package poussecafe.util;

import java.util.Objects;

public class StringId {

    private String id;

    public StringId(String value) {
        Objects.requireNonNull(value);
        id = value;
    }

    public String stringValue() {
        return id;
    }

    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + ((id == null) ? 0 : id.hashCode());
        return result;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj) {
            return true;
        }
        if (obj == null) {
            return false;
        }
        if (getClass() != obj.getClass()) {
            return false;
        }
        StringId other = (StringId) obj;
        if (id == null) {
            if (other.id != null) {
                return false;
            }
        } else if (!id.equals(other.id)) {
            return false;
        }
        return true;
    }

    @Override
    public String toString() {
        return id;
    }
}

package poussecafe.util;

import java.util.Objects;

public class StringId implements Comparable<StringId> {

    private String id;

    public StringId(String value) {
        Objects.requireNonNull(value);
        id = value;
    }

    public String stringValue() {
        return id;
    }

    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + ((id == null) ? 0 : id.hashCode());
        return result;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj) {
            return true;
        }
        if (obj == null) {
            return false;
        }
        if (getClass() != obj.getClass()) {
            return false;
        }
        StringId other = (StringId) obj;
        if (id == null) {
            if (other.id != null) {
                return false;
            }
        } else if (!id.equals(other.id)) {
            return false;
        }
        return true;
    }

    @Override
    public String toString() {
        return id;
    }

    @Override
    public int compareTo(StringId o) {
        return id.compareTo(o.id);
    }
}

Add compareTo to StringId.