为什么try-Catch无法按预期工作？

Question

I wanted to make a try-catch block.

In my try block there's a method which checks if a radioButton is pressed, if so it should set an enum and go forward and if no radioButton is pressed, a NullPointerException should be thrown and the app closes (there're coming some extra features which aren't finished yet so I removed them for now).

Everything I saw so far didnt fit to my problem because they just forgot a catch or they missspelled a method or something like this.

It works if I don't press a radioButton my app closes, but if I press one radioButton my app closes again, but it should go to my next layout and set an enum (depending on which radioButton is pressed.

My java-code:

   public void setKnowledge() {
    setContentView(R.layout.knowledge_level);

    Button checking = (Button) findViewById(R.id.buttonChecking);

    checking.setOnClickListener(new View.OnClickListener(){
        @Override
        public void onClick(View v) {
            try {
                checkSelection();
                setContentView(R.layout.category_auswahl);
            }catch(NullPointerException n) {
                android.os.Process.killProcess(android.os.Process.myPid());
                System.exit(0);
            }
        }
    });
}

public knowledge checkSelection() {
    RadioButton rbNoob = (RadioButton) findViewById(R.id.radioButtonNoob);
    RadioButton rbBeginner = (RadioButton) findViewById(R.id.radioButtonBeginner);
    RadioButton rbAdvanced = (RadioButton) findViewById(R.id.radioButtonAdv);
    RadioButton rbPro = (RadioButton) findViewById(R.id.radioButtonPro);
    RadioButton rbGrandMaster = (RadioButton) findViewById(R.id.radioButtonGM);

    advertisment();

    if(rbNoob.isChecked()) {
        return knowledge.NOOB;
    }
    else if(rbBeginner.isChecked()) {
        return knowledge.BEGINNER;
    }
    else if(rbAdvanced.isChecked()) {
        return knowledge.ADVANCED;
    }
    else if(rbPro.isChecked()) {
        return knowledge.PRO;
    }
    else if(rbGrandMaster.isChecked()) {
        return knowledge.GM;
    }
    return null;
}

My XML: (EDIT: knowledge_level.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"
android:background="#081929" >

<RadioGroup
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginTop="32dp"
    app:layout_constraintTop_toBottomOf="@+id/testWissenstandText"
    android:layout_marginLeft="8dp"
    app:layout_constraintLeft_toLeftOf="parent"
    android:id="@+id/radioGroup"
    app:layout_constraintRight_toLeftOf="@+id/buttonChecking"
    android:layout_marginRight="8dp"
    app:layout_constraintHorizontal_bias="0.538">

    <RadioButton
        android:id="@+id/radioButtonNoob"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginLeft="0dp"
        android:layout_marginRight="0dp"
        android:layout_marginTop="8dp"
        android:checked="false"
        android:text="Keine Vorkenntnisse"
        android:textAlignment="center"
        android:textColor="#E0E0E0"
        android:textSize="18sp"
        android:textStyle="bold"
        app:layout_constraintHorizontal_bias="0.505"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toLeftOf="@+id/buttonChecking"
        app:layout_constraintTop_toBottomOf="@+id/testWissenstandText"/>

    <RadioButton
        android:id="@+id/radioButtonBeginner"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginLeft="0dp"
        android:layout_marginRight="0dp"
        android:layout_marginTop="8dp"
        android:checked="false"
        android:text="Anfänger"
        android:textAlignment="center"
        android:textColor="#E0E0E0"
        android:textSize="18sp"
        android:textStyle="bold"
        app:layout_constraintHorizontal_bias="0.0"
        app:layout_constraintLeft_toLeftOf="@+id/radioButtonNoob"
        app:layout_constraintRight_toRightOf="@+id/radioButtonNoob"
        app:layout_constraintTop_toBottomOf="@+id/radioButtonNoob"/>

