java - Nullpointer on NavigationView

Question

I don't know why I always get this Nullpointer exception:

 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.google.android.material.navigation.NavigationView.setNavigationItemSelectedListener(com.google.android.material.navigation.NavigationView$OnNavigationItemSelectedListener)' on a null object reference
    at com.sys.photolocations.start_and_navigate.onCreate(start_and_navigate.java:32)

I overworked the code like five times but I still can't find the error :/ I also tried not implementing NavigationView.OnNavigationItemSelectedListener and instead of navigationView.setNavigationItemSelectedListener(this) I used navigationView.setNavigationItemSelectedListener(new OnNavigationItemSelectedListener......

but it still didn't work. Any ideas?

public class start_and_navigate extends AppCompatActivity implements NavigationView.OnNavigationItemSelectedListener {


    //Navigationbar
    private DrawerLayout drawerLayout;


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

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

        drawerLayout = findViewById(R.id.drawlayout);
        NavigationView navigationView = findViewById(R.id.nav_view);
        navigationView.setNavigationItemSelectedListener(this);


        ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(this, drawerLayout, toolbar,
                R.string.navigation_drawer_open, R.string.navigation_drawer_close);
        drawerLayout.addDrawerListener(toggle);
        toggle.syncState();

        if (savedInstanceState == null){
        getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container,
                new Map_Fragment()).commit();
        navigationView.setCheckedItem(R.id.nav_map);}

    }





    @Override
    public void onBackPressed() {
        if (drawerLayout.isDrawerOpen(GravityCompat.START)) {
            drawerLayout.closeDrawer(GravityCompat.START);
        } else {
            super.onBackPressed();
        }
    }

    @Override
    public boolean onNavigationItemSelected(@NonNull MenuItem item) {
        switch (item.getItemId()){
            case R.id.nav_add:
                getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container,
                        new Add_Location_Fragment()).commit();
                break;

            case R.id.nav_bug_report:
                getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container,
                        new Bug_Fragment()).commit();
                break;

            case R.id.nav_map:
                getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container,
                        new Map_Fragment()).commit();
                break;

            case R.id.nav_profile:
                getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container,
                        new Profile_Fragment()).commit();
                break;

            case R.id.nav_settings:
                getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container,
                        new Settings_Fragment()).commit();
                break;
        }

        drawerLayout.closeDrawer(GravityCompat.START);
        return true;
    }
}

XML-Code:

<?xml version="1.0" encoding="utf-8"?>
<androidx.drawerlayout.widget.DrawerLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/drawlayout"
    android:fitsSystemWindows="true"
    tools:context=".start_and_navigate">


    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical">

        <androidx.appcompat.widget.Toolbar
            android:id="@+id/toolbar"
            android:layout_width="match_parent"
            android:layout_height="?attr/actionBarSize"
            android:background="#C9C9C9"
            android:theme="@style/ThemeOverlay.AppCompat.Dark"
            app:popupTheme="@style/ThemeOverlay.AppCompat.Light"
            android:elevation="4dp"/>

        <FrameLayout
            android:id="@+id/fragment_container"
            android:layout_width="wrap_content"
            android:layout_height="match_parent" />

    </LinearLayout>

    <com.google.android.material.navigation.NavigationView
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:id="@+id/nav_view"
        android:layout_gravity="start"
        app:headerLayout="@layout/nav_header"
        app:menu="@menu/draw_menu"/>


</androidx.drawerlayout.widget.DrawerLayout>

question from:https://stackoverflow.com/questions/65648552/nullpointer-on-navigationview

Answer 1

The error message states clearly that function setNavigationItemSelectedListener is called on a null object. So, variable navigationView is null.

In function onCreate add at the beginning this

setContentView(R.layout.XmlFileName);

to connect your xml to the activity. Now findViewById will be able to return the correct view id.