我试图建立一个免费的图书交换应用程序几个月。卡在指向空对象的NavigationView代码中。我已经为你的persual提取了完整的java。这是我第一次求助。
我不明白的是，有：
NavigationView navigationView = findViewById（R.id.nav_view）; navigationView.setNavigationItemSelectedListener（本）
引用于：
公共类MainmainActivity扩展AppCompatActivity实现NavigationView.OnNavigationItemSelectedListener
但错误消息指向NullPointerException。如下：
java.lang.RuntimeException：无法启动活动ComponentInfo {booksavenue.co.booksavenue / booksavenue.co.booksavenue.MainmainActivity}：java.lang.NullPointerException：尝试调用虚方法＆＃39; void android .support.design.widget.NavigationView.setNavigationItemSelectedListener（android.support.design.widget.NavigationView $ OnNavigationItemSelectedListener）＆＃39;在空对象引用上
package booksavenue.co.booksavenue;
import android.support.annotation.NonNull;
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.os.Bundle;
import android.support.v7.widget.Toolbar;
import android.view.MenuItem;
import android.widget.Toast;
public class MainmainActivity extends AppCompatActivity implements NavigationView.OnNavigationItemSelectedListener {
private DrawerLayout drawer;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
drawer = findViewById(R.id.drawer_layout);
NavigationView navigationView = findViewById(R.id.nav_view);
navigationView.setNavigationItemSelectedListener(this);
ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(this, drawer, toolbar,
R.string.navigation_drawer_open, R.string.navigation_drawer_close);
drawer.addDrawerListener(toggle);
toggle.syncState();
if (savedInstanceState == null) {
getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container,
new MessageFragment()).commit();
navigationView.setCheckedItem(R.id.nav_message);
}
}
@Override
public boolean onNavigationItemSelected(@NonNull MenuItem item) {
switch (item.getItemId()) {
case R.id.nav_message:
getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container,
new MessageFragment()).commit();
break;
case R.id.nav_chat:
getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container,
new ChatFragment()).commit();
break;
case R.id.nav_profile:
getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container,
new ProfileFragment()).commit();
break;
case R.id.nav_share:
Toast.makeText(this, "Share", Toast.LENGTH_SHORT).show();
break;
case R.id.nav_send:
Toast.makeText(this, "Send", Toast.LENGTH_SHORT).show();
break;
}
drawer.closeDrawer(GravityCompat.START);
return true;
}
@Override
public void onBackPressed() {
if (drawer.isDrawerOpen(GravityCompat.START)) {
drawer.closeDrawer(GravityCompat.START);
} else {
super.onBackPressed();
}
}
}
最初我认为它是android：主题NoActionBar问题。但似乎事实并非如此。附上剧本文件供你仔细阅读
<?xml version="1.0" encoding="utf-8"?>
<application
android:allowBackup="true"
android:icon="@mipmap/ic_launcher"
android:label="@string/app_name"
android:roundIcon="@mipmap/ic_launcher_round"
android:supportsRtl="true"
android:theme="@style/AppTheme">
<activity android:name=".SplashscreenActivity">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity android:name=".MainActivity" />
<activity android:name=".forgetpassActivity" />
<activity android:name=".NewuserActivity" />
<activity android:name=".SecondActivity" />
<activity android:name=".TncActivity" />
<activity
android:name=".MainmainActivity"
android:theme="@style/AppTheme.NoActionBar">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
</intent-filter>
</activity>
</application>
**
**
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="176dp"
android:background="@color/colorAccent"
android:gravity="bottom"
android:orientation="vertical"
android:padding="16dp"
android:theme="@style/ThemeOverlay.AppCompat.Dark">
<ImageView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:src="@mipmap/ic_launcher_round" />
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:paddingTop="8dp"
android:text="BooksAvenue"
android:textAppearance="@style/TextAppearance.AppCompat.Body1" />
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="booksavenue@yahoo.com.sg" />
</LinearLayout>
Activity_mainmain.xml
<?xml version="1.0" encoding="utf-8"?>
<android.support.v4.widget.DrawerLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="@+id/drawer_layout"
android:fitsSystemWindows="true"
tools:context="booksavenue.co.booksavenue.MainmainActivity"
tools:openDrawer="start">
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">
<android.support.v7.widget.Toolbar
android:id="@+id/toolbar"
android:layout_width="match_parent"
android:layout_height="?android:attr/actionBarSize"
android:background="@color/colorPrimary"
android:elevation="4dp"
android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar"
app:popupTheme="@style/ThemeOverlay.AppCompat.Light" />
<FrameLayout
android:id="@+id/fragment_container"
android:layout_width="match_parent"
android:layout_height="match_parent" />
</LinearLayout>
<android.support.design.widget.NavigationView
android:id="@+id/nav_view"
android:layout_width="wrap_content"
android:layout_height="match_parent"
android:layout_gravity="start"
app:headerLayout="@layout/nav_header"
app:menu="@menu/drawer_menu" />
</android.support.v4.widget.DrawerLayout>
styles.xml
<resources>
<!-- Base application theme. -->
<style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">
<!-- Customize your theme here. -->
<item name="colorPrimary">@color/colorPrimary</item>
<item name="colorPrimaryDark">@color/colorPrimaryDark</item>
<item name="colorAccent">@color/colorAccent</item>
</style>
<style name="AppTheme.NoActionBar">
<item name="windowActionBar">false</item>
<item name="windowNoTitle">true</item>
</style>
</resources>