    <RadioButton
        android:id="@+id/radioButtonAdv"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginLeft="0dp"
        android:layout_marginRight="0dp"
        android:layout_marginTop="8dp"
        android:text="Fortgeschrittener"
        android:textAlignment="center"
        android:textColor="#E0E0E0"
        android:textSize="18sp"
        android:textStyle="bold"
        app:layout_constraintHorizontal_bias="0.0"
        app:layout_constraintLeft_toLeftOf="@+id/radioButtonBeginner"
        app:layout_constraintRight_toRightOf="@+id/radioButtonBeginner"
        app:layout_constraintTop_toBottomOf="@+id/radioButtonBeginner"/>

    <RadioButton
        android:id="@+id/radioButtonPro"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginLeft="0dp"
        android:layout_marginRight="0dp"
        android:layout_marginTop="8dp"
        android:text="Pro"
        android:textAlignment="center"
        android:textColor="#E0E0E0"
        android:textSize="18sp"
        android:textStyle="bold"
        app:layout_constraintHorizontal_bias="0.0"
        app:layout_constraintLeft_toLeftOf="@+id/radioButtonAdv"
        app:layout_constraintRight_toRightOf="@+id/radioButtonAdv"
        app:layout_constraintTop_toBottomOf="@+id/radioButtonAdv"/>

    <RadioButton
        android:id="@+id/radioButtonGM"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginLeft="0dp"
        android:layout_marginRight="0dp"
        android:layout_marginTop="8dp"
        android:text='"Grand Master"'
        android:textAlignment="center"
        android:textColor="#E0E0E0"
        android:textSize="18sp"
        android:textStyle="bold"
        app:layout_constraintHorizontal_bias="0.0"
        app:layout_constraintLeft_toLeftOf="@+id/radioButtonPro"
        app:layout_constraintRight_toRightOf="@+id/radioButtonPro"
        app:layout_constraintTop_toBottomOf="@+id/radioButtonPro"/>

</RadioGroup>

<ImageView
    android:id="@+id/kastenKnowledge"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginEnd="8dp"
    android:layout_marginStart="8dp"
    android:layout_marginTop="16dp"
    app:layout_constraintHorizontal_bias="0.517"
    app:layout_constraintLeft_toLeftOf="parent"
    app:layout_constraintRight_toRightOf="parent"
    app:layout_constraintTop_toTopOf="parent"
    app:srcCompat="@drawable/kasten"/>

<TextView
    android:id="@+id/testWissenstandHeader"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginBottom="8dp"
    android:layout_marginEnd="8dp"
    android:layout_marginStart="8dp"
    android:layout_marginTop="8dp"
    android:text="Dein Wissensstand?"
    android:textAlignment="center"
    android:textColor="#511613"
    android:textSize="32sp"
    android:textStyle="bold"
    app:layout_constraintBottom_toBottomOf="@+id/kastenKnowledge"
    app:layout_constraintLeft_toLeftOf="@+id/kastenKnowledge"
    app:layout_constraintRight_toRightOf="@+id/kastenKnowledge"
    app:layout_constraintTop_toTopOf="@+id/kastenKnowledge"/>

<TextView
    android:id="@+id/testWissenstandText"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginEnd="16dp"
    android:layout_marginStart="16dp"
    android:layout_marginTop="24dp"
    android:text='Wie würdest du deinen Wissenstand\nbezüglich dem Spieleinhalt\nvon "Overwatch" beurteilen?'
    android:textAlignment="center"
    android:textColor="#E0E0E0"
    android:textSize="20sp"
    app:layout_constraintHorizontal_bias="0.512"
    app:layout_constraintLeft_toLeftOf="parent"
    app:layout_constraintRight_toRightOf="parent"
    app:layout_constraintTop_toBottomOf="@+id/testWissenstandHeader"/>

<Button
    android:id="@+id/buttonChecking"
    android:layout_width="100dp"
    android:layout_height="100dp"
    android:layout_marginEnd="16dp"
    android:layout_marginStart="0dp"
    android:background="@drawable/button_box"
    android:text="C\nL\nI\nC\nK"
    android:textAllCaps="false"
    android:textColor="#511613"
    android:textSize="16sp"
    android:textStyle="bold"
    app:layout_constraintRight_toRightOf="parent"
    android:layout_marginRight="16dp"
    app:layout_constraintBottom_toBottomOf="@+id/radioGroup"
    android:layout_marginBottom="8dp"
    app:layout_constraintTop_toTopOf="@+id/radioGroup"
    android:layout_marginTop="8dp"
    app:layout_constraintVertical_bias="0.5"/>



