Update

package com.example.tugassqlfirebase;

import android.content.Intent;
import android.database.Cursor;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;

import androidx.appcompat.app.AppCompatActivity;

public class UpdateActivity extends AppCompatActivity {

    Button _Confirm, _Cancel;
    EditText _fnInput, _unInput, _agInput, _stInput;
    String _fn, _un, _ag, _st, _id;

    DatabaseHelper _DB;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_update);

        _fnInput = findViewById(R.id.FullnameInput);
        _unInput = findViewById(R.id.UsernameInput);
        _agInput = findViewById(R.id.AgeInput);
        _stInput = findViewById(R.id.StatusInput);
        _Confirm = findViewById(R.id.Update);
        _Cancel = findViewById(R.id.Cancel);

        _DB = new DatabaseHelper(UpdateActivity.this);

        getData();

        _Confirm.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                _fn = _fnInput.getText().toString();
                _un = _unInput.getText().toString();
                _ag = _agInput.getText().toString();
                _st = _stInput.getText().toString();

                _DB.updateData(_id, _fn, _un, _ag, _st);

                Intent Menu = new Intent(UpdateActivity.this, MainActivity.class);
                startActivity(Menu);
                finish();
            }
        });

        _Cancel.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                finish();
            }
        });
    }

    void getData() {
        DatabaseHelper DB = new DatabaseHelper(UpdateActivity.this);
        Cursor res = DB.readData();
        while(res.moveToNext()) {

            _fnInput.setText(res.getString(1));
            _unInput.setText(res.getString(2));
            _agInput.setText(res.getString(3));
            _stInput.setText(res.getString(4));

        }
    }
}