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.

Answer 1

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);
        }
    }
}

Answer 2

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);
    }
}

Answer 3

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());
    }
}

Answer 4

use:

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