将枚举列表转换为具有匹配枚举值的逗号分隔字符串

Question

I have a List of Enum values like MON, TUE, WED, etc., same need to convert to comma-separated String. Need to use Java 8 to convert the same in an efficient way. For example.

Arrays.stream(Days.values())
    .map(MON -> TimeRangeConstants.MON)
    .collect(Collectors.joining(","));

enum Days {
   MON, TUE, WED, THU, FRI, SAT, SUN;
}

main() {
   Days v1 = Days.MON;
   Days v2 = Days.WED;
   Days v3 = Days.FRI;
   List<Days> days = new ArrayList<>();
   days.add(v1);
   days.add(v2);
   days.add(v3);
    String str = convertToString(days);
}


convertToString(List<Days> list) {      
   // need to return String as "Monday, Wednesday, Friday" 
}

For the given above example, I need output as "Monday, Wednesday, Friday"

Answer 1

You can declare a new method in the enum to map the day to the name of the day and then use the java-8 streams like this:

import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;

public class DaysToCsv {

    enum Days {
        MON, TUE, WED, THU, FRI, SAT, SUN;

        public static String getFullName(Days day) {
            switch (day) {
                case MON:
                    return "Monday";
                case TUE:
                    return "Tuesday";
                case WED:
                    return "Wednesday";
                case THU:
                    return "Thursday";
                case FRI:
                    return "Friday";
                case SAT:
                    return "Saturday";
                case SUN:
                    return "Sunday";
                default:
                    throw new IllegalArgumentException("Unexpected day");
            }
        }
    }

    public static void main(String[] args) {
        Days v1 = Days.MON;
        Days v2 = Days.WED;
        Days v3 = Days.FRI;
        List<Days> days = new ArrayList<>();
        days.add(v1);
        days.add(v2);
        days.add(v3);
        String str = convertToString(days);
        System.out.println(str);
    }

    public static String convertToString(List<Days> list) {
        return list.stream().map(day -> Days.getFullName(day)).collect(Collectors.joining(", "));
    }
}

Answer 2

You would have to edit the enum to:

enum Days {
    MON("Monday"), TUE("Tuesday"), WED("Wednesday")
    ;
    private String val;
    Days(String val) {
        this.val = val;
    }
    @Override
    public String toString() {
        return val;
    }
}

If you have access to the newer stream() method, you can do this:

final String s = String.join(",", list.stream().map(Object::toString).collect(Collectors.toList());
System.out.println("s = " + s);