<TextView
    android:id="@+id/knowledgeHint"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginLeft="0dp"
    android:layout_marginRight="0dp"
    android:text="Wenn du einen Wissenstand\nfestgelegt hast, drücke den\nKnopf daneben!"
    android:textAlignment="center"
    android:textColor="#FF4400"
    android:textSize="20sp"
    android:textStyle="bold|italic"
    app:layout_constraintLeft_toLeftOf="parent"
    app:layout_constraintRight_toRightOf="parent"
    android:layout_marginTop="32dp"
    app:layout_constraintTop_toBottomOf="@+id/radioGroup"/>

</android.support.constraint.ConstraintLayout>

Error message(s):

08-14 21:01:26.469 4312-4312/? I/Process: Sending signal. PID: 4312 SIG: 9
08-14 21:01:26.470 1521-1590/system_process W/AudioTrack: AUDIO_OUTPUT_FLAG_FAST denied by client

                                                      --------- beginning of system
08-14 21:01:26.476 1521-1773/system_process I/WindowState: WIN DEATH: Window{ab30a43 u0 com.example.alexander.hashtagnevereverbuggy/com.example.alexander.hashtagnevereverbuggy.MainActivity}
08-14 21:01:26.480 1521-2538/system_process I/ActivityManager: Process com.example.alexander.hashtagnevereverbuggy (pid 4312) has died
08-14 21:01:26.480 1521-2538/system_process W/ActivityManager: Force removing ActivityRecord{ed88b90 u0 com.example.alexander.hashtagnevereverbuggy/.MainActivity t137}: app died, no saved state
08-14 21:01:26.491 1128-1128/? W/SurfaceFlinger: couldn't log to binary event log: overflow.
08-14 21:01:27.104 1776-1958/com.android.launcher D/EGL_emulation: eglMakeCurrent: 0x7fd4ff628200: ver 2 0
08-14 21:01:27.135 1521-1773/system_process W/InputMethodManagerService: Got RemoteException sending setActive(false) notification to pid 4312 uid 10058

Answer 1

Remove setContentView(R.layout.category_auswahl); inside onclick listener.

Answer 2

Change:

public knowledge checkSelection() {
    RadioButton rbNoob = (RadioButton) findViewById(R.id.radioButtonNoob);
    RadioButton rbBeginner = (RadioButton) findViewById(R.id.radioButtonBeginner);

    // etc

    else if(rbGrandMaster.isChecked()) {
        return knowledge.GM;
    }
    return null;
}

To this:

public knowledge checkSelection() {
    RadioButton rbNoob = (RadioButton) findViewById(R.id.radioButtonNoob);
    RadioButton rbBeginner = (RadioButton) findViewById(R.id.radioButtonBeginner);

    // etc

    else if(rbGrandMaster.isChecked()) {
        return knowledge.GM;
    }
    throw new NullPointerException();
}

Answer 3

    try {
            if (checkSelection() == null) {
            throw new NullPointerException("checkSelection is null");
               }
            setContentView(R.layout.category_auswahl);
        }catch(NullPointerException n) {
            android.os.Process.killProcess(android.os.Process.myPid());
            System.exit(0);
        }

Answer 4

Man, try-catch is not your problem in this code.

It works fine and closes your app: 08-14 21:01:26.469 4312-4312/? I/Process: Sending signal. PID: 4312 SIG: 9 08-14 21:01:26.469 4312-4312/? I/Process: Sending signal. PID: 4312 SIG: 9

You are not printing NPE, this is why you do not have it in the log.

You need to use Fragments instead of setting new content view.

You would never need System.exit(0) in Android beacuse it is against the system, you have to use finish from your Activity class.

Instead of using checking button, you can set onClickListener into your RadioGroup and you will get events every time selection has been made, so you can proceed to the next Screen (Activity or Fragment, but threre is no reason to use another one setContentView).

Here is good explanation about setContentView: Android: is using setContentView multiple times bad while changing layouts?