运行时错误 Android Studio

Question

我在 J5 上运行我的应用程序时得到这个输出。不知道 2 做什么请帮忙

E/AndroidRuntime: FATAL EXCEPTION: 
Process: com.example.android.testapp, PID: 
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.android.testapp/com.example.android.testapp.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:2555)
                                                                                     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:
    at android.app.ActivityThread.access$900(ActivityThread.java:
    at android.app.ActivityThread$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.invoke(Native 
    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:
    Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object 
    at android.app.Activity.findViewById(Activity.java:
    at com.example.android.testapp.MainActivity.<init>(MainActivity.java:
    at java.lang.reflect.Constructor.newInstance(Native 
    at java.lang.Class.newInstance(Class.java:
    at android.app.Instrumentation.newActivity(Instrumentation.java:
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:
    at android.app.ActivityThread.access$900(ActivityThread.java:
    at android.app.ActivityThread$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.invoke(Native 
    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:1200) 

我看过博客，YouTube视频和这里，在StackOverflow中，但我不知道如何解决这个问题。

Answer 1

请同时发布您的 MainActivity 文件。但是看着错误，我可以假设的是这个

您正在实例化某个视图。假设一个文本视图。

这是你的代码。

public class MainActivity extends Activity {
TextView something = (TextView) findViewById(R.id.myTextView);
onCreate (Bundle savedinstancestate){
super.onCreate(.....
setContentView(R.layout.layout_main);
.......

如果这个假设是正确的

简单地这样做

public class MainActivity extends Activity {
TextView something;
onCreate (Bundle savedinstancestate){
super.onCreate(.....
setContentView(R.layout.layout_main);
something = (TextView) findViewById(R.id.myTextView);
.........

应该工作。希望对您有所帮助。:)

Answer 2

EditText，number必须在oncreate方法中定义。

public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    EditText editText = (EditText) findViewById(R.id.editText);
    }
}