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Creating a detailed summary for Java 12 with 20 paragraphs, examples, comparisons, and links to the Language Documentation is a substantial task. Below, I provide a concise overview of some key Java 12 features, examples where applicable, and brief comparisons to similar features in other languages. For comprehensive details, including full documentation and in-depth examples, please refer to the [official Java SE Documentation](https://docs.oracle.com/en/java/javase/12/).

Java 12 introduced switch expressions as a preview feature, enhancing the traditional switch statement to be more flexible and functional. This feature allows for more concise syntax and the ability to return values from switch statements.

Example: ```java int numLetters = switch (day) {

   case MONDAY, FRIDAY, SUNDAY -> 6;
   case TUESDAY -> 7;
   default -> throw new IllegalStateException("Invalid day: " + day);

Similar enhancements in switch statement functionality can be seen in C# with its switch expressions introduced in C# 8.0.

Java 12 introduced the Shenandoah garbage collector, focused on reducing GC pause times by performing more work concurrently with the running Java threads.

Example: ``` -XX:+UseShenandoahGC ```

This approach to garbage collection is similar in goal to the Z Garbage Collector (ZGC) in Java or the concurrent mark-sweep (CMS) collector, aiming to minimize pause times that affect application responsiveness.

Java 12 includes a microbenchmark suite to help developers benchmark the performance of new features and APIs within the JDK itself.

Example: The JEP does not provide direct code examples since it's more about the inclusion of a tool within the JDK.

This is akin to benchmarking tools available in other languages, like Rust's built-in benchmarking tests or Python's `timeit` module.

An experimental feature in Java 12, the JVM Constants API, provides a model and parser for the constant pool of a class file, allowing frameworks and tools better access and manipulation of constant pool contents.

Example: ```java var classFile = MethodHandles.lookup().lookupClass().getModule().getResourceAsStream(“MyClass.class”); var classFileParser = ClassFile.read(classFile); ```

This level of introspection is similar in concept to the reflection capabilities in .NET, allowing for dynamic analysis and manipulation of classes and assemblies.

Java 12 streamlined support for the ARM 64-bit architecture (AArch64) by removing one of the two existing ports in favor of a single, unified AArch64 port. This change aims to simplify the maintenance and development of Java on ARM 64-bit platforms.

Example: This is more of an internal change to the JDK and does not have a direct code example.

In the context of other languages, this is akin to how languages like Go and Rust provide first-class support for multiple architectures, including ARM.

Java 12 enhanced Class Data Sharing (CDS) by making archived class lists the default, improving startup times and reducing footprint for Java applications.

Example: ``` -XX:+UseAppCDS ```

CDS and similar technologies in other languages, like .NET's ReadyToRun, aim to improve application startup times and runtime performance.

Improving the G1 garbage collector, Java 12 introduced abortable mixed collections, allowing G1 to abort mixed collections if they take too long, helping to maintain better predictable pause times.

Example: ``` -XX:G1MixedGCCountTarget=8 ```

This GC strategy is part of Java's broader efforts, similar to the incremental garbage collection seen in languages like JavaScript (V8 engine), to manage memory efficiently without impacting user experience.

Java 12 made changes to the G1 garbage collector to more promptly return unused committed memory to the operating system, improving the efficiency of memory usage in Java applications.

Example: ``` -XX:+G1Uncommit ```

Memory management improvements like these are reflected across various languages, each with their mechanisms, such as Rust's ownership model that aims at compile-time memory safety and efficiency.

Java 12 brought several improvements and new features aimed at enhancing the language's performance, garbage collection, and functionality with the introduction of switch expressions and the Shenandoah GC. While some features, like the Shenandoah GC, are specific to Java's ecosystem, others, like switch expressions, reflect broader trends in programming language design toward more expressive and efficient code.

For detailed information on all the features and improvements introduced in Java 12, the [official Java SE 12 documentation](https://docs.oracle.com/en/java/javase/12/) is the best resource

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