How to prevent my app from crashing?

Question

I am a newbie and trying to make an android application for Online Voting. I have just designed the layouts so far but, unfortunately, I am facing rendering problems and the null pointer exception.

When I open an activity from MainActivity, I can only see a white screen! The sequence of activities is as follows:

MainActivity --> Instructions or AdminLogin --> UserLogin or RegisterCandidate

Even after hours of googling, I am not able to figure out the exact problem. I have checked several answers on StackOverflow itself but failed to figure out what's wrong.

The error trace is as follows:

E/AndroidRuntime: FATAL EXCEPTION: main
  java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.palakjain.onlinevotingschool/com.example.palakjain.onlinevotingschool.AdminLogin}: java.lang.NullPointerException
      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2360)
      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2412)
      at android.app.ActivityThread.access$600(ActivityThread.java:162)
      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1364)
      at android.os.Handler.dispatchMessage(Handler.java:107)
      at android.os.Looper.loop(Looper.java:194)
      at android.app.ActivityThread.main(ActivityThread.java:5388)
      at java.lang.reflect.Method.invokeNative(Native Method)
      at java.lang.reflect.Method.invoke(Method.java:525)
      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:833)
      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:600)
      at dalvik.system.NativeStart.main(Native Method)
   Caused by: java.lang.NullPointerException
      at com.example.suniljain.onlinevotingschool.AdminLogin.onCreate(AdminLogin.java:39)
      at android.app.Activity.performCreate(Activity.java:5141)
      at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1084)
      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2307)
      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2412) 
      at android.app.ActivityThread.access$600(ActivityThread.java:162) 
      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1364) 
      at android.os.Handler.dispatchMessage(Handler.java:107) 
      at android.os.Looper.loop(Looper.java:194) 
      at android.app.ActivityThread.main(ActivityThread.java:5388) 
      at java.lang.reflect.Method.invokeNative(Native Method) 
      at java.lang.reflect.Method.invoke(Method.java:525) 
      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:833) 
      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:600) 
      at dalvik.system.NativeStart.main(Native Method)

MainActivity code:

public class MainActivity extends AppCompatActivity {

Button btnConductVoting, btnInstructions, btnSkipInstructions;

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

    btnConductVoting = (Button) findViewById(R.id.btnConductVoting);
    btnInstructions = (Button) findViewById(R.id.btnInstructions);
    btnSkipInstructions = (Button) findViewById(R.id.btnSkipInstructions);

    btnConductVoting.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent i = new Intent(MainActivity.this, AdminLogin.class);
            startActivity(i);
        }
    });

    btnInstructions.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent i = new Intent(MainActivity.this, Instructions.class);
            startActivity(i);
        }
    });

    btnSkipInstructions.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent i = new Intent(MainActivity.this, UserLogin.class);
            startActivity(i);
        }
    });

}
}

Instructions code:

public class Instructions extends AppCompatActivity{
Button btnStartVoting;

@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    btnStartVoting = (Button) findViewById(R.id.btnStartVoting);

    btnStartVoting.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent i = new Intent(Instructions.this, UserLogin.class);
            startActivity(i);
        }
    });
}
}

AdminLogin code:

public class AdminLogin extends AppCompatActivity{

Button btnAdminLogin;
EditText etUsername;
EditText etPassword;

@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    btnAdminLogin = (Button) findViewById(R.id.btnAdminLogin);
    etUsername = (EditText) findViewById(R.id.etAdminPassword);
    etPassword = (EditText) findViewById(R.id.etAdminPassword);

    btnAdminLogin.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            if(etUsername.getText().toString().equals("administrator") && etPassword.getText().toString().equals("password")){
                Intent i = new Intent(AdminLogin.this, RegisterCandidate.class);
                startActivity(i);
            }
        }
    });

}
}

UserLogin code:

public class UserLogin extends AppCompatActivity{

//TextView tvUserForgotPassword;
TextView tvNewUserSignUp;
//Button btnUserLogIn;

