Error- Attempt to invoke Virtual Method___________setOnClickListener(android.view.View$OnClickListener)' on a null object reference

Question

I couldn't find anything that was not initialized, and can't work my way up further. The same bit works for another piece of code that I've written, both are totally identical, and I can't figure out the problem. The complete error code is - Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference.

The log file-

2021-07-30 21:29:33.613 17989-17989/com.elc.hostel E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.elc.hostel, PID: 17989
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.elc.hostel/com.elc.hostel.LoginActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3449)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:223)
        at android.app.ActivityThread.main(ActivityThread.java:7656)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at com.elc.hostel.LoginActivity.onCreate(LoginActivity.java:50)
        at android.app.Activity.performCreate(Activity.java:8000)
        at android.app.Activity.performCreate(Activity.java:7984)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1309)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3422)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:223) 
        at android.app.ActivityThread.main(ActivityThread.java:7656) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947) 
2021-07-30 21:29:35.004 17989-18005/com.elc.hostel W/System: A resource failed to call close. 
 

The code-

public class LoginActivity extends AppCompatActivity {

    EditText mEmail, mPassword;
    Button mLogin;
    ProgressBar mProgressBar;
    FirebaseAuth fAuth;

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

        requestWindowFeature(Window.FEATURE_NO_TITLE);
        this.getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,WindowManager.LayoutParams.FLAG_FULLSCREEN);
        Objects.requireNonNull(getSupportActionBar()).hide();

        mEmail = findViewById(R.id.LoginEmail);
        mPassword = findViewById(R.id.LoginPassword);
        mLogin = findViewById(R.id.LoginButton);

        mProgressBar = findViewById(R.id.progressBarLog);
        fAuth = FirebaseAuth.getInstance();

        mLogin.setOnClickListener(new View.OnClickListener() {------------------Error
            @Override
            public void onClick(View v) {
                String email = mEmail.getText().toString().trim();
                String password = mPassword.getText().toString().trim();

                if (TextUtils.isEmpty(email)){
                    mEmail.setError("Email is required");
                    return;
                }
                if (TextUtils.isEmpty(password)){
                    mPassword.setError("Password is required");
                    return;
                }
                if (password.length() < 6){
                    mPassword.setError("Password must be at least 6 characters");
                    return;
                }
                mProgressBar.setVisibility(View.VISIBLE);

Answer 1

 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 com.elc.hostel.LoginActivity.onCreate(LoginActivity.java:50)

Above stack indicates that mLogin is null.

Few cases in which findViewById() returns null are :

1. findViewById() is called before setContentView().
2. id passed in findViewById() is not present in layout set in setContentView().

In this case, setContentView() is not called. Add setContentView() before accessing the views will solve the problem.