Java 尝试在空对象引用上调用虚拟方法 'android.view.Window$Callback android.view.Window.getCallback()'
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/36666091/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
提问by
When my SplashActivityopens the LoginActivitymy app crashes.
当我SplashActivity打开时，LoginActivity我的应用程序崩溃了。
The following is my SplashActivity.java:
以下是我的SplashActivity.java：
package com.example.android.appName;
import android.content.Intent;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import java.util.Timer;
import java.util.TimerTask;
public class SplashActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_splash);
Timer timer = new Timer();
timer.schedule(new TimerTask() {
public void run() {
Intent intent = new Intent(SplashActivity.this, LoginActivity.class);
startActivity(intent);
finish();
}
}, 1500);
}
}
and my LoginActivity.java:
和我的LoginActivity.java：
package com.example.android.appName;
import android.content.Intent;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.KeyEvent;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.view.View;
import android.widget.EditText;
import android.widget.ProgressBar;
import android.widget.TextView;
public class LoginActivity extends AppCompatActivity {
private EditText usernameField = (EditText)findViewById(R.id.username),
passwordField = (EditText)findViewById(R.id.password);
private TextView error = (TextView)findViewById(R.id.error);
private ProgressBar progress = (ProgressBar)findViewById(R.id.progress);
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_login);
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
MenuInflater inflater = getMenuInflater();
inflater.inflate(R.menu.login_menu, menu);
return true;
}
@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
if (android.os.Build.VERSION.SDK_INT > 5
&& keyCode == KeyEvent.KEYCODE_BACK
&& event.getRepeatCount() == 0) {
onBackPressed();
return true;
}
return super.onKeyDown(keyCode, event);
}
public void exit(MenuItem item) {
finish();
}
public void signIn(View view) {
Intent intent = new Intent(this, MainActivity.class);
startActivity(intent);
finish();
}
}
Part of AndroidManifest.xml:
部分AndroidManifest.xml：
<activity android:name=".SplashActivity"
android:theme="@style/NoActionBar">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity
android:name=".LoginActivity"
android:label="@string/title_activity_login" />
Error in logcat:
logcat 中的错误：
04-16 23:24:16.124 4015-4015/com.example.android.appName E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.android.appName, PID: 4015
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.android.appName/com.example.android.appName.LoginActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2993)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3248)
at android.app.ActivityThread.access00(ActivityThread.java:197)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1681)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:145)
at android.app.ActivityThread.main(ActivityThread.java:6872)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1404)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1199)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
at android.support.v7.app.AppCompatDelegateImplBase.<init>(AppCompatDelegateImplBase.java:68)
at android.support.v7.app.AppCompatDelegateImplV7.<init>(AppCompatDelegateImplV7.java:145)
at android.support.v7.app.AppCompatDelegateImplV11.<init>(AppCompatDelegateImplV11.java:28)
at android.support.v7.app.AppCompatDelegateImplV14.<init>(AppCompatDelegateImplV14.java:42)
at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:186)
at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:168)
at android.support.v7.app.AppCompatActivity.getDelegate(AppCompatActivity.java:508)
at android.support.v7.app.AppCompatActivity.findViewById(AppCompatActivity.java:180)
at com.example.android.appName.LoginActivity.<init>(LoginActivity.java:20)
at java.lang.reflect.Constructor.newInstance(Native Method)
at java.lang.Class.newInstance(Class.java:1690)
at android.app.Instrumentation.newActivity(Instrumentation.java:1080)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2983)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3248)?
at android.app.ActivityThread.access00(ActivityThread.java:197)?
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1681)?
at android.os.Handler.dispatchMessage(Handler.java:102)?
at android.os.Looper.loop(Looper.java:145)?
at android.app.ActivityThread.main(ActivityThread.java:6872)?
at java.lang.reflect.Method.invoke(Native Method)?
at java.lang.reflect.Method.invoke(Method.java:372)?
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1404)?
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1199)
采纳答案by George Mulligan
An Activityis not fully initialized and ready to look up views until after setContentView(...)is called in onCreate().
AnActivity尚未完全初始化并准备好查找视图，直到在setContentView(...)中调用之后onCreate()。
Only declare the fields like the following:
只声明如下字段：
private EditText usernameField, passwordField;
private TextView error;
private ProgressBar progress;
and then assign the values in onCreate:
然后分配值onCreate：
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_login);
usernameField = (EditText)findViewById(R.id.username);
passwordField = (EditText)findViewById(R.id.password);
error = (TextView)findViewById(R.id.error);
progress = (ProgressBar)findViewById(R.id.progress);
}
Might not be part of the problem but as an extra bit of advice a Timerruns the TimerTaskon a background thread and that should be avoided in this case. Replace the Timerwith a Handlerinstead to run it on the UI thread.
可能不是问题的一部分，但作为一个额外的建议， a在后台线程上Timer运行，TimerTask在这种情况下应该避免。替换为TimeraHandler以在 UI 线程上运行它。
new Handler().postDelayed(new Runnable() {
@Override
public void run() {
Intent intent = new Intent(SplashActivity.this, LoginActivity.class);
startActivity(intent);
finish();
}
}, 1500);
回答by Gowtham Gowda
Views defined in xml file can only be accessed in java code, after setting the ContentViewas the required xml file using:
在 xml 文件中定义的视图只能在 java 代码中访问，ContentView使用以下命令将其设置为所需的 xml 文件后：
setContentView(R.layout.xml_file_name);
So 1st call above method inside onCreatemethod and then initialize the Viewinstances inside onCreateor inside the methods in which the instance will be used.
所以第一次在方法内部调用上面的方法onCreate，然后在将使用实例的方法View内部onCreate或内部初始化实例。
