Getting the error Cannot resolve method 'join' in 'String' when using the join() method

Question

Essentially all I want to do is put each array element on a new line but when using the join() method I get the error, Cannot resolve method 'join' in 'String'.

public class revision_testing extends AppCompatActivity {

    start_timetable start_timetable = new start_timetable();
    revision_time revision_time = new revision_time();
    public int append_counter = 0;
    public int revision_days = 25; //FOR TEST
    String[] all_dates = new String[revision_days];

    String date=start_timetable.clicked_date;

    public int day=start_timetable.day;
    public int month=start_timetable.month;
    public int year=start_timetable.year;
    int [] days_in_months = {31,28,31,30,31,30,31,31,30,31,30,31};

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

        for(int i=revision_days;i>0;i--){
            if (day >0){
                all_dates[append_counter] = year+"/"+month+"/"+day;
                day--;
                append_counter++;

            }else {
                day=days_in_months[month];
                month++;
                all_dates[append_counter] = year+"/"+month+"/"+day;
                day--;
                append_counter++;
            }
        }

        all_dates= all_dates.join("\n",all_dates);     //This line is the issue 
        }

Answer 1

Most likely you wanted to use the String.join method instead of all_dates.join:

String allDatesJoined = String.join("\n", all_dates);

You are also assigning the result back into the all_dates array. You need to tell Java which position in the array you want to use. If you want to assign to the first array position, use all_dates[0]:

all_dates[0] = allDatesJoined;