Custom type that acts like boxed primitive in Java

Question

I want to create custom type that acts like Integer or Double . Lest say I have multilevel navigable map:

// [UserLevel: [UserBalance: Multiplier]]
NavigableMap<Integer, NavigableMap<Double, Double>> multiplierByBalanceAndLevel

It's absolutely not readable, if you don't look into the comment. And also always when I work with such Map I should go to the declaration and check comment. It's absolutely annoying, and I want something like:

NavigableMap<UserLevel, NavigableMap<UserBalance, Multiplier>> multiplierByBalanceAndLevel

For example in scala it's possible to use:

type UserLevel = Int

Does it possible to implement in java somehow? Maybe there are some libraries for such purposes?

PS I can write custom implementation with lombok help

import java.util.NavigableMap;
import com.google.common.collect.ImmutableSortedMap;
import lombok.Value;
import lombok.experimental.Delegate;

public class TestExample {

    @Value
    static class UserLevel extends Number implements Comparable<UserLevel> {
        @Delegate
        Integer userLevel;

        public int compareTo(UserLevel another) {
            return this.userLevel.compareTo(another.getUserLevel());
        }
    }

    @Value
    static class UserBalance extends Number implements Comparable<UserBalance> {
        @Delegate
        Double userBalance;

        public int compareTo(UserBalance another) {
            return this.userBalance.compareTo(another.getUserBalance());
        }
    }

    @Value
    static class Multiplier extends Number implements Comparable<Multiplier> {
        @Delegate
        Double multiplier;

        public int compareTo(Multiplier another) {
            return this.multiplier.compareTo(another.getMultiplier());
        }
    }

    public static void main(String[] args) {
        double originalAmount = 10;
        NavigableMap<UserLevel, NavigableMap<UserBalance, Multiplier>> multiplierByBalanceAndLevel =
                ImmutableSortedMap.of(new UserLevel(1),
                                      ImmutableSortedMap.of(new UserBalance(2.0), new Multiplier(3.0)));
        Multiplier multiplier = multiplierByBalanceAndLevel
                .floorEntry(new UserLevel(13)).getValue()
                .ceilingEntry(new UserBalance(1.8)).getValue();
        double amount = originalAmount * multiplier.getMultiplier();
    }
}

But the use of wrapped classes looks ugly and produces a lot of boilerplate code, because they don't act like auto-boxed ones

Answer 1

Java does not support auto-boxing or auto-unboxing of custom classes.

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Boxing and unboxing conversions in Java are specified in JLS 5.1.7 and JLS 5.1.8 respectively. They state that conversions only work between primitive types and the corresponding wrapper types.