NPE 在添加抽屉监听器 [重复]

Question

我目前正在尝试编写一个小型应用程序，现在我正在尝试在我的应用程序中使用导航抽屉。这不适用于我在drawer.addDrawerListener(toggle); 获得 NPE 的原因。据我所知，其他一切似乎都很好，并且应用程序正常工作，直到添加抽屉。提前谢谢。

import android.content.Intent;
import android.content.res.Configuration;
import android.net.ConnectivityManager;
import android.net.NetworkInfo;
import android.os.Bundle;
import android.view.View;
import android.support.design.widget.NavigationView;
import android.support.v4.view.GravityCompat;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBarDrawerToggle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.TextView;
import de.nocompany.gotthold.gw2companion.Tools.GwApiAccess;

public class MainActivity extends AppCompatActivity implements NavigationView.OnNavigationItemSelectedListener {

    private DrawerLayout drawer;
    private Toolbar toolbar;
    private ActionBarDrawerToggle toggle;
    private NavigationView navigationView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        toolbar = (Toolbar) findViewById(R.id.toolbar);
        drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        navigationView = (NavigationView) findViewById(R.id.nav_view);
        setSupportActionBar(toolbar);
        setContentView(R.layout.activity_main);

        toggle = new ActionBarDrawerToggle(this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close) {

            /** Called when a drawer has settled in a completely closed state. */
            public void onDrawerClosed(View view) {
                super.onDrawerClosed(view);
                //getActionBar().setTitle(mTitle);
                invalidateOptionsMenu(); // creates call to onPrepareOptionsMenu()
            }

            /** Called when a drawer has settled in a completely open state. */
            public void onDrawerOpened(View drawerView) {
                super.onDrawerOpened(drawerView);
                //getActionBar().setTitle(mDrawerTitle);
                invalidateOptionsMenu(); // creates call to onPrepareOptionsMenu()
            }
        };

        drawer.addDrawerListener(toggle);
        navigationView.setNavigationItemSelectedListener(this);

        ConnectivityManager connMgr = (ConnectivityManager) this.getSystemService(CONNECTIVITY_SERVICE);
        NetworkInfo networkInfo = connMgr.getActiveNetworkInfo();
        View navHeader = navigationView.getHeaderView(0);
        if (networkInfo != null && networkInfo.isConnected()) {
            TextView connectionStatus = (TextView) navHeader.findViewById(R.id.connection_status);
            connectionStatus.setText(R.string.common_online);
        } else {
            TextView connectionStatus = (TextView) navHeader.findViewById(R.id.connection_status);
            connectionStatus.setText(R.string.common_offline);
        }

        toggle.syncState();

        GwApiAccess gwApiAccess = new GwApiAccess(this,"https://api.guildwars2.com/v2/items/30704?lang=de");
    }

    @Override
    protected void onPostCreate(Bundle savedInstanceState) {
        super.onPostCreate(savedInstanceState);
        // Sync the toggle state after onRestoreInstanceState has occurred.
        toggle.syncState();
    }

    @Override
    public void onConfigurationChanged(Configuration newConfig) {
        super.onConfigurationChanged(newConfig);
        toggle.onConfigurationChanged(newConfig);
    }


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

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    public boolean onPrepareOptionsMenu(Menu menu) {
        // If the nav drawer is open, hide action items related to the content view
        boolean drawerOpen = drawer.isDrawerOpen(GravityCompat.START);
        return super.onPrepareOptionsMenu(menu);
    }


    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        //int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        //if (id == R.id.action_settings) {
        //    return true;
        //}

        return super.onOptionsItemSelected(item);
    }

    @SuppressWarnings("StatementWithEmptyBody")
    @Override
    public boolean onNavigationItemSelected(MenuItem item) {
        // Handle navigation view item clicks here.
        int id = item.getItemId();

        if (id == R.id.nav_profile) {
            Intent intent = new Intent(this,profile.class);
            startActivity(intent);
        } else if (id == R.id.nav_itemDB) {
            Intent intent = new Intent(this,items.class);
            startActivity(intent);
        } else if (id == R.id.nav_guilds) {
            Intent intent = new Intent(this,guilds.class);
            startActivity(intent);
        } else if (id == R.id.nav_maps) {
            Intent intent = new Intent(this,map.class);
            startActivity (intent);
        } else if (id == R.id.settings) {
            Intent intent = new Intent(this,SettingsActivity.class);
            startActivity (intent);
        }

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

Answer 1

先改变这部分

 @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        toolbar = (Toolbar) findViewById(R.id.toolbar);
        drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        navigationView = (NavigationView) findViewById(R.id.nav_view);
        setSupportActionBar(toolbar);
        setContentView(R.layout.activity_main);

到这里

 @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);       
        setContentView(R.layout.activity_main);
        toolbar = (Toolbar) findViewById(R.id.toolbar);
        drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        navigationView = (NavigationView) findViewById(R.id.nav_view);
        setSupportActionBar(toolbar);

因为在此之前您没有将布局设置为 Actiivty，所以您必须访问它的内部元素，以便 Studio 找不到它，它会为您提供 NullpointerExeception。