How to proper hashCode() for keys.

[John Smith]

Hi, getting java.lang.IllegalArgumentException: Key group 39 is not in
KeyGroupRange{startKeyGroup=96, endKeyGroup=103}. Unless you're directly
using low level state access APIs, this is most likely caused by
non-deterministic shuffle key (hashCode and equals implementation).

This is my class, is my hashCode deterministic?

public final class MyEventCountKey {
    private final String countDateTime;
    private final String domain;
    private final String event;

    public MyEventCountKey(final String countDateTime, final String
domain, final String event) {
        this.countDateTime = countDateTime;
        this.domain = domain;
        this.event = event;
    }

    public String getCountDateTime() {
        return countDateTime;
    }

    public String getDomain() {
        return domain;
    }

    public String getEven() {
        return event;
    }

    @Override
    public String toString() {
        return countDateTime + "|" + domain + "|" + event;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        MyEventCountKey that = (MyEventCountKey) o;
        return countDateTime.equals(that.countDateTime) &&
                domain.equals(that.domain) &&
                event.equals(that.event);
    }

    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + countDateTime.hashCode();
        result = prime * result + domain.hashCode();
        result = prime * result +  event.hashCode();
        return result;
    }
}