Question

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.ferryfinal/com.example.ferryfinal.Timings}: java.lang.NullPointerException

我在调用i函数时得到空指针异常，即（在 Timings.java 中调用 display_all ）我想。

// CommentsDataSource.java

     public List<Comment> display_all()
      {
         // String args[]={ref_id};
          Comment comment=new Comment();
          //String q = "SELECT * FROM timings where ref_id = ?";
            String q= " select * from timings ";
          Cursor cursor=database.rawQuery(q, null);

          List<Comment> list = new ArrayList<Comment>(); 
           cursor.moveToFirst();
            while (!cursor.isAfterLast()) {
              comment = cursorToComment(cursor);
              list.add(comment);
              cursor.moveToNext();

            }
            // make sure to close the cursor
           // cursor.close();

            Log.w(MySQLiteHelper.class.getName(), "in display fn"+comment.get_Ref_Id());

            return list;
            }

// Route1.java

    ferry.setOnClickListener(new View.OnClickListener() {

                @Override
                public void onClick(View v) {
                    String ref_id="1";
                    Intent intent = new Intent(Route1.this, Timings.class);
                   intent.putExtra(EXTRA_MESSAGE, ref_id);
                   Log.w(MySQLiteHelper.class.getName(),intent.getStringExtra(EXTRA_MESSAGE));
                    startActivity(intent);

                }
            });



        }

// Timings.java

public class Timings extends Activity {
     final CommentsDataSource dt=null;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_timings);

        Intent intent = getIntent();
       String message = intent.getStringExtra(Route1.EXTRA_MESSAGE);
       Log.w(MySQLiteHelper.class.getName(),message+"hello");
        //String message = "1";
        List<Comment> values = dt.display_all();
        Log.w(MySQLiteHelper.class.getName(),values.toString()+"hello1");
        StringBuilder builder = new StringBuilder();
        Log.w(MySQLiteHelper.class.getName(),builder+"hello2");
        for (Comment c: values)

        {   
             Log.w(MySQLiteHelper.class.getName(),message+"hello3");
            builder.append(c.get_Ref_Id()).append(";")
                .append(c.get_Starting_From()).append(";")
                .append(c.get_Departure_Time()).append(";")
                .append(c.get_Destination()).append("_");
            Log.w(MySQLiteHelper.class.getName(),builder+"hello4");
        }
        //tv.setText(builder.toString());
......

Answer 1

根据您的代码，您将获得Timings.java类中的其中一行。

List<Comment> values = dt.display_all();

您尚未初始化dt ..首先初始化

Answer 2

在 Timings.java 中，您尚未初始化final CommentsDataSource dt=null;。

在dt中声明onCreate()，如下所示：

    dt=new CommentsDataSource();

android ...获取空指针异常

2 个答案: