Java 尝试调用虚拟方法';android.view.Window$Callback android.view.Window.getCallback()';关于空对象引用
当myJava 尝试调用虚拟方法';android.view.Window$Callback android.view.Window.getCallback()';关于空对象引用,java,android,findviewbyid,Java,Android,Findviewbyid,当mySplashActivity打开LoginActivity时,我的应用程序崩溃 下面是我的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.
SplashActivity
打开LoginActivity
时,我的应用程序崩溃
下面是我的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);
}
}
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();
}
}
以及我的LoginActivity.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);
}
}
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();
}
}
AndroidManifest.xml的一部分:
<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" />
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.access$1000(ActivityThread.java:197)
at android.app.ActivityThread$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.access$1000(ActivityThread.java:197)
at android.app.ActivityThread$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$MethodAndArgsCaller.run(ZygoteInit.java:1404)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1199)
04-16 23:24:16.124 4015-4015/com.example.android.appName E/AndroidRuntime:FATAL EXCEPTION:main
进程:com.example.android.appName,PID:4015
java.lang.RuntimeException:无法实例化活动组件信息{com.example.android.appName/com.example.android.appName.LoginActivity}:java.lang.NullPointerException:尝试在空对象引用上调用虚拟方法“android.view.Window$Callback android.view.Window.getCallback()”
在android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2993)上
位于android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3248)
在android.app.ActivityThread.access$1000(ActivityThread.java:197)
在android.app.ActivityThread$H.handleMessage(ActivityThread.java:1681)
位于android.os.Handler.dispatchMessage(Handler.java:102)
位于android.os.Looper.loop(Looper.java:145)
位于android.app.ActivityThread.main(ActivityThread.java:6872)
位于java.lang.reflect.Method.invoke(本机方法)
位于java.lang.reflect.Method.invoke(Method.java:372)
在com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run上(ZygoteInit.java:1404)
位于com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1199)
原因:java.lang.NullPointerException:尝试对空对象引用调用虚拟方法“android.view.Window$Callback android.view.Window.getCallback()”
位于android.support.v7.app.AppCompateDelegateImplbase。(AppCompateDelegateImplbase.java:68)
在android.support.v7.app.AppCompateDelegateImplv7。(AppCompateDelegateImplv7.java:145)
在android.support.v7.app.AppCompateDelegateImplv11。(AppCompateDelegateImplv11.java:28)
在android.support.v7.app.AppCompateDelegateImplv14。(AppCompateDelegateImplv14.java:42)
在android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:186)上
在android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:168)上
位于android.support.v7.app.AppCompatActivity.getDelegate(AppCompatActivity.java:508)
位于android.support.v7.app.AppCompatActivity.findViewById(AppCompatActivity.java:180)
位于com.example.android.appName.LoginActivity。(LoginActivity.java:20)
位于java.lang.reflect.Constructor.newInstance(本机方法)
位于java.lang.Class.newInstance(Class.java:1690)
位于android.app.Instrumentation.newActivity(Instrumentation.java:1080)
在android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2983)上
位于android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3248)
在android.app.ActivityThread.access$1000(ActivityThread.java:197)
在android.app.ActivityThread$H.handleMessage(ActivityThread.java:1681)
位于android.os.Handler.dispatchMessage(Handler.java:102)
位于android.os.Looper.loop(Looper.java:145)
位于android.app.ActivityThread.main(ActivityThread.java:6872)
位于java.lang.reflect.Method.invoke(本机方法)
位于java.lang.reflect.Method.invoke(Method.java:372)
在com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run上(ZygoteInit.java:1404)
位于com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1199)
在onCreate()
中调用setContentView(…)
之前,活动
未完全初始化并准备好查找视图
仅声明如下字段:
private EditText usernameField, passwordField;
private TextView error;
private ProgressBar progress;
然后在onCreate
中指定值:
@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);
}
可能不是问题的一部分,但作为额外的建议,Timer
在后台线程上运行TimerTask
,在这种情况下应该避免。将计时器
替换为处理程序
,以便在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);
使用以下方法将
ContentView
设置为所需的xml文件后,只能在java代码中访问xml文件中定义的视图:
setContentView(R.layout.xml_file_name);
因此,首先在
onCreate
方法内调用上述方法,然后在onCreate
内或实例将要使用的方法内初始化视图
实例。您是否在清单中声明了您的登录活动?@NicolasSimon是的,我在UI线程上执行了TimerTask.run()
。如果您从UI线程开始活动,那么它是否工作?@geogemulligan抱歉,但我不太确定“在UI线程上执行”是什么意思:(您能否显示LoginActivity
?的完整代码,或者至少显示类上所有声明的字段和完整的onCreate()
开始。这并没有解决问题,但感谢处理程序(当然是:)的确如此!非常感谢:P顺便说一句,我真的希望Android Studio有一天会给出比这更有用、更清晰的错误消息……我同意特定的堆栈跟踪对确定如何解决问题没有多大帮助。生命周期回调和项目配置之外的大多数其他错误消息更容易理解。“在onCreate()中调用setContentView(…)之前,活动未完全初始化,也未准备好查找视图。”现场显示。感谢您即使在onCreate()
中将值分配给我的按钮id后,我仍然会收到相同的错误@GeorgeMulligan