ListView.set.adapter导致NullPointerException

我正在尝试检索SQLite值并加载到listView MainActivity.我正在按照本教程.

但是,当我的应用程序刚刚开始时,它崩溃并显示出来Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference.

好像我还没有初始化listView.setAdapter.我检查了教程,仍然无法弄清楚我在这里错过了什么.

活动A.

public class MainActivity extends AppCompatActivity {

    InfoAPI sqlcon;
    private SimpleCursorAdapter dataAdapter;
    private SQLiteDatabase database;
    private MyDatabaseHelper dbHelper;
    private ListView listView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        listView=(ListView)findViewById(R.id.listView2);
        setContentView(R.layout.activity_main);
        dbHelper = new MyDatabaseHelper(this);
        sqlcon = new InfoAPI(this);
        BuildList();

    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        MenuInflater inflater=getMenuInflater();
        inflater.inflate(R.menu.create_menu, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        switch(item.getItemId()) {
            case R.id.add: // create new file
                View menuItemView = findViewById(R.id.add);
                PopupMenu po = new PopupMenu(MainActivity.this, menuItemView); //for drop-down menu
                po.getMenuInflater().inflate(R.menu.popup_menu, po.getMenu());
                po.setOnMenuItemClickListener(new PopupMenu.OnMenuItemClickListener() {
                    public boolean onMenuItemClick(MenuItem item) {
                        Toast.makeText(MainActivity.this, "You Clicked : " + item.getTitle(), Toast.LENGTH_SHORT).show();
                        if ("Create New File".equals(item.getTitle()) ) {
                            Intent intent = new Intent(MainActivity.this, Information.class);  // go to Information class
                            startActivity(intent);

                        }
                        return true;
                    }
                });
                po.show(); //showing popup menu
        }
        return super.onOptionsItemSelected(item);
    }

    public void BuildList()
    {
        sqlcon.open();
        Cursor cursor1=sqlcon.readData();

        String[] columns=new String[]{
        MyDatabaseHelper.Weather,MyDatabaseHelper.Date,MyDatabaseHelper.Status,MyDatabaseHelper.TimeIn_Info,MyDatabaseHelper.TimeOut_Info
        };

        int[] to=new int[]
         {
          R.id.weather, R.id.date,R.id.status,R.id.in,R.id.out
         };

        dataAdapter = new SimpleCursorAdapter(this, R.layout.retrieve_data,
                cursor1,
                columns,
                to,
                0);

        listView.setAdapter(dataAdapter);
    }

}

错误Logcat

12-03 05:53:49.622    2588-2588/? E/AndroidRuntime? FATAL EXCEPTION: main
    Process: com.example.project.myapplication, PID: 2588
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.project.myapplication/com.example.project.myapplication.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)

这两个线路,其中代码是指 BuildList();和 listView.setAdapter(dataAdapter);我知道这是一个常见的错误,并很快将被标记为复制,但我真的想不通.有人可以帮帮我吗 ？谢谢

在setContentView()之后初始化listview.

更改你的onCreate()方法:

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    listView=(ListView)findViewById(R.id.listView2);
    dbHelper = new MyDatabaseHelper(this);
    sqlcon = new InfoAPI(this);
    BuildList();

}