Java Nullpunkt exxception auftritt, während der Listenansicht

Question

Fehler beim Versuch, aus:

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference

Nicht alles bekommen, sind verwirrt!

Java-Code:

package com.listview; 

import android.app.Activity; 
import android.support.v7.app.AppCompatActivity; 
import android.os.Bundle; 
import android.widget.ArrayAdapter; 
import android.widget.ListView; 


public class MainActivity extends Activity { 
    ListView lvAndroid; 
    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    String[] arrAndroid= {"Cupcake","Donut","Jellybean","Kitkat","Lollipop","Marshmellow","Nougat"}; 



    ArrayAdapter adapt = new ArrayAdapter<>(this,R.layout.lv,arrAndroid); 
    lvAndroid=(ListView)findViewById(R.id.lvAndroid); 

    lvAndroid.setAdapter(adapt); 
    } 
} 

Lv.xml

<?xml version="1.0" encoding="utf-8"?> 
<!-- Single List Item Design --> 

<TextView xmlns:android="http://schemas.android.com/apk/res/android" 
    android:id="@+id/label" 
    android:layout_width="fill_parent" 
    android:layout_height="fill_parent" 
    android:padding="10dip" 
    android:textSize="10sp" 
    android:textStyle="bold" > 
</TextView> 

Haupttätigkeit:

<?xml version="1.0" encoding="utf-8"?> 
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" 
xmlns:tools="http://schemas.android.com/tools" 
android:id="@+id/activity_main" 
android:layout_width="match_parent" 
android:layout_height="match_parent" 
android:paddingBottom="@dimen/activity_vertical_margin" 
android:paddingLeft="@dimen/activity_horizontal_margin" 
android:paddingRight="@dimen/activity_horizontal_margin" 
android:paddingTop="@dimen/activity_vertical_margin" 
tools:context="com.listview.MainActivity"> 

<ListView 
    android:layout_width="match_parent" 
    android:layout_height="match_parent" 
    android:id="@+id/lvAndroid" 
    android:layout_alignParentTop="true" 
    android:layout_alignParentStart="true" /> 
</RelativeLayout> 

Answer 1

Sie sind nicht Methode setContentView() in oncreate aufrufen.

setContentView(R.layout.activity_main); 

Answer 2

verpasst Sie dies in onCreate

setContentView(R.layout.activity_main); 

Answer 3

Zum einen fügen setContentView (YourLayout); dann,

ArrayAdapter<String> adapt = new ArrayAdapter<String>(Activity.this,android.R.layout.simple_list_item,arrAndroid);