java - 错误 : Unable to instantiate activity ComponentInfo. 应用程序每次在 LOgCAT 中出现此错误时都会崩溃

Question

她是MainActivity.java

public class MainActivity extends AppCompatActivity implements View.OnClickListener{

    FrameLayout cat1 = (FrameLayout) findViewById(R.id.cat1);

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

    cat1.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Intent i = new Intent(MainActivity.this,Wallpapers.class);
            startActivity(i);
        }
    });
    }

@Override
public void onClick(View view) {

}

}

这是 list 代码

<?xml version="1.0" encoding="utf-8"?>

<application
    android:allowBackup="true"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:supportsRtl="true"
    android:theme="@style/AppTheme">
    <activity android:name="com.example.akkie.customize.MainActivity">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>

    <activity android:name="com.example.akkie.customize.Wallpapers">

       </activity>
</application>
<supports-screens android:largeScreens="true"
    android:smallScreens="true"
    android:normalScreens="true"
    android:xlargeScreens="true"
    android:compatibleWidthLimitDp="320"/>

这是日志猫

E/AndroidRuntime: FATAL EXCEPTION: main
              Process: com.example.akkie.customize, PID: 3969
              java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.akkie.customize/com.example.akkie.customize.MainActivity}: 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:3133)
                  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3416)
                  at android.app.ActivityThread.access$1100(ActivityThread.java:230)
                  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1822)
                  at android.os.Handler.dispatchMessage(Handler.java:102)
                  at android.os.Looper.loop(Looper.java:148)
                  at android.app.ActivityThread.main(ActivityThread.java:7409)
                  at java.lang.reflect.Method.invoke(Native Method)
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)
               Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
                  at android.support.v7.app.AppCompatDelegateImplBase.<init>(AppCompatDelegateImplBase.java:116)
                  at android.support.v7.app.AppCompatDelegateImplV9.<init>(AppCompatDelegateImplV9.java:147)
                  at android.support.v7.app.AppCompatDelegateImplV11.<init>(AppCompatDelegateImplV11.java:27)
                  at android.support.v7.app.AppCompatDelegateImplV14.<init>(AppCompatDelegateImplV14.java:53)
                  at android.support.v7.app.AppCompatDelegateImplV23.<init>(AppCompatDelegateImplV23.java:29)
                  at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:203)
                  at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:185)
                  at android.support.v7.app.AppCompatActivity.getDelegate(AppCompatActivity.java:525)
                  at android.support.v7.app.AppCompatActivity.findViewById(AppCompatActivity.java:193)
                  at com.example.akkie.customize.MainActivity.<init>(MainActivity.java:16)
                  at java.lang.Class.newInstance(Native Method)
                  at android.app.Instrumentation.newActivity(Instrumentation.java:1096)
                  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3123)
                  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3416) 
                  at android.app.ActivityThread.access$1100(ActivityThread.java:230) 
                  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1822) 
                  at android.os.Handler.dispatchMessage(Handler.java:102) 
                  at android.os.Looper.loop(Looper.java:148) 
                  at android.app.ActivityThread.main(ActivityThread.java:7409) 
                  at java.lang.reflect.Method.invoke(Native Method) 
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230) 
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120) 

I/Process:发送信号。 PID:3969 SIG:9应用程序终止。

我已经在 stackoverflow 上搜索了正确的答案，但没有一个符合我的场景。尝试了所有我能做的解决方案和改变。请帮忙。

Answer 1

移动:

FrameLayout cat1 = (FrameLayout) findViewById(R.id.cat1);

在 onCreate() 中调用 setContentView() 后。您无法找到尚不存在的事物。