Dynamically return a list of all class variable values in Java

Question

I am creating a helper class in parsing XML elements, so the developer do not need to know the exact name and capitalization of the XML fields.

private static class TagNames{
      public static String RESOURCE_ID = "ResourceId";
      public static String RESOURCE_NAME = "ResourceName";
      public static String RESOURCE_PRICE = "ResourcePrice";
}

This makes it easier to do things like:

someXMLParser.getValueByTagName(TagNames.RESOURCE_ID);

My question is this. If I want to iterate over all the fields declared in class TagNames, how do I do that? Pseudocode:

For tag in TagNames:
   someXMLParser.getValueByTagName(tag)

I know I will probably have to restructure all of this. But I can't figure out a way to make the names easily accessible as well as iterable, without any duplication.

Any suggestions?

Community · Accepted Answer · 2017-05-23 12:04:26Z

2

You're literally asking for a solution based on reflection, but I think a Java Enum may be a better choice in this case. Building on Frederick's example:

public class EnumTest {
    public enum Tags {
        RESOURCE_ID("ResourceId"), 
        REOURCE_NAME("ResourceName"), 
        RESOURCE_PRICE("ResourcePrice");

        private final String tagName;
        Tags(String tagName) {
            this.tagName = tagName;
        }

        public String getTagName() {
            return tagName;
        }
    }

    public static void main(String[] args) {
        for(Tags tag : Tags.values()) {
            System.out.println("const:" + tag.name() 
                    + " tagName:" + tag.getTagName());
        }
        // API user might do e.g.:
        // document.getValueForTag(Tags.REOURCE_NAME);
    }
}

edited May 23, 2017 at 12:04

CommunityBot

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answered Nov 23, 2010 at 5:58

Mike Clark

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2 Comments

Mike Clark Over a year ago

@maerics - so similar to your answer!

Fredrick Pennachi Over a year ago

thanks for the mention, I do think your way is better though :)

Sean Patrick Floyd · Accepted Answer · 2010-11-23 09:19:14Z

2

Although I agree that you should probably use enums or ResourceBundles, here's a solution to your actual question. A method that generates a Map name -> value from all public constants in a given class (the only thing that's missing should be try / catch or throws)

public static Map<String, Object> getConstantValues(Class<?> clazz){

    Map<String, Object> constantValues = new LinkedHashMap<String, Object>();
    for(Field field : clazz.getDeclaredFields()){
        int modifiers = field.getModifiers();
        if(Modifiers.isPublic(mod)
            && Modifiers.isStatic(mod) && Modifiers.isFinal(mod)){
            constantValues.put(field.getName(), field.get(null));
        }
    }
    return constantValues;
}

edited Nov 23, 2010 at 9:19

answered Nov 23, 2010 at 5:58

Sean Patrick Floyd

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2 Comments

Mike Clark Over a year ago

public static constant? :-) Is that some C#! But you're right, I suppose reflection is the most literal answer to his question.

Sean Patrick Floyd Over a year ago

Oh lord there was another awful bug. That's what you get for answering SO questions before breakfast :-)

phill · Accepted Answer · 2010-11-23 02:22:52Z

1

You may want to consider using a ResourceBundle instead of a class to store the tag names. May require a little bit of reworking of your code but it will be easier to produce a list of tags compared to what you are doing now, and adding a new tag won't require much work other then adding a line to the properties file.

answered Nov 23, 2010 at 2:22

phill

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Comments

Fredrick Pennachi · Accepted Answer · 2010-11-23 05:42:20Z

0

You can do this quite easily using enum and an accompanying array:

public class Main {

    public enum TagName { RESOURCE_ID, REOURCE_NAME, RESOURCE_PRICE }

    private static String[] tags = {"ResourceID", "ResourceName", "ResourcePrice"};

    public static String getValueByTagName(TagName tag) {

    return tags[tag.ordinal()];
    }

    public static void main(String[] args) {

    System.out.println("Calling by getValueByTagName:");
    System.out.println(getValueByTagName(TagName.RESOURCE_ID));

    System.out.println("Calling TagName.values() for loop:");

    for (TagName t : TagName.values()) {

        System.out.println(getValueByTagName(t));
    }
    }
}

edited Nov 23, 2010 at 5:42

answered Nov 23, 2010 at 5:36

Fredrick Pennachi

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1 Comment

Mike Clark Over a year ago

Good, but I would suggest storing the tag names in the enum itself, so you don't have to worry about lining up the enum constant ordinals with the indexing into the tagName array.

maerics · Accepted Answer · 2010-11-23 05:57:45Z

Using an enum is a good fit, especially if you use a custom constructor and the built in "values" method:

public class Main {
  public static enum TagName {
    RESOURCE_ID("ResourceId"),
    RESOURCE_NAME("ResourceName"),
    RESOURCE_PRICE("ResourcePrice"),
    ;
    private String s;
    private TagName(String s) { this.s = s; }
    public String toString() { return this.s; }
    public static String[] strings() {
      List<String> ss = new ArrayList<String>();
      for (TagName tagName : TagName.values()) {
        ss.add(tagName.toString());
      }
      return ss.toArray(new String[ss.size()]);
    }
  }
  public static void main(String[] args) {
    // Use TagName.values() for the enums, or for strings...
    for (String s : TagName.strings()) {
      System.out.println(s);
    }
  }
}

This way you can simply add new tags and they'll automatically get picked up by the "strings" method; for extra performance you could compute that string array just once, statically, since you can't change the set of enums dynamically. You could get even fancier by auto-generating the tag strings from their constant values, if they are really normalized...

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