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Hi David!</div>
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Thank you for the encouragement. </div>
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As it turns out, you still get the constant-folding benefit if you create a Map using (for example) Map.of or Collectors.toUnmodifiableMap and then provide individual </div>
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ComputedConstans.of(size, mapper) as a replacement for your LargeList. But maybe you want to create your LargeList in one sweep?</div>
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I have experimented a bit with Maps containing ComputedConstant instances for a while. There is nothing preventing us from adding it later on. Maybe not in the first incarnation of ComputedConstants.</div>
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Here is a sketch of one of the many Map variants contemplated:<br>
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<span style="font-family: Consolas, Courier, monospace;"> /**</span>
<div class="ContentPasted1"><span style="font-family: Consolas, Courier, monospace;"> * {@return a new unmodifiable Map of {@link ComputedConstant } values with the provided</span></div>
<div class="ContentPasted1"><span style="font-family: Consolas, Courier, monospace;"> * {@code keys} and provided {@code presetMapper}}</span></div>
<div class="ContentPasted1"><span style="font-family: Consolas, Courier, monospace;"> * <p></span></div>
<div class="ContentPasted1"><span style="font-family: Consolas, Courier, monospace;"> * The Map and its values are eligible for constant folding optimizations by the JVM.</span></div>
<div class="ContentPasted1"><span style="font-family: Consolas, Courier, monospace;"> * <p></span></div>
<div class="ContentPasted1"><span style="font-family: Consolas, Courier, monospace;"> * Below, an example of how to cache values in a Map is shown:</span></div>
<div class="ContentPasted1"><span style="font-family: Consolas, Courier, monospace;"> * {@snippet lang = java:</span></div>
<div class="ContentPasted1"><span style="font-family: Consolas, Courier, monospace;"> * class DemoMap {</span></div>
<div class="ContentPasted1"><span style="font-family: Consolas, Courier, monospace;"> *</span></div>
<div class="ContentPasted1"><span style="font-family: Consolas, Courier, monospace;"> * private static final Map<Integer, ComputedConstant<User>> USER_ID_CACHE =</span></div>
<div class="ContentPasted1"><span style="font-family: Consolas, Courier, monospace;"> * ComputedConstant.of(List.of(0, 1, 1000), DB::findUserById);</span></div>
<div class="ContentPasted1"><span style="font-family: Consolas, Courier, monospace;"> *</span></div>
<div class="ContentPasted1"><span style="font-family: Consolas, Courier, monospace;"> * public User userFromCache(int userId) {</span></div>
<div class="ContentPasted1"><span style="font-family: Consolas, Courier, monospace;"> * return USER_ID_CACHE.get(userId);</span></div>
<div class="ContentPasted1"><span style="font-family: Consolas, Courier, monospace;"> * }</span></div>
<div class="ContentPasted1"><span style="font-family: Consolas, Courier, monospace;"> * }</span></div>
<div class="ContentPasted1"><span style="font-family: Consolas, Courier, monospace;"> *}</span></div>
<div class="ContentPasted1"><span style="font-family: Consolas, Courier, monospace;"> *</span></div>
<div class="ContentPasted1"><span style="font-family: Consolas, Courier, monospace;"> * @param <K> the type of the keys</span></div>
<div class="ContentPasted1"><span style="font-family: Consolas, Courier, monospace;"> * @param <V> the type of the values</span></div>
<div class="ContentPasted1"><span style="font-family: Consolas, Courier, monospace;"> * @param keys the keys to associate with ComputedConstant instances</span><br>
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<div class="ContentPasted1"><span style="display: inline !important; font-family: Consolas, Courier, monospace; background-color: rgb(255, 255, 255);" class="ContentPasted5"> * @param presetMapper the mapper to apply when computing values</span><br>
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<div class="ContentPasted1"><span style="font-family: Consolas, Courier, monospace;"> */</span></div>
<div class="ContentPasted1"><span style="font-family: Consolas, Courier, monospace;"> static <K, V> Map<K, ComputedConstant<V>> of(Collection<K> keys, Function<? super K, ? extends V) presetMapper) {</span></div>
<div class="ContentPasted1"><span style="font-family: Consolas, Courier, monospace;"> Objects.requireNonNull(keys);</span><br>
