Android - A Checkbox when isChecked, shows button

Question

I'm trying to make an app which, in this certain activity I have a checkbox that if isChecked then show a button to the user, which will be used to launch the camera. I've tried looking for examples in other overflow topics and could find some, but everytime I try them, my app crashes. Could you please help me saying what am I doing wrong? Much thanks in advance.

package com.example.report;


public class started_report extends AppCompatActivity {

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

        final Button buttonCamera;
        buttonCamera = (Button)this.findViewById(R.id.buttonCamera);

        CheckBox checkbox = (CheckBox)this.findViewById(R.id.checkbox);
        checkbox.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() {
            @Override
            public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) {

            buttonCamera.setVisibility(View.VISIBLE);
        }
    });
}

public void openCamera(View view){
    //For posterior use to call the camera
}

}

This is my errorlog: Is it saying "on a null object reference" because I'm using void?

09-11 00:20:58.199 10913-10913/com.example.report E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.report, PID: 10913
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.report/com.example.report.started_report}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.CheckBox.setOnCheckedChangeListener(android.widget.CompoundButton$OnCheckedChangeListener)' on a null object reference
      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2325)
      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$MethodAndArgsCaller.run(ZygoteInit.java:903)
      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
    Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.CheckBox.setOnCheckedChangeListener(android.widget.CompoundButton$OnCheckedChangeListener)' on a null object reference
      at com.example.relatoriodeobras.started_alvara.onCreate(started_alvara.java:21)
      at android.app.Activity.performCreate(Activity.java:5990)
      at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)
      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2278)
      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)

my xml layout is here:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.example.report.started_report">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:text="@string/started_report"
        android:id="@+id/textView3"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true" />

    <CheckBox
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/placadeobra"
        android:id="@+id/checkBox"
        android:singleLine="false"
        android:layout_marginTop="40dp"
        android:layout_below="@+id/textView3"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/camera"
        android:id="@+id/buttonCamera"
        android:layout_above="@+id/radioGroup2"
        android:layout_centerHorizontal="true"
        android:visibility="gone"
        android:onClick="openCamera"/>

    <CheckBox
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/projetoexecutivo"
        android:id="@+id/checkBox2"
        android:layout_below="@+id/checkBox"
        android:layout_centerHorizontal="true"
        android:layout_alignParentStart="true"
        android:layout_alignParentEnd="false" />

    <RadioGroup
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/radioGroup2"
        android:layout_below="@+id/checkBox2"
        android:layout_alignRight="@+id/button"
        android:layout_alignEnd="@+id/button"
        android:layout_marginTop="49dp">

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/RT"
            android:id="@+id/textView5"
            android:layout_gravity="center_horizontal"
            android:textSize="20sp" />

        <RadioButton
            android:layout_width="wrap_content"
            android:layout_height="39dp"
            android:text="@string/ART"
            android:id="@+id/radioButton3"
            android:layout_marginTop="35dp" />

        <RadioButton
            android:layout_width="wrap_content"
            android:layout_height="36dp"
            android:text="@string/RRT"
            android:id="@+id/radioButton4" />

    </RadioGroup>

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceMedium"
        android:text="@string/notobs"
        android:id="@+id/textView4"
        android:layout_below="@+id/radioGroup2"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        android:layout_marginTop="39dp" />

    <EditText
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:id="@+id/editText"
        android:layout_alignParentStart="true"
        android:layout_below="@+id/textView4"
        android:layout_alignParentLeft="true"
        android:inputType="none"/>

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/buttonfinalize"
        android:id="@+id/button"
        android:layout_alignParentBottom="true"
        android:layout_alignRight="@+id/editText"
        android:layout_alignEnd="@+id/editText" />

</RelativeLayout>

"started_alvara.java:21" What is line 21 of started_alvara.java? — Code-Apprentice, Sep 11 '16 at 00:38
it's started_report* but it's the line where I call the OnCheckedChangedListener checkbox.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() — HextechEnthusiast, Sep 11 '16 at 00:39
The error message says the problem is in a file called started_alvara.java. Please post the code from this file or the real error message. — Code-Apprentice, Sep 11 '16 at 00:47
The started_alvara.java is the started_report.java file shown above — HextechEnthusiast, Sep 11 '16 at 00:48
You should post the exact code you are running along with the exact error message. Otherwise you risk causing a lot of confusion and wasting time getting the answers to your question. — Code-Apprentice, Sep 11 '16 at 00:54
Pleased edit your question to include `activity_started_report.xml` — Code-Apprentice, Sep 11 '16 at 00:56
Possible duplicate of [Unfortunately MyApp has stopped. How can I solve this?](http://stackoverflow.com/questions/23353173/unfortunately-myapp-has-stopped-how-can-i-solve-this) — Code-Apprentice, Sep 11 '16 at 00:58

score 1 · Answer 1 · answered Sep 11 '16 at 04:06

I suggest you use setOnClickListener method instead of, like this

final Button buttonCamera;
buttonCamera = (Button)this.findViewById(R.id.buttonCamera);

final CheckBox checkbox = (CheckBox)this.findViewById(R.id.checkbox);    

checkbox.setOnClickListener(new OnClickListener() {

      @Override
      public void onClick(View v) {

        if (checkBox.isChecked()) {
            buttonCamera.setVisibility(View.VISIBLE);
        }
        else {
            buttonCamera.setVisibility(View.INVISIBLE);
        }

      }
    });

score 0 · Answer 2 · answered Sep 11 '16 at 00:43

0

This is my errorlog: Is it saying "on a null object reference" because I'm using void?

You get this message because a reference variable is null. In this case it is the CheckBox. Be sure you call findViewById() to initialize your variable before calling methods on it.

answered Sep 11 '16 at 00:43

Code-Apprentice

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But I already did that... Isn't what's done here? CheckBox checkbox = (CheckBox)this.findViewById(R.id.checkbox); – HextechEnthusiast Sep 11 '16 at 01:09
@HextechEnthusiast Yes, you did. I was making a suggestion based on incomplete information. Please provide the additional information I have requested. – Code-Apprentice Sep 11 '16 at 01:13
Nevermind...I found the problem right after posting the .xml I should use R.id.checkBox instead of R.id.checkbox – HextechEnthusiast Sep 11 '16 at 01:22
@HextechEnthusiast yup they must be the same – Code-Apprentice Sep 11 '16 at 01:23

Android - A Checkbox when isChecked, shows button

2 Answers2