Thread: Creating generic version of methods

Well, your enhanced for() loop is not correct, so start there. I'm not sure that'll be the only fix, but maybe that'll get you moving forward.

Yeah? Did this correct the error?

	private T getEnum(T type, String name) {
		for ( T t : type.values( ) ) {
			if(t.title.equals(name)){
				return t;
			}
		}
		return null;
	}

Other than that, I can't spot my error...

Anyhow, this is still the complaint:

horoscope\Enums.java:234: error: cannot find symbol
                for ( T t : type.values( ) ) {
                                ^
  symbol:   method values()
  location: variable type of type T
  where T is a type-variable:
    T extends Object declared in class EnumHelper
horoscope\Enums.java:235: error: cannot find symbol
                        if(t.title == name){
                            ^
  symbol:   variable title
  location: variable t of type T
  where T is a type-variable:
    T extends Object declared in class EnumHelper
2 errors

So basically, how do I make the compiler accept that the names type and t will have these fields called values and title respectively at runtime?

--- Update ---

Going forward with my Java studies, I now realize I really don't need a generic class for this. Instead I added a generic method in my Enums class:

abstract class Enums {
 
	static private <E> Class getEnum(Class<E> type, String name) {
		for ( type t : type.values( ) ) {
			if(t.title.equals(name)){
				return t;
			}
		}
		return null;
	}

Well, this still doesn't compute:

horoscope\Enums.java:6: error: cannot find symbol
                for ( type t : type.values( ) ) {
                      ^
  symbol:   class type
  location: class Enums
horoscope\Enums.java:6: error: cannot find symbol
                for ( type t : type.values( ) ) {
                                   ^
  symbol:   method values()
  location: variable type of type Class<E>
  where E is a type-variable:
    E extends Object declared in method <E>getEnum(Class<E>,String)
2 errors

I just guessing here as to the generics stuff... First of all, nor the return value or the parameter should really be Class, right? I tried Object and Enum and they didn't work, so I'm stuck again.

Once again, your enhanced for() loop is incorrect. Review the required structure and fix it.

Compare your statement:

for ( type t : type.values( ) )

to the documentation for the enhanced for() loop.

Hints: Is 'type' really a Type? Using proper capitalization per Java's naming conventions will cause these kind of errors to jump off the page at you when you're more familiar.

I'm afraid I'm a bit lost. This is what I got now:

abstract class Enums {
 
	static private <E> Class getEnum(Class<E> enumType, String name) {
		for ( enumType t : enumType.values( ) ) {
			if(t.title.equals(name)){
				return t;
			}
		}
		return null;
	}	
 
	static private Landmass getLandmass(String name) {
		for ( Landmass l : Landmass.values( ) ) {
			if(l.title.equals(name)){
				return l;
			}
		}
		return null;
	}

The difference between these two methods, as far as I can fathom, is that the first one takes an additional parameter (EnumType) corresponding to a Enum type, while the second one deals with Landmass Enum types exclusively.

Sure, I'm not getting the whole generics thing yet, but fail to see anything wrong with my for() loops. The second method seems to work, at least.

Regarding "Type" and "type", I might have used a keyword or a built-in class name by mistake. I changed the identifier to enumType, which shouldn't cause any issues, right?

I'm not sure what you're trying to do. Can you post a full program - even a broken one - focused on simply doing the one thing you're trying to do that could be used to understand what the objective is?

Sure

abstract class Enums{
 
	public enum TimeOWeek{
		MONDAY ("måndag"),
		TUESDAY ("tisdag"),
		WEDNESDAY ("onsdag"),
		THURSDAY ("tordag"),
		FRIDAY ("fredag"),
		SATURDAY ("lördag"),
		SUNDAY ("söndag"),;
 
		public final String title;
 
		TimeOWeek (String name) {
			this.title = name;
		}
	}
 
	public enum TimeODay{
		DAWN ("gryning"),
		DAY ("dag"),
		DUSK ("skymning"),
		NIGHT ("natt"),;
 
		public final String title;
 
