Solution to Java Enum Generics Problem Posted on StackOverflow

org.public_domain.enum_ops_plus_id.DbIdEnum.java

package org.public_domain.enum_ops_plus_id;

public interface DbIdEnum<T> {
  T getDbId();
}

org.public_domain.enum_ops_plus_id.DbIdEnumOps.java

package org.public_domain.enum_ops_plus_id;

import java.util.Collections;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Objects;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public final class DbIdEnumOps<E extends Enum<E> & DbIdEnum<T>, T> {
  private final Map<T, E> enumValueByDbId;
  private final Map<String, E> enumValueByNameLowerCaseOrDbId;

  public static <E extends Enum<E> & DbIdEnum<T>, T> DbIdEnumOps<E, T> from(Class<E> enumClassE) {
    return new DbIdEnumOps<>(enumClassE);
  }

  private DbIdEnumOps(Class<E> enumClassE) {
    //how to validate AT COMPILE TIME `enumClassE` also conforms to `Class<EE>`?
    var enumOpsList =
        EnumOps.from(Objects.requireNonNull(enumClassE))
            .toList();
    this.enumValueByDbId =
        Collections.unmodifiableMap(
            enumOpsList
                .stream()
                .map(e -> Map.entry(e.getDbId(), e))
                .collect(Collectors.toMap(Entry::getKey, Entry::getValue)));
    this.enumValueByNameLowerCaseOrDbId =
        Collections.unmodifiableMap(
            enumOpsList
                .stream()
                .flatMap(e -> Stream.of(
                    Map.entry(e.name().toLowerCase(), e),
                    Map.entry(e.getDbId().toString(), e)))
                .collect(Collectors.toMap(Entry::getKey, Entry::getValue)));
  }

  public Map<T, E> getEnumValueByDbId() {
    return this.enumValueByDbId;
  }

  public Map<String, E> getEnumValueByNameLowerCaseOrDbId() {
    return this.enumValueByNameLowerCaseOrDbId;
  }
}

org.public_domain.enum_ops_plus_id.EnumOps.java

package org.public_domain.enum_ops_plus_id;

import java.util.Arrays;
import java.util.Collections;
import java.util.List;

public final class EnumOps<E extends Enum<E>> {
  private final Class<E> enumClass;
  private final List<E> enumsValues;

  private EnumOps(Class<E> enumClass) {
    this.enumClass = enumClass;
    this.enumsValues = Collections.unmodifiableList(Arrays.asList(enumClass.getEnumConstants()));
  }

  public static <E extends Enum<E>> EnumOps<E> from(Class<E> enumClass) {
    return new EnumOps<>(enumClass);
  }

  public Class<E> getEnumClass() {
    return this.enumClass;
  }

  public List<E> toList() {
    return this.enumsValues;
  }

  //omitted lots of other helpful enum utilities that ought to have been provided by the Java compiler by default
}

org.public_domain.enum_ops_plus_id.Main.java

package org.public_domain.enum_ops_plus_id;

public class Main {

  public static void main(String[] args) {
    var a = DbIdEnumOps.from(TrafficLight.class);
    var b = DbIdEnumOps.<TrafficLight, Integer>from(TrafficLight.class);
    DbIdEnumOps<TrafficLight, Integer> c = DbIdEnumOps.from(TrafficLight.class);
    System.out.println("use me to set a breakpoint to examine the contents of the maps");
  }
}

org.public_domain.enum_ops_plus_id.TrafficLight.java

package org.public_domain.enum_ops_plus_id;

public enum TrafficLight implements DbIdEnum<Integer> {
  GREEN(1),
  YELLOW(2),
  RED(3);

  private final Integer dbId;

  TrafficLight(Integer dbId) {
    this.dbId = dbId;
  }

  private static final EnumOps<TrafficLight> enumOps =
      EnumOps.from(TrafficLight.class);
  private static final DbIdEnumOps<TrafficLight, Integer> dbIdEnumOpsA =
      DbIdEnumOps.from(TrafficLight.class);

  public static EnumOps<TrafficLight> ops() {
    return enumOps;
  }

  public static DbIdEnumOps<TrafficLight, Integer> dbIdOps() {
    return dbIdEnumOpsA;
  }

  public Integer getDbId() {
    return this.dbId;
  }
}

Overview.txt

//StackOverflow Question: https://stackoverflow.com/q/77362860/501113

//Updated/fixed the 5 code files to incorporate the answer by Turing85: https://stackoverflow.com/users/4216641/turing85