终生无法解决 GetSupportActionBar().setDisplayHomeAsUpEnabled 可能会产生 NullPointerException

Question

我知道这已经被讨论了很多，但我已经尝试了所有方法，将 getSupportActionBar 更改为 getActionBar，将我的布局更改为片段 activity、操作栏 activity，将我的布局更改为工具栏。似乎没有什么可以消除该警告。感谢您的帮助。

哦，如果对 main_menu.xml 和 menu_main.xml 感到困惑，那是因为我有一个溢出菜单，我正试图将它切换到滑动抽屉菜单。

MainActivity.java

package me.paxana.alerta;

import android.app.Activity;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentTransaction;
import android.support.v4.app.FragmentManager;
import android.support.v7.app.ActionBar;
import android.support.v7.app.ActionBarDrawerToggle;
import android.app.Fragment;
import android.content.Intent;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBarActivity;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import android.view.MenuInflater;
import android.view.View;
import android.widget.AdapterView;
import android.widget.Button;
import android.widget.ListView;
import android.widget.RelativeLayout;
import android.widget.Toolbar;


import com.parse.ParseUser;

import java.util.ArrayList;
import java.util.List;

import butterknife.Bind;
import butterknife.ButterKnife;
import me.paxana.alerta.adapter.SlidingMenuAdapter;
import me.paxana.alerta.fragment.Fragment1;
import me.paxana.alerta.fragment.Fragment2;
import me.paxana.alerta.fragment.Fragment3;
import me.paxana.alerta.model.ItemSlideMenu;

public class MainActivity extends AppCompatActivity {
    public static final String TAG = MainActivity.class.getSimpleName();
    private List<ItemSlideMenu> listSliding;
    private SlidingMenuAdapter adapter;
    private ListView listViewSliding;
    private DrawerLayout drawerLayout;
    private android.support.v7.app.ActionBarDrawerToggle actionBarDrawerToggle;
    private Toolbar mToolbar;

    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        ButterKnife.bind(this);


        ParseUser currentUser = ParseUser.getCurrentUser();
             if (currentUser == null) {
                navigateToLogin();
            }

        else {
                Log.i(TAG, currentUser.getUsername());
            }

        listViewSliding = (ListView)findViewById(R.id.lv_sliding_menu);
        drawerLayout = (DrawerLayout)findViewById(R.id.drawer_layout);
        listSliding = new ArrayList<>();
        //add item for sliding list
        listSliding. add(new ItemSlideMenu(R.drawable.ic_action_settings, "Settings"));
        listSliding.add(new ItemSlideMenu(R.drawable.ic_action_about, "About"));
        listSliding.add(new ItemSlideMenu(R.mipmap.ic_launcher, "Android"));
        adapter = new SlidingMenuAdapter(this, listSliding);
        listViewSliding.setAdapter(adapter);


        //display icon to open/close slider
        mToolbar = (Toolbar) findViewById(R.id.toolbar);

        if (mToolbar != null)
        {
            setActionBar(mToolbar);
            getSupportActionBar().setDisplayHomeAsUpEnabled(true);
            getSupportActionBar().setElevation(0); // or other
        }

        //set title
        setTitle(listSliding.get(0).getTitle());
        //item selected
        listViewSliding.setItemChecked(0, true);
        //close menu
        drawerLayout.closeDrawer(listViewSliding);
        //handle on item click
        listViewSliding.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                //set title
                setTitle(listSliding.get(position).getTitle());
                //item selected
                listViewSliding.setItemChecked(position, true);
                //close menu
                drawerLayout.closeDrawer(listViewSliding);
            }

        });

        actionBarDrawerToggle = new ActionBarDrawerToggle(this, drawerLayout, R.string.drawer_opened, R.string.drawer_closed) {
            @Override
            public void onDrawerOpened(View drawerView) {
                super.onDrawerOpened(drawerView);
                invalidateOptionsMenu();
            }

            @Override
            public void onDrawerClosed(View drawerView) {
                super.onDrawerClosed(drawerView);
                invalidateOptionsMenu();
            }
        };

        drawerLayout.setDrawerListener(actionBarDrawerToggle);

    }

    private void navigateToLogin() {
        Intent intent = new Intent(this, LoginActivity.class);
        intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
        intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK);
        startActivity(intent);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        MenuInflater inflater = getMenuInflater();
        inflater.inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        if(actionBarDrawerToggle.onOptionsItemSelected(item)) {
            return true;
        }

        int itemId = item.getItemId();
        if (itemId == R.id.action_logout) {
            ParseUser.logOut();
            navigateToLogin();

        }
        return super.onOptionsItemSelected(item);
    }

    @Override
    protected void onPostCreate(Bundle savedInstanceState) {
        super.onPostCreate(savedInstanceState);
        actionBarDrawerToggle.syncState();
    }
//create method replace fragment
    private void replaceFragment(int pos) {
        android.support.v4.app.Fragment fragment = null;
        switch (pos) {
            case 0:
                fragment = new Fragment1();
                    break;
            case 1:
                fragment = new Fragment2();
                break;
            case 2:
                fragment = new Fragment3();
                break;
            default:
                fragment = new Fragment1();
                break;
        }
        if(null != fragment) {
            android.support.v4.app.FragmentManager fragmentManager = getSupportFragmentManager();
            android.support.v4.app.FragmentTransaction transaction = fragmentManager.beginTransaction();
            transaction.replace(R.id.main_content, fragment);
            transaction.addToBackStack(null);
            transaction.commit();

        }
    }
}

Activity_Main.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.v4.app.DrawerLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/drawer_layout"
    tools:context="me.paxana.alerta.MainActivity">

    <RelativeLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/main_content">
    </RelativeLayout>

    <ListView
        android:layout_width="200dp"
        android:layout_height="match_parent"
        android:id="@+id/lv_sliding_menu"
        android:background="#FFFFFF"
        android:choiceMode="singleChoice"
        android:layout_gravity="start" />
</android.support.v4.app.DrawerLayout>

Answer 1

setActionBar(mToolbar);
getSupportActionBar().foo;

将 setActionBar() 更改为 setSupportActionBar()。在您为 appcompat 库的包装器调用 'set' 方法之前，相应的 get 方法将为 return null。

终生无法解决 GetSupportActionBar().setDisplayHomeAsUpEnabled 可能会产生 NullPointerException

cannot for the life of me resolve GetSupportActionBar().setDisplayHomeAsUpEnabled may produce NullPointerException

java

android

slidingdrawer

slidingmenu

android-studio