You simply have to use a DateFormatter class. There are default formats. You just instantiate it, then call format on it, passing the Date.

Here is the documentation: DateFormatter.

Dates in Android are a special drag because war broke out between Oracle and Google about the Java licensing terms. So Java was not updated for a while, including Java 8 which included a complete rewrite of dates and times. You can now use Java 8 Dates/Times in Android but if you do you lose compatability with older OSes.

Here is your code, with the formatter added:

   public void onPositiveButtonClick(Pair<Long, Long> selection) {
       tv1.setText(materialDatePicker.getHeaderText());
        startDate = selection.first;
        endDate = selection.second;
        DateFormatter formatter = SimpleDateFormatter("dd-MM-yyyy", Locale.US);
        System.out.println("this is start date: " + formatter.format(startDate));
        System.out.println("this is end date " + formatter.format(endDate)));

    }

Of course you could use the default Locale.

what I should do if i want to print these dates? Because using System.out.println(startDate); System.out.println(endDate); gives me this output : 1595203200000 1596153600000 and I want date as output

public class MainActivity extends AppCompatActivity {

private Button bt1;
private TextView tv1;
private Long startDate;
private Long endDate;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    
    bt1 = findViewById(R.id.bt1);
    tv1 = findViewById(R.id.tv1);

    MaterialDatePicker.Builder<Pair<Long, Long>> builder = MaterialDatePicker.Builder.dateRangePicker();
    final MaterialDatePicker<Pair<Long,Long>> materialDatePicker = builder.build();

    bt1.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            materialDatePicker.show(getSupportFragmentManager(),"date picker");
        }
    });

    /*materialDatePicker.addOnPositiveButtonClickListener(new MaterialPickerOnPositiveButtonClickListener() {
        @Override
        public void onPositiveButtonClick(Object selection) {
           tv1.setText(materialDatePicker.getHeaderText());
        }
    });*/
    materialDatePicker.addOnPositiveButtonClickListener(new MaterialPickerOnPositiveButtonClickListener<Pair<Long, Long>>() {
        @RequiresApi(api = Build.VERSION_CODES.N)
        @Override
        public void onPositiveButtonClick(Pair<Long, Long> selection) {
           tv1.setText(materialDatePicker.getHeaderText());
            startDate = selection.first;
            endDate = selection.second;
            System.out.println("this is start date: " + startDate);
            System.out.println("this is end date " + endDate);

        }
    });

}

}

the current output is : I/System.out: this is start date: 1594598400000 this is end date 1596672000000