我是一个新的安卓程序员。我有4个活动：a b c d。使用按钮的顺序是a->b->c->d->a和a->d。我想将数据保存在活动d中的arraylist中。问题是，当我从d移到a并返回d时，arraylist中的数据没有保存。此处为d活动的代码：

public class SchedulerActivity extends AppCompatActivity {

    public String name = "";
    public String number = "";
    public String date = "";
    public String hour = "";
    public ArrayList<EventClass> scheduler = new ArrayList<>();

    @RequiresApi(api = Build.VERSION_CODES.N)
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_scheduler);
        Bundle extras = getIntent().getExtras();

        FloatingActionButton fab = findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent intent = new Intent(SchedulerActivity.this, MainActivity.class);
                intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP | Intent.FLAG_ACTIVITY_SINGLE_TOP);
                startActivity(intent);
                finish();
            }
        });
        if (extras != null) {
            String sender = extras.getString("sender");
            if(sender.compareTo("Hours") == 0) {
                name = extras.getString("name");
                number = extras.getString("number");
                date = extras.getString("date");
                hour = extras.getString("hour");
                Date real_date = new Date();
                SimpleDateFormat formatter1=new SimpleDateFormat("dd/MM/yyyy");
                try {
                    real_date = formatter1.parse(date);
                } catch (ParseException e) {
                    e.printStackTrace();
                }
                scheduler.add(new EventClass(real_date, name, number, "", hour));
                for (EventClass event : scheduler){
                    LinearLayout.LayoutParams params = new LinearLayout.LayoutParams(
                            LinearLayout.LayoutParams.MATCH_PARENT,
                            LinearLayout.LayoutParams.WRAP_CONTENT);
                    final TextView t = new TextView(this);
                    t.setText(event.toString());
                    LinearLayout linearLayout = (LinearLayout) findViewById(R.id.linearLayout);
                    linearLayout.addView(t, params);
                }

            }
            else{
                for (EventClass event : scheduler){
                    LinearLayout.LayoutParams params = new LinearLayout.LayoutParams(
                            LinearLayout.LayoutParams.MATCH_PARENT,
                            LinearLayout.LayoutParams.WRAP_CONTENT);
                    final Button btn = new Button(this);
                    final TextView t = new TextView(this);
                    t.setText(event.toString());
                    LinearLayout linearLayout = (LinearLayout) findViewById(R.id.linearLayout);
                    linearLayout.addView(btn, params);
                }
            }
        }
    }

我想在c->d出现时更改我的arraylist并打印它，当d->a出现时我只想打印它。我知道我可以使用sharedpreferences，但是第一步，我想使用arraylist。最好的办法是什么？