为什么在课程开始时使用 findViewById(R.id.******) 初始化变量时，我的 Android 应用程序会因 NullPointerException 而崩溃? [英] Why does my Android app crash with a NullPointerException when initializing a variable with findViewById(R.id.******) at the beginning of the class?

问题描述

这段代码，注释了顶部的块，运行成功:

This code, with the block at the top commented, runs successfully:

public class MainActivity extends AppCompatActivity {

    /*
    EditText username = (EditText)findViewById(R.id.editText_Username);
    EditText password = (EditText)findViewById(R.id.editText_Password);
    TextView inputdata = (TextView)findViewById(R.id.textView_InputData);
    TextView welcome = (TextView)findViewById(R.id.textView_Welcome);
    Button login=(Button)findViewById(R.id.button_Login);
    Button anotherLogin=(Button)findViewById(R.id.button_Login_Another);
    */

    public void doLoginOnClick(View v)
    {
        String s1=username.getText().toString();
        inputdata.setText(s1);
    }

    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        FloatingActionButton fab = (FloatingActionButton)findViewById(R.id.fab); 
        fab.setOnClickListener(new View.OnClickListener()
        {
            @Override
            public void onClick(View view)
            {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });
    } 
 
}

---

我正在尝试使用

---

I am trying to capture id of component using

findViewById(R.id.***)

如您所见，我将这段代码放在代码开头的注释中.

as you can see, I put this code in comment at the very beginning of the code.

EditText username = (EditText)findViewById(R.id.editText_Username);
EditText password = (EditText)findViewById(R.id.editText_Password);
TextView inputdata = (TextView)findViewById(R.id.textView_InputData);
TextView welcome = (TextView)findViewById(R.id.textView_Welcome);
Button login=(Button)findViewById(R.id.button_Login);
Button anotherLogin=(Button)findViewById(R.id.button_Login_Another);

如果我删除上面的注释并运行它(在模拟器和真实设备中)，程序会立即崩溃，我很惊讶这里出了什么问题?

If I remove the comment above and run it (both in emulator and real device), the program crashes immediately, I am surprised what's wrong here?

即使我尝试使用构造函数初始化相同的东西.

even I have tried to initialize the same thing using constructor.

但是如果我把它放在 onCreate() 里面就不会崩溃吗?为什么?

But if I put it inside onCreate() there's no crash? Why?

我试图获取用户名和密码的信息并将其显示到 textView_Inputdata 中的 textview 使用

I was trying to fetch info of username and password and display it to the textview in textView_Inputdata using

EditText username = (EditText)findViewById(R.id.editText_Username);
EditText password = (EditText)findViewById(R.id.editText_Password);
TextView inputdata = (TextView)findViewById(R.id.textView_InputData);
inputdata.setText(username.getText.toString()+" "+password.getText.toString());

有没有更好或更简单的方法来做到这一点?

Is there better or easier way to do that?

推荐答案

实例成员变量在实例本身初始化时初始化.findViewById()

Instance member variables are initialized when the instance itself is initialized. It's too early for findViewById()

在 onCreate() 之前，您的 Activity 还没有 findViewById() 内部需要的 Window.在 setContentView()(您应该在 onCreate() 中调用)之前，也找不到视图.

Before onCreate() your activity does not yet have a Window that findViewById() needs internally. Before setContentView() (that you should be calling in onCreate()) there are no views to be found either.

因此在 onCreate() 和 setContentView() 之后初始化您的视图引用.

Therefore init your view references in onCreate() and after setContentView().

这篇关于为什么在课程开始时使用 findViewById(R.id.******) 初始化变量时，我的 Android 应用程序会因 NullPointerException 而崩溃?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！