Java - get fields names of certain types

Question

I have an example class below and I want to return all class fields of certain type, in this example of type Image.

public class Contact {
    private String surname, lastname, address;
    private int age, floor;
    private Image contactPhoto, companyPhoto;
    private boolean isEmployed;

    public String[] getAllImages() {
        String images[] = // missing code
        return images;
        // in this case, I want to return {"contactPhoto","companyPhoto"}
    }
}

I need a help here. How can I find all class fields of certain type. I will be calling this method in another class ofc.

I'm sure you mean you want to return the value of surname and not "surname" itself, right? — runDOSrun, Dec 11 '14 at 22:20
Ok. For such a simple problem you would never use Introspection (which you are asking about, you can google about it). Just use the string values instead of variable names. The actual name doesn't matter for you. — runDOSrun, Dec 11 '14 at 22:28

Kevin Bowersox · Accepted Answer · 2014-12-11T22:37:25.580

6

Use reflection to access the fields declared on the class. Then iterate through the fields and check to see if their type matches Image.

You could also create a more useful method by accepting two parameters a target Class and a searchType Class. The method would then searches for fields with the target of the type searchType.

I would also recommend making this method static, since it really doesn't depend on any of the classes state.

Example

public class Contact {
    private String surname, lastname, address;
    private int age, floor;
    private Image contactPhoto, companyPhoto;
    private boolean isEmployed;

    public static String[] getFieldsOfType(Class<?> target, Class<?> searchType) {

        Field[] fields  = target.getDeclaredFields();

        List<String> results = new LinkedList<String>();
        for(Field f:fields){
            if(f.getType().equals(searchType)){
                results.add(f.getName());
            }
        }
        return results.toArray(new String[results.size()]);
    }

    public static String[] getAllImages(){
        return getFieldsOfType(Contact.class, Image.class); 
    }

    public static void main(String[] args) {
        String[] fieldNames = getAllImages();

        for(String name:fieldNames){
            System.out.println(name);
        }
    }
}

edited Dec 11 '14 at 22:37

answered Dec 11 '14 at 22:26

Kevin Bowersox

93,289
19
159
189

Yes this should be it! I will try with this example – ononononon Dec 11 '14 at 22:29
@SotiriosDelimanolis Answer abstracted to better accommodate indecisiveness. :) – Kevin Bowersox Dec 11 '14 at 22:30
@MariánZekeŠedaj Glad I could help! I tried to modify the answer to make it a little more reusable. I would also advocate for making the method static, since you really don't need an instance of the class. – Kevin Bowersox Dec 11 '14 at 22:34
1

I really want to point out to OP that he could save himself all the trouble with reflection by simply putting the images in a List which can then be returned instead. It's much more elegant, faster, simpler to read and easier to expand. – runDOSrun Dec 11 '14 at 22:40
@runDOSrun We don't know the entire use case, maybe this is just for experimentation. I would also think a `Map` could work well. Use the `String` key to get the names. – Kevin Bowersox Dec 11 '14 at 22:42
1

@KevinBowersox Yes, Map would be ideal. True, we don't know. That's why it's good to make sure OP doesn't think this approach is the "standard way". – runDOSrun Dec 11 '14 at 22:48

score 2 · Answer 2 · answered Dec 11 '14 at 23:03

A simpler alternative to using reflection would be to use a map as the primary data type for the field you are interested in:

public class Contact {

    private static final String CONTACT_PHOTO = "contactPhoto";
    private static final String COMPANY_PHOTO = "companyPhoto";

    private String surname, lastname, address;
    private int age, floor;
    private HashMap<String, Image> images;
    private boolean isEmployed;

    public Contact() {
        images = new HashMap<String, Image>();
        images.put(CONTACT_PHOTO, null);
        images.put(COMPANY_PHOTO, null);
    }

    public String[] getAllImages() {
        Set<String> imageNames = images.keySet();
        return imageNames.toArray(new String[imageNames.size()]);
    }

    public void setContactPhoto(Image img) {
        images.put(CONTACT_PHOTO, img);
    }

    public Image getContactPhoto() {
        return images.get(CONTACT_PHOTO);
    }

    public void setCompanyPhoto(Image img) {
        images.put(COMPANY_PHOTO, img);
    }

    public Image getCompanyPhoto() {
        return images.get(COMPANY_PHOTO);
    }
}

null · Answer 3 · 2014-12-11T22:56:31.637

Using field names as values can cause headaches.

If you need to identify individual images with strings, you could create a HashMap. Use your current field names as keys and Image objects as values.

https://docs.oracle.com/javase/8/docs/api/java/util/HashMap.html

You can retrieve all key values (in your case,the names) via the method keySet()

Edit: Here's a working example class, derived from yours, but only with the relevant fields:

import java.awt.Image;
import java.util.HashMap;
import java.util.Set;

public class Contact
{
    private HashMap<String, Image> images;

    public Contact ()
    {
        images = new HashMap<String, Image>();
        images.put( "contactPhoto", null);
        images.put( "companyPhoto", null);
    }

    public Set<String> getAllImages()
    {
        return images.keySet();
    }

    public static void main(String[] args)
    {
        System.out.println(new Contact().getAllImages());
    }
}

score 0 · Answer 4 · answered Dec 11 '14 at 22:24

0

use:

Field[] fields=this.getClass().getFields();
...
 for (...){
  if ( fields[i].getType() == ?? ){
  ...
  }
 }

answered Dec 11 '14 at 22:24

diegoyucra

24
2

1

Might want to use `getDeclaredFields()` so it picks up the private fields. – Kevin Bowersox Dec 11 '14 at 22:38

Java - get fields names of certain types

4 Answers4