Which is best way to define constants in android, either static class, interface or xml resource?

There is a big difference between the two in that you can reference project resources in your XML layouts. They are available in the application context and are therefore accessible across the global application. The biggest advantages of using project resources is the ease of access and that they allow you to organize your project significantly.

static final constants are compiled into the java bytecode; project resources are compiled into a binary format within the apk. Accessing either is extremely efficient... if there is a difference between the two, it is trivial at most.

There isn't a set rule on how you should be using resources/constants in your project. That said, I personally use resources for values that I might need to use in my XML or java code. On the other hand, I typically use static final constants for values that will only be used by my java code and are specific to my implementation.

Also note that it is possible to load XML resources at runtime depending on the device's current configuration (i.e. screen size, locale, etc.). So you should take this into consideration when deciding whether or not you should declare the constant in XML or directly in your .java files.

For the people who want to see how we can use a Class to define our constants and call any where we need.

Constant.java

    package org.nrum.nrum;

/**
 * Created by rajdhami on 5/23/2017.
 */
public class Constant {
    public static final String SERVER = "http://192.168.0.100/bs.dev/nrum";
//    public static final String SERVER = "http://192.168.100.2/bs.dev/nrum";
    public static final String API_END = SERVER + "/dataProvider";
    public static final String NEWS_API = API_END + "/newsApi";
    public static final String BANNER_API = API_END + "/bannerApi/lists";
    public static final String NOTICE_API = API_END + "/noticeApi/lists";
    public static final String UPLOAD_PATH = SERVER + "/uploads";
    public static final String UPLOAD_PATH_BANNER = UPLOAD_PATH + "/company_1/banner";
    public static final String UPLOAD_PATH_NEWS = UPLOAD_PATH + "/company_1/news";
    public static final int BANNER_TRANSITION_DURATION = 5000;
    public static final int NOTICE_BUTTON_BLINK_DURATION = 5000;
    public static final int BANNER_FETCH_LIMIT = 3;
}

Now we can use above constants in following way.

Constant.NOTICE_BUTTON_BLINK_DURATION

In general case:

• XML values have the advantage of accessbilty in layout file and manifest file over Constants in java file
• XML values have the advantage for multi language support over Constants in java file