尝试在空对象引用上调用虚拟方法‘android.view.Window$回调-android.view.Window.getCallback()’？
尝试在空对象引用上调用虚拟方法‘android.view.Window$回调-android.view.Window.getCallback()’？
提问于 2018-08-19 10:53:28
当我SplashActivity打开LoginActivity我的应用程序崩溃。
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);
}
}
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();
}
}
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" />
logcat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)
回答 1
Stack Overflow用户
发布于 2018-08-19 20:40:18
Activity未完全初始化并准备好查看视图，直到setContentView(...)调用之后onCreate()。
仅声明如下字段：
private EditText usernameField, passwordField;
private TextView error;
private ProgressBar progress;
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);
}
可能不是问题的一部分，但作为一个额外的建议，Timer，TimerTask在后台线程上运行，在这种情况下应该避免。替换为Timer，Handler而不是在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);
页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：
https://stackoverflow.com/questions/-100002259
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