getActionBar().setDisplayHomeAsUpEnabled(true), ->java.lang.NullPointerException 无法解决 [英] getActionBar().setDisplayHomeAsUpEnabled(true), -> java.lang.NullPointerException Impossible to resolve

问题描述

我有以下问题:
getActionBar().setDisplayHomeAsUpEnabled(true);

java.lang.NullPointerException

java.lang.NullPointerException

我知道有很多帖子有同样的问题，但我可以解决这个问题.我尝试了我找到的所有解决方案，但没有人帮助我，我使用这个 Fragment and Drawer (Android)

I know that there are many post with the same problem, but this one i can resolve. I tried all solution that I found, but no one help me, I work this Fragment and Drawer (Android)

这是代码:MainActivity.java

this is the code : MainActivity.java

    package com.example.erik.test2;


import android.app.Activity;

import android.os.Bundle;
import android.support.design.widget.NavigationView;
import android.support.v4.app.ActionBarDrawerToggle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.view.GravityCompat;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.MenuItem;
import android.view.View;
import android.view.Window;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.TextView;


public class MainActivity extends AppCompatActivity {
    private DrawerLayout mDrawer;
    private Toolbar toolbar;
    private ActionBarDrawerToggle mDrawerToggle;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
       //requestWindowFeature(Window.FEATURE_ACTION_BAR);
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);


        // Set a Toolbar to replace the ActionBar.
        toolbar = (Toolbar) findViewById(R.id.toolbar);
        //setSupportActionBar(toolbar);
       // getSupportActionBar().setDisplayHomeAsUpEnabled(true);


        // Find our drawer view
        mDrawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        NavigationView nvDrawer = (NavigationView) findViewById(R.id.nvView);
        setupDrawerContent(nvDrawer);

        //---- Open Drawer on clic icon BUT CAUSE NULLPOINT EXEPTION--- //
       //*
        getActionBar().setDisplayHomeAsUpEnabled(true);
        getActionBar().setHomeButtonEnabled(true);//*/

        /*
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
        getSupportActionBar().setHomeButtonEnabled(true);//*/
//*
       mDrawerToggle = new ActionBarDrawerToggle(this, mDrawer,  R.drawable.logo_tam, R.string.ouverture, R.string.fermeture) {
            public void onDrawerClosed(View view) {
                getActionBar().setTitle(R.string.titre);
                invalidateOptionsMenu();
            }

            public void onDrawerOpened(View drawerView) {
                getActionBar().setTitle(R.string.titre_apres_ouverture);
                invalidateOptionsMenu();
            }
        };
        mDrawer.setDrawerListener(mDrawerToggle); //*

        //-----------------------------------------------------------------------------------------------------------------
/*
        mDrawerToggle = new ActionBarDrawerToggle(this, mDrawer, R.drawable.logo_tam, R.string.ouverture, R.string.fermeture ) {
            public void onDrawerClosed(View view) {
                super.onDrawerClosed(view);
                getActionBar().setTitle("Fermer");
            }
            public void onDrawerOpened(View drawerView) {
                super.onDrawerOpened(drawerView);
                getActionBar().setTitle("Ouvert");
            }
        };
        // Set the drawer toggle as the DrawerListener
        mDrawer.setDrawerListener(mDrawerToggle);*/

       // getActionBar().setDisplayHomeAsUpEnabled(true);
        //getActionBar().setHomeButtonEnabled(true);
    }


    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // The action bar home/up action should open or close the drawer.
        switch (item.getItemId()) {
            case android.R.id.home:
                mDrawer.openDrawer(GravityCompat.START);
                return true;
        }
        return super.onOptionsItemSelected(item);
    }