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<div class="ContentPasted1"><span style="display: inline !important; font-family: Consolas, Courier, monospace; background-color: rgb(255, 255, 255);" class="ContentPasted4"> Objects.requireNonNull(presetMapper);</span></div>
<div class="ContentPasted1"><span style="font-family: Consolas, Courier, monospace;"> ...</span></div>
<span style="font-family: Consolas, Courier, monospace;"> }</span><br>
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The above javadoc example will create a Map with the keys 0, 1, and, 100 for which the bound values of the ComputedConstants will be computed</div>
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by invoking the provided mapper that presumably will look up users in a database upon being referenced the first time.<br>
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This would allow you to do something like this:<br>
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<span style="font-family: Consolas, Courier, monospace;">Map<MyEnum, ComputedConstant<LargeList<MyResource>> map = </span></div>
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<span style="font-family: Consolas, Courier, monospace;"> ComputedContant.of(EnumSet.allOf( MyEnum.class ), SomeUtil::fetchLargeListOfMyResource);</span><br>
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<span style="font-family: Consolas, Courier, monospace;">MyResource myFooResource0 = map.get(MyEnum.FOO).get().get(0);</span><br>
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Let me know your thoughts on this.</div>
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Best, Per</div>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> leyden-dev <leyden-dev-retn@openjdk.org> on behalf of David Alayachew <davidalayachew@gmail.com><br>
<b>Sent:</b> Sunday, August 20, 2023 1:36 AM<br>
<b>To:</b> leyden-dev <leyden-dev@openjdk.org><br>
<b>Subject:</b> Computed Constant feature request</font>
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<div class="x_gmail_default" style="font-family:monospace">Hello Leyden Dev Team,</div>
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<div class="x_gmail_default" style="font-family:monospace">I am very excited for the ComputedConstant JEP that has been recently announced. There are lots of places in my current projects that could use a feature like this.</div>
<div class="x_gmail_default" style="font-family:monospace"><br>
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<div class="x_gmail_default" style="font-family:monospace">Today, for example, I want to lazy load a gigantic list of resources (>100). However, these resources are not fetched via an index, they are fetched via a key.</div>
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<div class="x_gmail_default" style="font-family:monospace">My current data type that I am storing them into is essentially a Map<MyEnum, LargeList<MyResource>>. I would like to be able to swap out this map for a ComputedConstant that can give me my LargeList<MyResource>
when I provide it an instance of MyEnum.<br>
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<div class="x_gmail_default" style="font-family:monospace">I could technically create a mapper that takes in MyEnum, turns it into its ordinal, and then does something like this.</div>
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<div class="x_gmail_default" style="font-family:monospace">ComputedConstant</div>
<div class="x_gmail_default" style="font-family:monospace"> .of</div>
<div class="x_gmail_default" style="font-family:monospace"> (</div>
<div class="x_gmail_default" style="font-family:monospace"> MyEnum.values().length,</div>
<div class="x_gmail_default" style="font-family:monospace"> i -> fetchLargeListOfMyResource(MyEnum.values()[i])<br>
</div>
<div class="x_gmail_default" style="font-family:monospace"> )</div>
<div class="x_gmail_default" style="font-family:monospace"> ;</div>
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<div class="x_gmail_default" style="font-family:monospace">But I'd like to avoid that if at all possible. It feels like I would end up right back into that error-prone territory from before.</div>
<div class="x_gmail_default" style="font-family:monospace"><br>
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<div class="x_gmail_default" style="font-family:monospace">Is this feature possible and worth doing as part of this JEP?<br>
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<div class="x_gmail_default" style="font-family:monospace"><br>
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<div class="x_gmail_default" style="font-family:monospace">Thank you for your time and help!</div>
<div class="x_gmail_default" style="font-family:monospace">David Alayachew<br>
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