我是Kotlin的新手,所以在解决此问题时遇到了麻烦。
我创建了一个底部导航视图,用于在活动之间进行切换。当我调用BottomNavigationView.OnNavigationItemSelectedListener时,我的应用程序崩溃了,我也不知道为什么。谁能帮忙吗?
我已经搜索过,但只找到片段的答案。
我的MainActivity代码:
class MainActivity : AppCompatActivity() {
private val navigasjonen = BottomNavigationView.OnNavigationItemSelectedListener { item ->
when (item.itemId) {
R.id.home -> {
return@OnNavigationItemSelectedListener false
}
R.id.leggTilNavigasjon -> {
val intent = Intent(this@MainActivity, AddRec::class.java)
startActivity(intent)
return@OnNavigationItemSelectedListener true
}
R.id.favoritter -> {
val intent = Intent(this@MainActivity, MyRecipes::class.java)
startActivity(intent)
return@OnNavigationItemSelectedListener true
}
R.id.utforsk -> {
return@OnNavigationItemSelectedListener true
}
}
false
}
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
val bottomNavigation = findViewById<BottomNavigationView>(R.id.bottom_navigation)
//bottomNavigation.setSelectedItemId(R.id.home)
bottomNavigation.setOnNavigationItemSelectedListener(navigasjonen)
}
}
我的xml代码:
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 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:background="@color/colorLys"
android:id="@+id/home">
<com.google.android.material.bottomnavigation.BottomNavigationView
android:id="@+id/navigationView"
android:layout_width="match_parent"
android:layout_height="wrap_content"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
android:background="?android:attr/windowBackground"
app:menu="@menu/navigasjon" />
</androidx.constraintlayout.widget.ConstraintLayout>
我的错误:
2020-01-27 14:20:06.197 20309-20309 / com.example.foodleaks E / AndroidRuntime:FATAL EXCEPTION:main流程:com.example.foodleaks,PID:20309java.lang.RuntimeException:无法启动活动ComponentInfo {com.example.foodleaks / com.example.foodleaks.MainActivity}:java.lang.NullPointerException:尝试调用虚拟方法'void com.google.android.material.bottomnavigation。空对象引用上的BottomNavigationView.setOnNavigationItemSelectedListener(com.google.android.material.bottomnavigation.BottomNavigationView $ OnNavigationItemSelectedListener)'在android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2913)在android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)在android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)在android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)在android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)在android.app.ActivityThread $ H.handleMessage(ActivityThread.java:1808)在android.os.Handler.dispatchMessage(Handler.java:106)在android.os.Looper.loop(Looper.java:193)在android.app.ActivityThread.main(ActivityThread.java:6669)在java.lang.reflect.Method.invoke(本机方法)在com.android.internal.os.RuntimeInit $ MethodAndArgsCaller.run(RuntimeInit.java:493)在com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)引起原因:java.lang.NullPointerException:尝试在空对象引用上调用虚拟方法“ void com.google.android.material.bottomnavigation.BottomNavigationView.setOnNavigationItemSelectedListener(com.google.android.material.bottomnavigation.BottomNavigationView $ OnNavigationItemSelectedListener)”在com.example.foodleaks.MainActivity.onCreate(MainActivity.kt:39)在android.app.Activity.performCreate(Activity.java:7136)在android.app.Activity.performCreate(Activity.java:7127)在android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271)在android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2893)在android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)在android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)在android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)在android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)在android.app.ActivityThread $ H.handleMessage(ActivityThread.java:1808)在android.os.Handler.dispatchMessage(Handler.java:106)在android.os.Looper.loop(Looper.java:193)在android.app.ActivityThread.main(ActivityThread.java:6669)在java.lang.reflect.Method.invoke(本机方法)在com.android.internal.os.RuntimeInit $ MethodAndArgsCaller.run(RuntimeInit.java:493)在com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)。
val bottomNavigation = findViewById<BottomNavigationView>(R.id.bottom_navigation)
with:
val bottomNavigation = findViewById<BottomNavigationView>(R.id.navigationView)
val bottomNavigation = findViewById<BottomNavigationView>(R.id.bottom_navigation)
并且在布局中具有ID:
android:id="@+id/navigationView"
您可以更改布局上的ID:
android:id="@+id/bottom_navigation"
或在findViewById中进行更改:
val bottomNavigation = findViewById<BottomNavigationView>(R.id.navigationView)
保持一致,以便它可以找到BottomNavigationView元素,并且不会因空对象引用而崩溃。
Attempt to invoke virtual method 'void com.google.android.material.bottomnavigation.BottomNavigationView.setOnNavigationItemSelectedListener(com.google.android.material.bottomnavigation.BottomNavigationView$OnNavigationItemSelectedListener)' on a null object reference
您的BottomNavigationView
为空。为什么?
查看您在布局中的声明:<com.google.android.material.bottomnavigation.BottomNavigationView
android:id="@+id/navigationView
这就是您查找视图的方式:
findViewById<BottomNavigationView>(R.id.bottom_navigation)
您使用了错误的视图ID。