Question

我是Android的新手，并且对数据库知之甚少。

我创建了一个名为＆＃34; topics＆＃34;在解析服务器中并手动添加＆＃34; topicName＆＃34;列。

现在我想在Listview中检索这个topicName值/字符串但我收到错误

尝试调用虚方法＆＃39; void android.widget.ListView.setAdapter（android.widget.ListAdapter）＆＃39;在...上 null对象引用

任何帮助都将不胜感激。

代码：

public class topics_class1 extends AppCompatActivity {

@Override
protected void onCreate(Bundle savedInstanceState){
    super.onCreate(savedInstanceState);
    final ArrayList<String> topics1 = new ArrayList<>();
    final ListView listView = findViewById(R.id.listView_topics_class1);
    final ArrayAdapter arrayAdapter = new ArrayAdapter(this, android.R.layout.simple_list_item_1, topics1);

    ParseQuery<ParseObject> query = ParseQuery.getQuery("topics");        
    query.findInBackground(new FindCallback<ParseObject>() {
        @Override
        public void done(List<ParseObject> objects, ParseException e) {
            if (e == null){

                for (int i=0; i< objects.size(); i++) {
                    topics1.add(objects.get(i).getString("topicName"));
                }
                listView.setAdapter(arrayAdapter);

                }

            else {
                e.printStackTrace();
            }
        }

    });
 }
}

XML：

<?xml version="1.0" encoding="utf-8"?>

<ListView
    android:id="@+id/listView_topics_class1"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true"
    android:layout_alignParentTop="true"
    android:layout_marginTop="89dp" />

Answer 1

错误本身解释

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

你没有setContentView(layout)。这就是你得错的原因尝试设置 setContentView(R.layout.layoutname);

并相应地获取ListView

final ListView listView = (ListView)findViewById(R.id.listView_topics_class1);

如果您有任何其他问题，请告诉我

Answer 2

public class MainActivity extends Activity {
    ListView lv;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        lv=(ListView) findViewById(R.id.listView);
        ArrayAdapter adapter = new ArrayAdapter(this,
                android.R.layout.simple_list_item_1, android.R.id.text1, planets);

               ParseQuery<ParseObject> query = ParseQuery.getQuery("topics");
//query.selectKeys(Arrays.asList("topicName"));
query.findInBackground(new FindCallback<ParseObject>() {
    @Override
    public void done(List<ParseObject> objects, ParseException e) {
        if (e == null){


           // Toast.makeText(topics_class1.this, "success", Toast.LENGTH_LONG).show();
            for (int i=0; i< objects.size(); i++) {
                topics1.add(objects.get(i).getString("topicName"));
            }
            listView.setAdapter(arrayAdapter);

            }

        //}
        else {
            e.printStackTrace();
        }
    }

});
    }
}

按照这种方式实施

XML布局

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" 

xmlns:tools="http://schemas.android.com/tools" 

android:layoutwidth="matchparent" 

android:layoutheight="matchparent" 

android:paddingBottom="@dimen/activityverticalmargin" 

android:paddingLeft="@dimen/activityhorizontalmargin" 

android:paddingRight="@dimen/activityhorizontalmargin" 

android:paddingTop="@dimen/activityverticalmargin" 

tools:context=".MainActivity" > 

<ListView android:layoutwidth="fillparent" 

android:layoutheight="fillparent" 

android:id="@+id/listView"> 

</ListView> 

</LinearLayout>

从Parse Server中重新获取单列数据并在列表视图中显示

2 个答案: