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I want to add a Back button in a toolbar that I created because I deleted the default one to have the material design effect with some tabs, but every time I add the code to make that button appear, Android gives an exception saying this:

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.juan.sdfnsdfnskdfnskdfmsfd, PID: 13348
java.lang.NullPointerException: Attempt to invoke virtual method 'int android.support.v4.widget.DrawerLayout.getDrawerLockMode(int)' on a null object reference
at android.support.v7.app.ActionBarDrawerToggle.toggle(ActionBarDrawerToggle.java:284)
at android.support.v7.app.ActionBarDrawerToggle$1.onClick(ActionBarDrawerToggle.java:202)
at android.view.View.performClick(View.java:6308)
at android.view.View$PerformClick.run(View.java:23969)
at android.os.Handler.handleCallback(Handler.java:751)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6823)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1557)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1445)

I do not understand what it means with a null object reference if I only write the corresponding code in the toolbar that I created.

This is the toolbar.xml:

<android.support.v7.widget.Toolbar xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:background="@color/toolbar_morado"
android:id="@+id/toolbar_lista_compras"
android:minHeight="?attr/actionBarSize">

<RelativeLayout
android:layout_width="match_parent"
android:layout_height="wrap_content">

<TextView
    android:id="@+id/contador_toolbar"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Lista de compras"
    android:textSize="18sp"
    android:textColor="#ffffff"/>

</RelativeLayout>

</android.support.v7.widget.Toolbar>

This is where I implement the toolbar:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">

    <android.support.design.widget.AppBarLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar"
        android:id="@+id/appBarLayoutCalcular">

        <include layout="@layout/toolbar"></include>

        <android.support.design.widget.TabLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:id="@+id/tablayoutCalcular"
            app:tabGravity="fill"
            app:tabMode="fixed"
            android:background="@color/toolbar_anaranjado"
            app:tabTextAppearance="@style/myTabSize"
            app:tabIndicatorColor="#5c1be1"/>

    </android.support.design.widget.AppBarLayout>

    <android.support.v4.view.ViewPager
        android:id="@+id/viewPagerCalcular"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

</LinearLayout>

This is my class:

public class CalculateMaterials extends AppCompatActivity{

DrawerLayout drawerLayout;
ActionBarDrawerToggle actionBarDrawerToggle;
Toolbar toolbar;
NavigationView navigationView;
TabLayout tabLayout;
ViewPager viewPager;
ViewPagerAdapter viewPagerAdapter;
String tabSeleccionado;

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

    toolbar = (Toolbar)findViewById(R.id.toolbar_shopping_list);
    toolbar.setBackgroundColor(ContextCompat.getColor(getApplicationContext(), R.color.toolbar_anaranjado));
    setSupportActionBar(toolbar);

    // text for the toolbar
    TextView textoToolbar = (TextView)findViewById(R.id.contador_toolbar);
    textoToolbar.setText("Calcular los materiales");

    // status bar color
    if(Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {

        getWindow().setStatusBarColor(ContextCompat.getColor(getApplicationContext(), R.color.statusBar_anaranjado));

    }


    // adding toolbar back button
    // this is where I have the error 
    if (toolbar != null) {
        toolbar.setNavigationIcon(R.drawable.backButton);
        toolbar.setNavigationOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                onBackPressed();
            }
        });
    }

    drawerLayout = (DrawerLayout)findViewById(R.id.drawer_layout);

    actionBarDrawerToggle = new ActionBarDrawerToggle(this, drawerLayout, toolbar, R.string.drawer_open, R.string.drawer_close);

    navigationView = (NavigationView)findViewById(R.id.navigationView);

    tabLayout = (TabLayout)findViewById(R.id.tablayoutCalcular);

    viewPager = (ViewPager)findViewById(R.id.viewPagerCalcular);

    // adding the tabs
    viewPagerAdapter = new ViewPagerAdapter(getSupportFragmentManager());
    viewPagerAdapter.addFragments(new FragmentCalcular(), "Con area");
    viewPagerAdapter.addFragments(new FragmentCalcular2(), "Con metros");

    viewPager.setAdapter(viewPagerAdapter);

    tabLayout.setupWithViewPager(viewPager);

I also try with: 

// add back arrow to toolbar
if (getSupportActionBar() != null){
    getSupportActionBar().setDisplayHomeAsUpEnabled(true);
    getSupportActionBar().setDisplayShowHomeEnabled(true);
}

That also gives the same error.

So, Can anyone help me with this problem?

Edit

Thanks to everyone who tried to help me with this problem. I finally found the error.

Some time ago, I tried to add a navigationView in all the activities, but I forgot to delete the necessary code in the class, that's why the drawerLayout doesnt appears in the xml.

So, when I delete the drawerLayout variable, everything works!

juan
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    Possible duplicate of [What is a NullPointerException, and how do I fix it?](https://stackoverflow.com/questions/218384/what-is-a-nullpointerexception-and-how-do-i-fix-it) – AskNilesh Dec 26 '17 at 06:14
  • Where is drawer_layout located ? I do not see in your xml above . – ADM Dec 26 '17 at 06:16
  • Thanks for the answer, but I already solved the problem deleting some extra variables that I dont use! – juan Dec 26 '17 at 17:55

6 Answers6

5

Try this

    toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    getSupportActionBar().setDisplayHomeAsUpEnabled(true);
    toolbar.setNavigationOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            onBackPressed();
        }
    });
akash bs
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1

Try this.

   toolbar = (Toolbar) findViewById(R.id.toolbar_editprofile);
    setSupportActionBar(toolbar);
    getSupportActionBar().setDisplayShowTitleEnabled(false):
    toolbar.setNavigationIcon(getResources().getDrawable(R.drawable.back_arrow));
    toolbar.setNavigationOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            finish();
        }
    });
mehul chauhan
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0

It seems like drawerLayout is null. I can't able to find find your "drawer_layout" in XML.

Make sure you are using the same id for the navigation drawer in findViewById(R.id.drawer_layout) and in the layout file.

Gowtham Subramaniam
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0

First thing where is your DrawerLayout ? i cant find it from your question may be you forgot to post it or not created yet.

make sure you have to declare correct ID for DrawerLayout so check drawerLayout = (DrawerLayout)findViewById(R.id.drawer_layout); of drawer_layout.xml

Error Line: i have found error in this line may be this is not your answer but should have to correct you 

TextView textoToolbar = (TextView)findViewById(R.id.contador_toolbar);

Try This

toolbar = (Toolbar)findViewById(R.id.toolbar_shopping_list);
    toolbar.setBackgroundColor(ContextCompat.getColor(getApplicationContext(), R.color.toolbar_anaranjado));
    setSupportActionBar(toolbar);


    // text for the toolbar
    TextView textoToolbar = (TextView)toolbar.findViewById(R.id.contador_toolbar);
    textoToolbar.setText("Calcular los materiales");
ND1010_
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0

In onCreate method paste this:

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

     Toolbar   toolbar = (Toolbar) findViewById(R.id.toolBarGig);
        setSupportActionBar(toolbar);
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
        getSupportActionBar().setDisplayShowHomeEnabled(true);
        getSupportActionBar().setDisplayShowTitleEnabled(false);

after this you have to override method:- 

@Override
    public boolean onSupportNavigateUp() {
        onBackPressed();
        return true;
    }

Try this. Hope it will helps you!

Kartik Shah
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0

First initialize the toolbar bar for back button

 Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
            setSupportActionBar(toolbar);
          getSupportActionBar().setDisplayHomeAsUpEnabled(true);

then call back button method this code is enough to go to previous activity else you can use intent method inside onBackPressed method to go back

//method on back button click

@Override
public void onBackPressed() {

    super.onBackPressed();

/*OR
  Intent forgot_password = new Intent( AddItemActivity.this,  HomeActivity.class);
                startActivity(forgot_password);
                finish();
        }*/

back button click functionality

 @Override
  public boolean onOptionsItemSelected(MenuItem item) {
    switch (item.getItemId()) {
        case android.R.id.home:
            onBackPressed();

            return true;

        default:
            return super.onOptionsItemSelected(item);
    }
}
Android Geek
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