java - Android 错误尝试在空对象引用上调用虚拟方法

Question

这个问题已经有答案了:

I would like to set my variables at the top of my class instead of in the method (4 个回答)

已关闭 7 年前。

我是 Android Studio 的新手(并且讨厌从 Eclipse 的过渡体验)。我正在尝试编写一个当前有两个 Activity 的应用程序。我已经编写了主要逻辑的 Java 代码，并且运行良好，但尚未将其放入第二个 Activity 中。当我尝试编写并运行该应用程序时，它给了我一个错误。

有一个 LoginActivity 和 MainActivity。目前，LoginActivity 只有一个虚拟文本和一个将用户带到 MainActivity 的按钮。目前 MainActivity 中只有一个 TextView。当我单击它时，应用程序崩溃并给出以下错误。我可以看到它与启动 MainActivity 有关，但我似乎无法修复它。

09-10 20:16:43.012  21126-21126/com.projects.example.myapp E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.projects.example.myapp, PID: 21126
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.projects.example.myapp/com.projects.example.myapp.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object reference
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2267)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2418)
            at android.app.ActivityThread.access$900(ActivityThread.java:154)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1321)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5289)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:904)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:699)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object reference
            at android.app.Activity.findViewById(Activity.java:2072)
            at com.projects.example.myapp.MainActivity.<init>(MainActivity.java:22)
            at java.lang.reflect.Constructor.newInstance(Native Method)
            at java.lang.Class.newInstance(Class.java:1606)
            at android.app.Instrumentation.newActivity(Instrumentation.java:1066)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2257)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2418)
            at android.app.ActivityThread.access$900(ActivityThread.java:154)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1321)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5289)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:904)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:699)

我的 xml 代码:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
    android:layout_height="match_parent" android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin" tools:context=".LoginActivity"
    android:orientation="vertical" >

    <TextView
        android:text="@string/hello_world"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />

    <Button
        android:id="@+id/loginbtn"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Login"
        android:onClick="goToMainActivity" />

</LinearLayout>

LoginActivity相关代码:

public void goToMainActivity(View view)
    {
        Intent intent = new Intent(LoginActivity.this, MainActivity.class);
        startActivity(intent);
    }

请求的MainActivity代码(第22行由前缀**表示):

public class MainActivity extends AppCompatActivity {

    String currentWinnerName = "";

    **TextView currentWinnerLabel = (TextView)findViewById(R.id.currentWinnerLabel);**

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        currentWinnerLabel.setText("Finding...");
        //try
        //{
            //checkCurrentWinner(); //will implement this

        //}
        //catch (Exception e)
        //{
            //print e.getMessage() to log
        //}

    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }

Answer 1

原因是，您试图在创建 Activity 之前为 currentWinnerLabel 赋值。

除非创建了 Activity，否则上下文为 null，并且您将在 null 对象引用上调用 findViewById 方法。

只需更改您的代码:

TextView currentWinnerLabel;

及之后:

protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        currentWinnerLabel = (TextView)findViewById(R.id.currentWinnerLabel);

...