activity (R.layout.activity_main)，你不应该这样做:你用startActivity启动activity，如果你想用Intent extras发送一些值给它们。