Android Studio:Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object

Question

Please any help ???

It's true that there are questions similar to the one I'm asking, but I really tried all the solutions you proposed but it does not work for me.

The error that I'm having in the consol.

E/AndroidRuntime: FATAL EXCEPTION: main

              Process: ems.develop.megagame3, PID: 9574
              java.lang.RuntimeException: Unable to start activity ComponentInfo{ems.develop.megagame3/ems.develop.megagame3.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
                  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2778)
                  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2856)
                  at android.app.ActivityThread.-wrap11(Unknown Source:0)
                  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1589)
                  at android.os.Handler.dispatchMessage(Handler.java:106)
                  at android.os.Looper.loop(Looper.java:164)
                  at android.app.ActivityThread.main(ActivityThread.java:6494)
                  at java.lang.reflect.Method.invoke(Native Method)
                  at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)
               Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
                  at ems.develop.megagame3.MainActivity.onCreate(MainActivity.java:41)
                  at android.app.Activity.performCreate(Activity.java:7009)
                  at android.app.Activity.performCreate(Activity.java:7000)

#

here it is mainActivity.java

public class MainActivity extends AppCompatActivity {

private EditText TxtNumber=null ;
private Button lblCompare;
private TextView TxtResult;
private ProgressBar pgbScole;
private TextView TxtHistory;

private int searchedValue;
private int score;

public MainActivity() {
}


@Override
protected void onCreate(Bundle savedInstanceState) {
   super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    /* TxtNumber =(EditText) findViewById(R.id.TxtNumber); */
    TxtNumber =(EditText) findViewById(R.id.TxtNumber);
    lblCompare =(Button) findViewById(R.id.lblCompare);
    TxtResult =(TextView) findViewById(R.id.TxtResult);
    pgbScole =(ProgressBar) findViewById(R.id.pgbScole);
    TxtHistory= (TextView) findViewById(R.id.TxtHistory);
    lblCompare.setOnClickListener(btnCompareListner);

    init();

}

private void init() {

    score= 0;
    searchedValue = 1+ (int)(Math.random() *100);
    Log.i("DEBUG", " Valeur cherché "+searchedValue);
    // réinitialisation des fields
    TxtNumber.setText("");
    pgbScole.setProgress(score);
    TxtResult.setText("");
    TxtHistory.setText("");
// Mettre le focus au field TxtNumber;
    TxtNumber.requestFocus();

}

private View.OnClickListener btnCompareListner;

{
    btnCompareListner = new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Log.i("Debug", "Bouton cliqué ");
            String txtNumber = TxtNumber.getText().toString();
            if (txtNumber.equals("")) return;

            TxtHistory.append(TxtNumber +"\r\n");
            pgbScole.incrementProgressBy(1);
            score++;

            int enteredValue = Integer.parseInt(txtNumber);
            if (enteredValue == searchedValue) {
                congratulations();

            } else if (enteredValue < searchedValue) {
                TxtResult.setText(R.string.strGreater);
            }else{
                TxtResult.setText(R.string.strLower);
            }
            TxtNumber.setText("");
            TxtNumber.requestFocus();
        }
    };
}

private void congratulations() {
    TxtResult.setText(R.string.congra);

    AlertDialog retryAlert= new AlertDialog.Builder(this).create();
    retryAlert.setTitle(R.string.app_name);
    retryAlert.setMessage(getString( R.string.strMessage,score));
    retryAlert.setButton(AlertDialog.BUTTON_POSITIVE, getString(R.string.strYes), new AlertDialog.OnClickListener(){
        @Override
        public void onClick(DialogInterface dialogInterface, int i) {
            init();
        }
    });
    retryAlert.setButton(AlertDialog.BUTTON_NEGATIVE, getString(R.string.strNo), new AlertDialog.OnClickListener(){
        @Override
        public void onClick(DialogInterface dialogInterface, int i) {
            finish();
        }
    });
    retryAlert.show();
}

and this is "activity_main.xml"

   <?xml version="1.0" encoding="utf-8"?><android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity">

<android.support.constraint.Guideline
    android:id="@+id/guideline3"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginBottom="281dp"
    android:orientation="vertical"
    app:layout_constraintBottom_toTopOf="@+id/pgbScole"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintGuide_end="-44dp"
    app:layout_constraintTop_toTopOf="parent">

    <ProgressBar
        android:id="@+id/pgbScole"
        style="?android:attr/progressBarStyleHorizontal"
        android:layout_width="448dp"
        android:layout_height="14dp"
        android:layout_marginTop="180dp"
        android:max="10"
        android:progress="5"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.68"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <TextView
        android:id="@+id/lbltitre"
        style="@android:style/Animation"
        android:layout_width="0dp"
        android:layout_height="42dp"
        android:layout_marginBottom="8dp"
        android:layout_marginEnd="29dp"
        android:layout_marginLeft="29dp"
        android:layout_marginRight="29dp"
        android:layout_marginStart="29dp"
        android:text="@string/lblTitle"
        android:textAppearance="@android:style/TextAppearance.Large"
        android:textColor="@android:color/background_dark"
        android:textColorHighlight="@android:color/background_dark"
        android:textColorHint="@android:color/background_dark"
        android:textColorLink="@android:color/holo_purple"
        app:layout_constraintBottom_toTopOf="@+id/TxtNumber"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="1.0"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="1.0"
        tools:height="?android:attr/lightRadius"
        tools:textCursorDrawable="@android:color/holo_blue_dark"
        tools:textSize="18sp" />

    <EditText
        android:id="@+id/TxtNumber"
        android:layout_width="0dp"
        android:layout_height="43dp"
        android:layout_marginBottom="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginLeft="28dp"
        android:layout_marginRight="8dp"
        android:layout_marginStart="28dp"
        android:layout_marginTop="56dp"
        android:ems="10"
        android:inputType="numberSigned"
        app:layout_constraintBottom_toTopOf="@+id/TxtResult"
        app:layout_constraintEnd_toStartOf="@+id/lblCompare"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        tools:ignore="UnknownId" />

    <Button
        android:id="@+id/lblCompare"
        android:layout_width="159dp"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:layout_marginEnd="28dp"
        android:layout_marginRight="28dp"
        android:layout_marginTop="64dp"
        android:text="@string/lblComapre"
        app:layout_constraintBottom_toBottomOf="@+id/TxtResult"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.036" />

    <TextView
        android:id="@+id/TxtHistory"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:layout_marginBottom="24dp"
        android:layout_marginEnd="12dp"
        android:layout_marginLeft="12dp"
        android:layout_marginRight="12dp"
        android:layout_marginStart="12dp"
        android:layout_marginTop="8dp"
        android:background="@color/colorAccent"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/pgbScole" />


    <EditText
        android:id="@+id/TxtResult"
        android:layout_width="299dp"
        android:layout_height="47dp"
        android:layout_marginBottom="4dp"
        android:layout_marginEnd="56dp"
        android:layout_marginLeft="8dp"
        android:layout_marginRight="56dp"
        android:layout_marginStart="8dp"
        android:ems="10"
        android:inputType="textPersonName"
        app:layout_constraintBottom_toTopOf="@+id/pgbScole"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="@+id/guideline3" />
</android.support.constraint.Guideline>

Answer 1

Are sure that you haven't any another xml with same name and you are using this another xml.

An very similar scenario is when your have two layouts to differents configurations, e.g, landscape and portrait where one have the views and another one haven't them