Getting error when changing other activity's TextView

Question

I'm a beginner at Android and Java and I have an error when trying to change an other activity's TextView. I've created MainActivity, and another one (PlayerNames).

I've set PlayerNames to be the Launcher activity, maybe the error is that, I don't know. I post the error and the code:

FATAL EXCEPTION: main

Process: com.example.android.mapamundi, PID: 1423
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.android.mapamundi/com.example.android.mapamundi.PlayerNames}: 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:2236)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
at android.app.ActivityThread.access$800(ActivityThread.java:151)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5254)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$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:146)
at android.support.v7.app.AppCompatDelegateImplV11.<init>(AppCompatDelegateImplV11.java:28)
at android.support.v7.app.AppCompatDelegateImplV14.<init>(AppCompatDelegateImplV14.java:41)
at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:190)
at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:172)
at android.support.v7.app.AppCompatActivity.getDelegate(AppCompatActivity.java:512)
at android.support.v7.app.AppCompatActivity.findViewById(AppCompatActivity.java:184)
at com.example.android.mapamundi.PlayerNames.<init>(PlayerNames.java:12)
at java.lang.reflect.Constructor.newInstance(Native Method)
at java.lang.Class.newInstance(Class.java:1606)
at android.app.Instrumentation.newActivity(Instrumentation.java:1066)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2226)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387) 
at android.app.ActivityThread.access$800(ActivityThread.java:151) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303) 
at android.os.Handler.dispatchMessage(Handler.java:102) 
at android.os.Looper.loop(Looper.java:135) 
at android.app.ActivityThread.main(ActivityThread.java:5254) 
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:903) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)

MAINACTIVITY.JAVA

Well, on MainActivity I only cast the TextViews with public static TextView tv1, so I don't post it

PLAYERNAMES.JAVA

public class PlayerNames extends AppCompatActivity implements View.OnClickListener {

Button accept = (Button) findViewById(R.id.changeNames);



EditText playerInput1 = (EditText) findViewById(R.id.PlayerNameInput1);
EditText playerInput2 = (EditText) findViewById(R.id.PlayerNameInput2);
EditText playerInput3 = (EditText) findViewById(R.id.PlayerNameInput3);
EditText playerInput4 = (EditText) findViewById(R.id.PlayerNameInput4);


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



    accept.setOnClickListener(this);
}

@Override
public void onClick(View view) {
    switch (view.getId()){
        case (R.id.changeNames):

            MainActivity.playerName1.setText(playerInput1.toString());
            MainActivity.playerName2.setText(playerInput2.toString());
            MainActivity.playerName3.setText(playerInput3.toString());
            MainActivity.playerName4.setText(playerInput4.toString());

            Intent intent = new Intent(PlayerNames.this, MainActivity.class);

            startActivity(intent);
    }
}

}

I post AndroidManifest.xml, just in case I have an error or something that causes the error

<activity
        android:name=".MainActivity"
        android:label="@string/app_name"
        android:theme="@style/AppTheme.NoActionBar">
        <intent-filter>
            <action android:name="com.example.android.mapamundi.MainActivity" />

            <category android:name="android.intent.category.DEFAULT" />
        </intent-filter>
    </activity>
    <activity android:name=".PlayerNames">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

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

Can someone please tell me what am I doing wrong? TKS!!

Answer 1

You are calling findViewById() to early, before your activity is fully created. It cannot be in a field initialiser. It has to be in (or after) onCreate(). And also after setContentView(), because you first have to set the content of your activity, before you can try to find views inside of it.