我有一个程序如下,谁能告诉我获得 NPE 的原因是什么。
public class MainActivity extends Activity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
secondClass x = new secondClass();
x.sText("Test Spring");
}
}
public class secondClass extends MainActivity {
public void sText(String s) {
TextView text = (TextView) findViewById(R.id.text);
text.setText(s);
}
}
主祭
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
...
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/hello_world"
android:id="@+id/text"
/>
</RelativeLayout>
日志猫
08-16 01:31:11.320 15032-15032/? E/Trace: error opening trace file: No such file or directory (2)
08-16 01:31:11.420 836-987/? E/EmbeddedLogger: App crashed! Process: com.example.myapplication
08-16 01:31:11.420 836-987/? E/EmbeddedLogger: App crashed! Package: com.example.myapplication v1 (1.0)
08-16 01:31:11.420 836-987/? E/EmbeddedLogger: Application Label: My Application
08-16 01:31:11.420 15032-15032/? E/AndroidRuntime: FATAL EXCEPTION: main
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.myapplication/com.example.myapplication.MainActivity}: java.lang.NullPointerException
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java)
at android.app.ActivityThread.access$600(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java)
at android.os.Handler.dispatchMessage(Handler.java)
at android.os.Looper.loop(Looper.java)
at android.app.ActivityThread.main(ActivityThread.java)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java)
at de.robv.android.xposed.XposedBridge.main(XposedBridge.java:110)
at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
at android.app.Activity.findViewById(Activity.java)
at com.example.myapplication.secondClass.sText(secondClass.java:13)
at com.example.myapplication.MainActivity.onCreate(MainActivity.java:14)
at android.app.Activity.performCreate(Activity.java)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java)
据我了解,这应该实例化并初始化一个 secondClass 对象,然后调用它的 sText() 方法。然后 sText 方法应该在 TextView 中存储一个引用。接下来,它调用 TextView 上的 setText() 方法,该方法将 TextView 中的文本更改为传递给 sText() 的内容。
logcat 说它抛出了 NullPointerException 但我不完全确定为什么。如果这是菜鸟的错误,那是因为我是菜鸟。谢谢你。