java - OnClickListener 在 startActivity 上抛出 NullPointerException

Question

我正在将 Android API 17 与 Android 开发工具 (ADT) 一起使用，我有一个正在监听点击事件的帮助按钮。 HelpButtonListener 正在监听 Help 按钮的点击事件，如下所示:

摘自MainActivity.java

private final class HelpButtonListener implements OnClickListener {
    public void onClick(View v) {           
        Intent mainToHelpIntent = new Intent(MainActivity.this, HelpActivity.class);
        startActivity(mainToHelpIntent);        
    }
}

以下HelpActivity代码工作得很好，并向我展示了一些有用的东西..关于HelpActivity创建:

public class HelpActivity extends RoboActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        TextView helpTextView = new TextView(HelpActivity.this);
        helpTextView.setText("Some helpful stuff..");
        setContentView(helpTextView);
    }
}

但是，如果我尝试在使用 RoboGuice 2 注入(inject)的 TextView View 组件中设置文本，以使用以下代码注入(inject) TextView，它会给我一个 空指针异常:

HelpActivity.java

public class HelpActivity extends RoboActivity {
    @InjectView(R.id.helpTextView)
    private TextView helpTextView;

    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        helpTextView.setText("Some helpful stuff..");
        setContentView(R.layout.activity_help);
    }
}

以下是当我单击“帮助”按钮并尝试在 activity_help.xml 布局资源上创建的 TextView 中设置文本时出现的错误:

05-16 18:00:52.591: E/AndroidRuntime(9880): FATAL EXCEPTION: main
05-16 18:00:52.591: E/AndroidRuntime(9880): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.wickedlynotsmart.myfirstapp/com.wickedlynotsmart.myfirstapp.activity.HelpActivity}: java.lang.NullPointerException
05-16 18:00:52.591: E/AndroidRuntime(9880):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
05-16 18:00:52.591: E/AndroidRuntime(9880):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
05-16 18:00:52.591: E/AndroidRuntime(9880):     at android.app.ActivityThread.access$600(ActivityThread.java:141)
05-16 18:00:52.591: E/AndroidRuntime(9880):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
05-16 18:00:52.591: E/AndroidRuntime(9880):     at android.os.Handler.dispatchMessage(Handler.java:99)
05-16 18:00:52.591: E/AndroidRuntime(9880):     at android.os.Looper.loop(Looper.java:137)
05-16 18:00:52.591: E/AndroidRuntime(9880):     at android.app.ActivityThread.main(ActivityThread.java:5041)
05-16 18:00:52.591: E/AndroidRuntime(9880):     at java.lang.reflect.Method.invokeNative(Native Method)
05-16 18:00:52.591: E/AndroidRuntime(9880):     at java.lang.reflect.Method.invoke(Method.java:511)
05-16 18:00:52.591: E/AndroidRuntime(9880):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
05-16 18:00:52.591: E/AndroidRuntime(9880):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
05-16 18:00:52.591: E/AndroidRuntime(9880):     at dalvik.system.NativeStart.main(Native Method)
05-16 18:00:52.591: E/AndroidRuntime(9880): Caused by: java.lang.NullPointerException
05-16 18:00:52.591: E/AndroidRuntime(9880):     at com.wickedlynotsmart.myfirstapp.activity.HelpActivity.onCreate(HelpActivity.java:22)
05-16 18:00:52.591: E/AndroidRuntime(9880):     at android.app.Activity.performCreate(Activity.java:5104)
05-16 18:00:52.591: E/AndroidRuntime(9880):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
05-16 18:00:52.591: E/AndroidRuntime(9880):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
05-16 18:00:52.591: E/AndroidRuntime(9880):     ... 11 more

有人可以帮助我理解为什么会出现此错误吗？

谢谢。

Answer 1

首先设置activity的内容。然后初始化textview。然后将文本设置为textview。您还没有初始化textview。因此你得到了 NullPointerException。

应该是这样的:

setContentView(R.layout.activity_help);
TextView helpTextView = (TextView)findViewById(R.id.helpTextView);
helpTextView.setText("Some helpful stuff..");