Append java static variables

Question

I want to append list of elements to next list array, here are code example:

public class AuthoritiesConstant {

    public static List<String> USER_ROLES = Arrays.asList("authenticated", "user", "article:read", "article:offer");
    public static List<String> MODERATOR = Arrays.asList( "moderator", "article:create", "article:update", "article:approve");
    public static List<String> ADMIN_ROLES = Arrays.asList("admin", "article:delete");


    AuthoritiesConstant() {
        // Each role get previous role authorities
        MODERATOR.addAll(USER_ROLES);
        ADMIN_ROLES.addAll(MODERATOR);
    }
}

When I use in code ADMIN_ROLES variable only two roles are registered: "admin", "article:delete".

How can I append roles from previous role to next. So that moderator has all user roles, and administrator all moderator roles?

Answer 1

tl;dr

    public static final List < String > USER_ROLES, MODERATOR_ROLES, ADMIN_ROLES;

    static
    {
        USER_ROLES = List.of( "authenticated" , "user" , "article:read" , "article:offer" );

        List < String > mod = new ArrayList <>( USER_ROLES );
        mod.addAll( List.of( "moderator" , "article:create" , "article:update" , "article:approve" ) );
        MODERATOR_ROLES = List.copyOf( mod );

        List < String > admin = new ArrayList <>( MODERATOR_ROLES );
        admin.addAll( List.of( "admin" , "article:delete" ) );
        ADMIN_ROLES = List.copyOf( admin );
    }

Cannot add to fixed-size list

As khelwood commented, the Arrays.asList returns a fixed-size list. You cannot add or delete elements.

We have a more modern alternative for a compact literal declaration of a unmodifiable list in Java 9 and later: List.of.

List < String > furryAnimals = List.of( "dog" , "cat" ) ;
List< String > featheredAnimals = List.of( "cockatiel" , "canary" ) ;

You can make a modifiable list from those unmodifiable lists by passing them to the constructor and methods of other List implementations.

Here we instantiate a modifiable ArrayList object. In the constructor we pass one unmodifiable list. Then we add the contents of a second unmodifiable list by passing to addAll method.

List< String > allCreaturesGreatAndSmall = new ArrayList<>( furryAnimals ) ;
allCreaturesGreatAndSmall.addAll( featheredAnimals ) ;

You can make an unmodifiable list by passing the modifiable one to List.copyOf in Java 10 and later.

List < String > animals = List.copyOf( allCreaturesGreatAndSmall ) ;  // Make an unmodifiable list of a modifiable list. 

See also the correct Answer by M A for other issues.

By the way, in Java conventions, we use all-uppercase only for constants. Your lists are changing, so names such as USER_ROLES should be userRoles.

Constants

If you did indeed intend for those lists to be constants, those lists should be:

• Unmodifiable
• Declared final so no other object can be later assigned.

Your code might look something like the following.

package work.basil.example;

import java.util.ArrayList;
import java.util.List;

public class Authority
{
    public static final List < String > USER_ROLES, MODERATOR_ROLES, ADMIN_ROLES;

    static
    {
        USER_ROLES = List.of( "authenticated" , "user" , "article:read" , "article:offer" );

        List < String > mod = new ArrayList <>( USER_ROLES );
        mod.addAll( List.of( "moderator" , "article:create" , "article:update" , "article:approve" ) );
        MODERATOR_ROLES = List.copyOf( mod );

        List < String > admin = new ArrayList <>( MODERATOR_ROLES );
        admin.addAll( List.of( "admin" , "article:delete" ) );
        ADMIN_ROLES = List.copyOf( admin );
    }

    public static void main ( String[] args )
    {
        System.out.println( "USER_ROLES = " + USER_ROLES );
        System.out.println( "MODERATOR_ROLES = " + MODERATOR_ROLES );
        System.out.println( "ADMIN_ROLES = " + ADMIN_ROLES );
    }
}

When run.

USER_ROLES = [authenticated, user, article:read, article:offer]
MODERATOR_ROLES = [authenticated, user, article:read, article:offer, moderator, article:create, article:update, article:approve]
ADMIN_ROLES = [authenticated, user, article:read, article:offer, moderator, article:create, article:update, article:approve, admin, article:delete]