shenandoah | Adds support for command-line and in-browser JavaScript | Application Framework library

You can do your task in this way:

Combine the all-states and abbreviations into a single column

This explanation will piggy-back on the first part and the second part of some answers I tried to put around Shenandoah 2.0.

To really answer this question we need to look at how the load reference barrier is implemented and how a GC cycle acts, in general.

When a certain GC cycle is triggered, it first chooses the regions with the most garbage; i.e.: objects that are in the collection set are very few (this will matter in the future). The simplest way to understand this topic is via an example. Suppose this is a scheme that now exists in a certain region:

First of all, every single java Object has two headers: klass and mark. They have been there in each instance since forever (they can slightly change how a JVM handles their flags internally with recent JVMs, for example) and are used for various reasons (will go into detail about only one of them a bit further in the answer).

The need for a forwarding pointer is literally in the second part of this answer. The forwarding pointer is needed in both read barrier and write barrier in Shenandoah 1.0 (though the read could skip the barrier for some field types - will not go into detail). In very simple words it simplifies concurrent copy very much. As said in that answer, it allows to atomically switch the forwarding pointer to the new copy of the Object and then concurrently update all references to point to that new Object.

Things have changed a bit in Shenandoah 2.0 where the "to-space invariant" is in place : meaning all the writes and reads are done via the to-space.This means one interesting thing : once the to-space copy is established, the from-copy is never used. Imagine a situation like this:

I will assume that the reader knows what a barriers is and why it is needed. For a very short intro here is another answer of mine on the topic.

In order to properly understand that, we need to first look at where the initial problem really was. Let's take a rather simple example:

From your description, your approach seems sound, and properly object-oriented. However, without additional information (e.g. specific aggregations which may dictate otherwise), it seems odd you would have multiple "duplicate" 'County' objects in your District objects. For example:

Since the input is quoted, we should specify the parameter sep (and not seq!) if we want scan to use 'non white space' as deliminator. From ?scan:

sep by default, scan expects to read ‘white-space’ delimited input fields. Alternatively, sep can be used to specify a character which delimits fields. A field is always delimited by an end-of-line marker unless it is quoted. If specified this should be the empty character string (the default) or NULL or a character string containing just one single-byte character.

Oracle does not support it, try downloading Oracle JDK and enabling it : -XX:+UseShenandoahGC, it will not work; it will though, if you get AdoptOpenJDK.

I think this Red Hat blog post is probably related, especially considering that Shenandoah works on your Linux machine and not macOS.

TL;DR: Red Hat led Shenandoah's development, so Oracle is hesitant to put it in OpenJDK. Not all builds support it as a result.

You see “test?” in url ? That must be replaced with the name of collection you try to connect to.