		TimeODay (String name) {
			this.title = name;
		}
	}
 
	static TimeOWeek getWeekday(String name) {
		for ( TimeOWeek t : TimeOWeek.values( ) ) {
			if(t.title.equals(name)){
				return t;
			}
		}
		return null;
	}
 
	static TimeODay getTime(String name) {
		for ( TimeODay t : TimeODay.values( ) ) {
			if(t.title.equals(name)){
				return t;
			}
		}
		return null;
	}
}
 
public class TimesAndDays {
 
	public static void main(String []args) {
		for( String name : args ){
			System.out.printf("\n");
			if(Enums.getWeekday(name) != null){
				System.out.printf(name + " corresponds with " + Enums.getWeekday(name));
			}else if(Enums.getTime(name) != null){
				System.out.printf(name + " corresponds with " + Enums.getTime(name));
			}
			else{
				System.out.printf(name + " corresponds with nothing");
			}
		}
	}
}

So if I run the application from Command Prompt I get:

$java TimesAndDays lördag midnatt skymning
$
$l÷rdag corresponds with SATURDAY
$midnatt corresponds with nothing
$skymning corresponds with DUSK

This verifies that the methods Enums.getWeekday() - which returns a TimeOWeek Enum Type - and Enums.getTime() - which returns a TimeODay Enum Type - works.

Now, I wanna get rid of both of these methods and instead have a generic Enums.getEnum() method:

	public static void main(String []args) {
		for( String name : args ){
			System.out.printf("\n");
			if(Enums.getEnum(Enums.TimeOWeek, name) != null){
				System.out.printf(name + " corresponds with " + Enums.getEnum(Enums.TimeOWeek, name));
			}else if(Enums.getEnum(Enums.TimeODay, name) != null){
				System.out.printf(name + " corresponds with " + Enums.getEnum(Enums.TimeODay, name));
			}
			else{
				System.out.printf(name + " corresponds with nothing");
			}
		}
	}

So I need to write a generic Enums.getEnum() method. This is what I tried:

	static <T> Enum getEnum(Enum<T> enumType, String name) {
		for ( Enum t : enumType.values( ) ) {
			if(t.title.equals(name)){
				return t;
			}
		}
		return null;
	}

Since I don't know what I'm doing, I get this from the compiler. Dunno what it means.

TimesAndDays.java:32: error: type argument T is not within bounds of type-variable E
        static <T> Enum getEnum(Enum<T> enumType, String name) {
                                     ^
  where T,E are type-variables:
    T extends Object declared in method <T>getEnum(Enum<T>,String)
    E extends Enum<E> declared in class Enum
TimesAndDays.java:33: error: cannot find symbol
                for ( Enum t : enumType.values( ) ) {
                                       ^
  symbol:   method values()
  location: variable enumType of type Enum<T>
  where T is a type-variable:
    T extends Object declared in method <T>getEnum(Enum<T>,String)
TimesAndDays.java:34: error: cannot find symbol
                        if(t.title.equals(name)){
                            ^
  symbol:   variable title
  location: variable t of type Enum
TimesAndDays.java:65: error: cannot find symbol
                        if(Enums.getEnum(Enums.TimeODay, name) != null){
                                              ^
  symbol:   variable TimeODay
  location: class Enums
TimesAndDays.java:66: error: cannot find symbol
                                System.out.printf(name + " corresponds with " + Enums.getEnum(Enums.TimeODay, name));
                                                                                                   ^
  symbol:   variable TimeODay
  location: class Enums
TimesAndDays.java:67: error: cannot find symbol
                        }else if(Enums.getEnum(Enums.TimeOWeek, name) != null){
                                                    ^
  symbol:   variable TimeOWeek
  location: class Enums
TimesAndDays.java:68: error: cannot find symbol
                                System.out.printf(name + " corresponds with " + Enums.getEnum(Enums.TimeOWeek, name));
                                                                                                   ^
  symbol:   variable TimeOWeek
  location: class Enums
7 errors

I think this article covers what you're trying to do, but I didn't work through it to verify. If this article is not quite right, you might try searching 'java generic enum' and mining those results.

Good luck!