应用程序正常打开，但是在打开此活动时显示“应用程序停止”。我不明白是什么问题。也许API级别与calendar.get（）不兼容？如果可以，我该如何解决？

public class SearchRide extends AppCompatActivity {

int p=1;
private int year,month,day;
Calendar calendar=Calendar.getInstance();
final EditText etDate=(EditText) findViewById(R.id.etDate);


protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_search_ride);






    TextView tFrom=(TextView) findViewById(R.id.tFrom);
    TextView tTo=(TextView) findViewById(R.id.tTo);
    EditText etFrom=(EditText) findViewById(R.id.etFrom);
    EditText etTo=(EditText) findViewById(R.id.etTo);
    Button bSearch=(Button) findViewById(R.id.bSearch);
    ImageView iplussign=(ImageView) findViewById(R.id.iplussign);
    ImageView iminussign=(ImageView) findViewById(R.id.iminussign);
    final TextView tNumber=(TextView) findViewById(R.id.tNumber);

    tNumber.setText(String.valueOf(p));
    iplussign.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            if(p<=9) {
                p++;
                tNumber.setText(String.valueOf(p));
            }
        }
    });
    iminussign.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            if(p>=2) {
                p--;
                tNumber.setText(String.valueOf(p));
            }
        }
    });
    etFrom.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent ListFrom=new Intent(SearchRide.this,ListFrom.class);
            SearchRide.this.startActivity(ListFrom);


        }
    });
    etTo.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent ToList =new Intent(SearchRide.this,ListTo.class);
            SearchRide.this.startActivity(ToList);
        }
    });

    etDate.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            new DatePickerDialog(SearchRide.this,listener,calendar.get(Calendar.DAY_OF_MONTH),calendar.get(Calendar.MONTH),calendar.get(Calendar.YEAR)).show();

        }
    });
}
DatePickerDialog.OnDateSetListener listener= new DatePickerDialog.OnDateSetListener() {
    @Override
    public void onDateSet(DatePicker view, int year, int month, int dayOfMonth) {

        etDate.setText("4");



    }
};

}