    // Make sure this is the method with just `Bundle` as the signature
    @Override
    protected void onPostCreate(Bundle savedInstanceState) {
        super.onPostCreate(savedInstanceState);
    }

    private void setupDrawerContent(NavigationView navigationView) {
        navigationView.setNavigationItemSelectedListener(
                new NavigationView.OnNavigationItemSelectedListener() {
                    @Override
                    public boolean onNavigationItemSelected(MenuItem menuItem) {
                        selectDrawerItem(menuItem);
                        return true;
                    }
                });
    }

    public void selectDrawerItem(MenuItem menuItem) {
        // Create a new fragment and specify the planet to show based on
        // position
        Fragment fragment = null;
        Class fragmentClass;
        switch(menuItem.getItemId()) {
            case R.id.nav_first_fragment:
                fragmentClass = FirstFragment.class;
                break;
            case R.id.nav_second_fragment:
                fragmentClass = SecondFragment.class;
                break;
            case R.id.nav_third_fragment:
                fragmentClass = ThirdFragment.class;
                break;
            default:
                fragmentClass = FirstFragment.class;
        }

        try {
            fragment = (Fragment) fragmentClass.newInstance();
        } catch (Exception e) {
            e.printStackTrace();
        }

        // Insert the fragment by replacing any existing fragment
        FragmentManager fragmentManager = getSupportFragmentManager();
        fragmentManager.beginTransaction().replace(R.id.flContent, fragment).commit();

        // Highlight the selected item, update the title, and close the drawer
        menuItem.setChecked(true);
        setTitle(menuItem.getTitle());
        mDrawer.closeDrawers();
    }

}

styles.xml 的代码(我已经尝试将主题更改为 Theme.AppCompat.Light 但它不起作用)

code of styles.xml (I already try to change theme to Theme.AppCompat.Light but it doesn't work)

<resources>

    <!-- Base application theme. -->
    <style name="AppTheme" parent="Theme.AppCompat.Light.NoActionBar">
        <!-- Customize your theme here. -->
        <item name="colorPrimary">@color/colorPrimary</item>
        <item name="colorPrimaryDark">@color/colorPrimaryDark</item>
        <item name="colorAccent">@color/colorAccent</item>
    </style>

</resources>

toolbar.xml 代码

code of 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:id="@+id/toolbar"
    android:layout_height="wrap_content"
    android:layout_width="match_parent"
    android:fitsSystemWindows="true"
    android:minHeight="?attr/actionBarSize"
    app:theme="@style/Theme.AppCompat.Light"
    android:background="?attr/colorPrimaryDark">
</android.support.v7.widget.Toolbar>

代码activity_main.xml

Code activity_main.xml

<!-- This DrawerLayout has two children at the root  -->
<android.support.v4.widget.DrawerLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/drawer_layout"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <!-- This LinearLayout represents the contents of the screen  -->
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical">

        <!-- The ActionBar displayed at the top -->
        <!--  <include
              layout="@layout/toolbar"
              android:layout_width="match_parent"
              android:layout_height="wrap_content" /> -->

          <!-- The main content view where fragments are loaded -->
        <FrameLayout
            android:id="@+id/flContent"
            android:layout_width="match_parent"
            android:layout_height="match_parent" />
    </LinearLayout>

    <!-- The navigation drawer that comes from the left -->
    <!-- Note that `android:layout_gravity` needs to be set to 'start' -->
    <android.support.design.widget.NavigationView
        android:id="@+id/nvView"
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:layout_gravity="start"
        android:background="@android:color/white"
        app:menu="@menu/drawer_view"
        app:headerLayout="@layout/nav_header"
        />
</android.support.v4.widget.DrawerLayout>

和清单代码

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.erik.test2">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity
            android:name=".MainActivity"
            android:label="@string/app_name">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

    </application>

</manifest>

我尝试了许多主题选项，但它在我的 ActivityMain 中不起作用，您可以在评论中看到我尝试过的所有测试.

I tried many options with the Theme but it doesn't work in my ActivityMain in the comment you can see all the tests that I tried.

推荐答案

你有两种方法:
1. 使用 getSupportActionBar() 代替 getActionBar()
2.更改您的活动以扩展ActionBarActivity

you have two ways:
1. use getSupportActionBar() instead getActionBar()
2. change your activity to extend ActionBarActivity

这篇关于getActionBar().setDisplayHomeAsUpEnabled(true), ->java.lang.NullPointerException 无法解决的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！