@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    tvNewUserSignUp = (TextView) findViewById(R.id.tvNewUserSignUp);        

    tvNewUserSignUp.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent i = new Intent(UserLogin.this, NewUser.class);
            startActivity(i);
        }
    });

}
}

RegisterCandidate code:

public class RegisterCandidate extends AppCompatActivity{
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
}
}

NewUser code:

public class NewUser extends AppCompatActivity{
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
}
}

Update1: I have made all the changes that are suggested in the answers, still my application is crashing whenever I click "SKIP INSTRUCTIONS" button in the MainActivity. Also, app crashes when I click "Log-in" button from AdminLogin Activity. What should I do?

Update2: i referred What is a NullPointerException, and how do I fix it? yet couldn't fix the error!

@Palak Jain and You can't use `==` on java for check variable values , you must use `.equals` instead of `==` — DolDurma, Jul 16 '17 at 05:41
@Palak Jain all of your activities such as `Instructions` - `AdminLogin` `UserLogin` - `RegisterCandidate` don't have setContentView — DolDurma, Jul 16 '17 at 05:43

Rahul · Accepted Answer · 2017-07-16T05:49:28.323

1

You didn't setContentView in AdminLogin Activity.

   @Override
   protected void onCreate(@Nullable Bundle savedInstanceState) {
   super.onCreate(savedInstanceState);
   // set contentview here before finding views otherwise they will be null
    btnAdminLogin = (Button) findViewById(R.id.btnAdminLogin);
   }

Just a note, same issue is with your other activities, cross check that.

If you want to know what is setContentView then in a short term it associate View/UI which will display when Activity launch. For an example - If you want to show EditText and Button on UI then they must be present in layout file which have .xml extension. (You can also create View through java code)

edited Jul 16 '17 at 05:49

answered Jul 16 '17 at 05:36

Rahul

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what's setContentView? Actually I am a newbie – Palak Jain Jul 16 '17 at 05:40
in a setContentView you have to pass layout_id of your activity. Check your MainActivity there you wrote setContentView(R.layout.activity_main) – Rahul Jul 16 '17 at 05:43
@PalakJain Use .equals to compare strings, not == – Rahul Jul 16 '17 at 05:55
yes, I have made all the changes...thanks a lot! – Palak Jain Jul 16 '17 at 05:56
Nice, happy coding! – Rahul Jul 16 '17 at 05:56
I have made all the changes that are suggested in the answer, still, my application is crashing whenever I click "SKIP INSTRUCTIONS" button in the MainActivity. Also, app crashes when I click "Log-in" button from AdminLogin Activity. What should I do? – Palak Jain Jul 16 '17 at 06:13
Or can you cross check, you associate correct layout with UserLogin Activity and it is registered in manifest file – Rahul Jul 16 '17 at 06:17
can we discuss the issue on mail? – Palak Jain Jul 16 '17 at 06:19

score 1 · Answer 2 · answered Jul 16 '17 at 05:42

1

You cannot compare Strings with == in your AdminLogin activity etUsername.getText().toString() == "administrator"

use

etUsername.getText().toString().equals("administrator")

also you didn't setContentView as stated in above answer

answered Jul 16 '17 at 05:42

Manohar

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score 1 · Answer 3 · answered Jul 16 '17 at 05:45

1

as far as you are newbie on android you must read this documents to set setContentView

What is setContentView(R.layout.main)?

https://developer.android.com/training/basics/firstapp/building-ui.html

answered Jul 16 '17 at 05:45

DolDurma

15,753
51
198
377

score 0 · Answer 4 · answered Jul 16 '17 at 07:27

0

There is no back end and front end language for Android. Both are done using Java. The XML file is just a layout or skeleton for the activity. To use the activity one must use

setContentView(R.id.layout_id)

answered Jul 16 '17 at 07:27

Praveen Thirumurugan

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How to prevent my app from crashing?

4 Answers4