Generics and Polymorphism

[Jason Cavett]

I'm having some issues with generics and polymorphism. I thought this
was possible in Java - maybe someone can clear up what I'm doing
wrong. Basically, when I actually try to use the preference, the code
will not compile and I get the following error. How can I do what I'm
trying to do?

Here is the code that has the error:

PreferencesEnum.DERIVED_PREFERENCE.getPreference().setValue(new
String());

The error is:
The method setValue(capture#2-of ? extends Object) in the type
Preference<capture#2-of ? extends Object> is not applicable for the
arguments (String)


Thanks,
Jason

--- CLASS LISTINGS ---

I have an enum:

PreferencesEnum {
DERIVED_PREFERENCE(new DerivedPreference());

private final Preference<? extends Object> pref;

private PreferencesEnum(Preference<? extends Object> pref) {
this.pref = pref;
}

public Preference<? extends Object> getPreference() {
return pref;
}
}

And I have the generic Preference:
public abstract class Preference<E extends Object> {

// provides access to the preferences per application, per user
protected static Preferences prefs =
Preferences.userNodeForPackage(Main.class);

/**
* Default constructor.
*/
public Preference() {
}

/**
* Perform a refresh when the preferences change.
*/
public abstract void refresh();

/**
* Set the value of the preference.
*
* @param value
* the value to set
*/
public abstract void setValue(E value);

/**
* Get the value of the preference.
*
* @return the associated preference value
*/
public abstract E getValue();
}

And here's a derived preference:

public class DerivedPreference extends Preference<String> {

private static final String KEY = "derived";

private static final String DEFAULT = "DEFAULT VALUE";

/**
* Default constructor
*/
public DerivedPreference() {
super();
}

@Override
public String getValue() {
return prefs.get(DerivedPreference.KEY,
DerivedPreference.DEFAULT);
}

@Override
public void refresh() {
}

@Override
public void setValue(String value) {
prefs.put(DerivedPreference.KEY, value);
}
}

 

[Daniel Pitts]

I'm having some issues with generics and polymorphism. I thought this
was possible in Java - maybe someone can clear up what I'm doing
wrong. Basically, when I actually try to use the preference, the code
will not compile and I get the following error. How can I do what I'm
trying to do?

Here is the code that has the error:

PreferencesEnum.DERIVED_PREFERENCE.getPreference().setValue(new
String());

The error is:
The method setValue(capture#2-of ? extends Object) in the type
Preference<capture#2-of ? extends Object> is not applicable for the
arguments (String)


Thanks,
Jason

--- CLASS LISTINGS ---

I have an enum:

PreferencesEnum {
DERIVED_PREFERENCE(new DerivedPreference());

private final Preference<? extends Object> pref;

private PreferencesEnum(Preference<? extends Object> pref) {
this.pref = pref;
}

public Preference<? extends Object> getPreference() {
return pref;
}
}

The problem is that DERIVED_PREFERENCE.getPreference() returns
Preference<? extends Object>, who's setValue() method accepts only E,
which can't be statically determined from the context...


Another issue is that enums can't have type parameters, so that makes
what you're trying to do specifically impossible using "enum"......


What you *can* do is instead of "enum", use a plain old class.

class PreferencesEnum<E> {
private final Preference<E> pref;

public static final DERIVED_PREFERENCE = new
PreferencesEnum<String>(new DerivedPreference());

private PreferenceEnum(Preference<E> pref) {
this.pref = pref;
}
}

etc...

Hope this helps.

 

[Jason Cavett]

The problem is that DERIVED_PREFERENCE.getPreference() returns
Preference<? extends Object>, who's setValue() method accepts only E,
which can't be statically determined from the context...

Another issue is that enums can't have type parameters, so that makes
what you're trying to do specifically impossible using "enum"......

What you *can* do is instead of "enum", use a plain old class.

class PreferencesEnum<E> {
    private final Preference<E> pref;

    public static final DERIVED_PREFERENCE = new
PreferencesEnum<String>(new DerivedPreference());

    private PreferenceEnum(Preference<E> pref) {
      this.pref = pref;
    }

}

etc...

Hope this helps.

Alright. It did help and I appreciate it.

The solution does seem a little clunky, however. Not being able to
paramaterize enums is kind of painful. Is there another possible way
of handling preferences that I'm not seeing? Basically, I want to
avoid having a huge file that contains every individual preference
(which is what was in place originally). Trying to edit that file was
a nightmare.

Either way, this solution works. Thanks again, Daniel.

 

[Daniel Pitts]

Alright. It did help and I appreciate it.

The solution does seem a little clunky, however. Not being able to
paramaterize enums is kind of painful. Is there another possible way
of handling preferences that I'm not seeing? Basically, I want to
avoid having a huge file that contains every individual preference
(which is what was in place originally). Trying to edit that file was
a nightmare.

Either way, this solution works. Thanks again, Daniel.

Are they truly preferences, or are they configuration? If its actually
configuration, you could try using Properties and/or a XML Spring container.

Alternatively, you can have a less generic Preferences class that has
fields and getters/setters for each preference that can be set.

The third approach is to use a EnumMap<PreferenceType, Object>, but you
don't get the type safety.

 

[Jason Cavett]

Are they truly preferences, or are they configuration?  If its actually
configuration, you could try using Properties and/or a XML Spring container.

Alternatively, you can have a less generic Preferences class that has
fields and getters/setters for each preference that can be set.

The third approach is to use a EnumMap<PreferenceType, Object>, but you
don't get the type safety.

I'm not sure what you mean by preferences vs. configuration.
(However, intuitively, I would say that these are preferences.)

When you say, "you can have less generic Preferences class..." do you
mean, I don't program to a generic interface and, instead, each
preference object has a similar naming scheme, but each knows exactly
what it has to set and get?

class Preference1 {
void set(String blah) ...
String get() ...
}

class Preference2 {
void set(Boolean blah) ...
Boolean get() ...
}


Something along those lines?

Thanks again for your help.

 

[Daniel Pitts]

I'm not sure what you mean by preferences vs. configuration.
(However, intuitively, I would say that these are preferences.)

When you say, "you can have less generic Preferences class..." do you
mean, I don't program to a generic interface and, instead, each
preference object has a similar naming scheme, but each knows exactly
what it has to set and get?

class Preference1 {
void set(String blah) ...
String get() ...
}

class Preference2 {
void set(Boolean blah) ...
Boolean get() ...
}


Something along those lines?

Thanks again for your help.

Actually, I was more along the lines of:

public class Preferences implements Serializable {
private static final long serialVersionUID = 1;
private Color favoriteColor;
private String explitive;
private boolean coldSoup;
public enum Animal {
cat, dog, bird, fish;
}
private Animal pet;


public Color getFavoriteColor() {
return favoriteColor;
}

public void setFavoriteColor(Color favoriteColor) {
this.favoriteColor = favoriteColor;
}

public String getExplitive() {
return explitive;
}

public void setExplitive(String explitive) {
this.explitive = explitive;
}

public boolean isColdSoup() {
return coldSoup;
}

public void setColdSoup(boolean coldSoup) {
this.coldSoup = coldSoup;
}

public Animal getPet() {
return pet;
}

public void setPet(Animal pet) {
this.pet = pet;
}
}

so, if you can use
preferences.setExplitive("Dag-nabbit!");
preferences.setColdSoup(false); // like my soups hot.
preferences.setPet(Preferences.Animal.cat); //
preferences.setFavoriteColor(Color.purple);