Question

错误发生在setAdapter行上。我找不到错误所在。它与空异常无关。某些事情不正常

java.lang.RuntimeException：无法启动活动 ComponentInfo {com.example.myapplication / com.example.myapplication.Namelist}： java.lang.NullPointerException：尝试调用虚拟方法'void android.widget.ListView.setAdapter（android.widget.ListAdapter）' 空对象引用在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

主类：

public class Namelist extends AppCompatActivity {
String[] mobileArray = {"Android","IPhone","WindowsMobile","Blackberry",
        "WebOS","Ubuntu","Windows7","Max OS X"};
ListView l1;
ArrayAdapter<String> arrayAdapter;

@Override
protected void onCreate(Bundle savedInstanceState) {
    l1 = (ListView) findViewById(R.id.list);

    arrayAdapter = new ArrayAdapter<String>(Namelist.this,R.layout.activity_namelist,mobileArray);

   l1.setAdapter(arrayAdapter);

    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_namelist);

}

}

活动xml：

<?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"
tools:context=".Namelist">

<ListView
    android:id="@+id/list"
    android:layout_width="409dp"
    android:layout_height="729dp"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

提前感谢

Answer 1

您不能在l1之前引用setContentView(R.layout.activity_namelist)

此处正确的代码：

public class Namelist extends AppCompatActivity {

    String[] mobileArray = {"Android","IPhone","WindowsMobile","Blackberry","WebOS","Ubuntu","Windows7","Max OS X"};
    ListView l1;
    ArrayAdapter<String> arrayAdapter;

    @Override
    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_namelist);

        l1 = findViewById(R.id.list);

        arrayAdapter = new ArrayAdapter<String>(Namelist.this,android.R.layout.simple_list_item_1,mobileArray);
       l1.setAdapter(arrayAdapter);
    }
}

您还将错误的layout R.layout.activity_namelist传递给ArrayAdapter的第二个参数。您没有声明布局名称activity_namelist。因此，您可以使用默认的内置布局android.R.layout.simple_list_item_1询问上面的代码。

空对象引用上的android.widget.ListView.setAdapter（android.widget.ListAdapter）'

1 个答案: