Desugaring Java 23 to 1.1: My 5 Insane Steps (2025)

Introduction: The Absurd Challenge

Ever stared at a piece of sleek, modern Java 23 code and wondered, "Could this run on a Pentium II?" No? Well, I did. In a fit of caffeine-fueled curiosity, I embarked on a journey that most would call pointless, and others, certifiably insane: desugaring modern Java 23 code, with all its bells and whistles, to be compatible with the Java 1.1 runtime. For context, Java 1.1 was released in 1997. It predates the Collections Framework, Generics, and even the `strictfp` keyword.

This isn't just about compiling with `-source 1.1`. This is about manually rewriting and polyfilling modern constructs to function in an environment that lacks them entirely. It's a deep dive into what the Java compiler does for us automatically, but taken to a horrifying extreme. Buckle up. This is how I made Java 23 run on a runtime from the last millennium, in five insane steps.

The 'Why': A Thought Experiment in Extreme Legacy Support

Before we dive into the technical abyss, let's address the elephant in the room: why? This project isn't for a real-world production system (I hope!). Instead, it serves two purposes. First, it's the ultimate thought experiment in backward compatibility and a testament to the foundational principles of the JVM. Second, it's an incredible educational tool. By manually deconstructing features like lambdas, virtual threads, and pattern matching, you gain an unparalleled appreciation for the elegance and power they provide. It forces you to understand not just what the code does, but what it represents at a much lower level.

The 5 Insane Steps to Java 1.1

This process is a minefield of manual labor, custom tooling, and debugging nightmares. Here’s the path I took through the madness.

Step 1: Building the Polyfill Universe

The first and most daunting hurdle is the complete absence of the Java Collections Framework, which was introduced in Java 1.2. This means no `List`, no `Map`, no `Set`, and certainly no `ArrayList` or `HashMap`. Your only tools are the primitive `Vector` and `Hashtable`.

My first step was to create a massive polyfill library. I had to:

• Re-implement Core Interfaces: Create my own `com.mypolyfills.List` and `com.mypolyfills.Map` interfaces.
• Build Concrete Classes: Implement `MyArrayList` using an underlying `Vector` and `MyHashMap` using an underlying `Hashtable`. This involved meticulously recreating methods like `get`, `put`, `remove`, and ensuring they matched the modern contract as closely as possible.
• Forge the `Iterable` Interface: Java 1.1 has no concept of the `for-each` loop because `java.lang.Iterable` doesn't exist. I had to create my own `Iterable` interface and a corresponding `Iterator`, then manually convert all `for-each` loops into old-school `Iterator` `while` loops.

This foundation took up 60% of the total effort. Without a functioning, albeit inefficient, collections library, no other desugaring is possible.

Step 2: Taming the Type System - A World Without Generics

With a polyfill library in place, the next challenge was Generics. Introduced in Java 5, generics provide compile-time type safety. Java 1.1 has none of this. Every collection is a raw type, holding `Object` instances.

This step involved a brute-force search-and-replace across the entire codebase:

• Type Erasure on Steroids: Every `List`, `Map`, or any other generic type had to be replaced with its raw polyfill equivalent (`MyList`, `MyMap`).
• The Casting Nightmare: Since every collection now returns `Object`, every single retrieval requires an explicit, manual cast. What was once a clean `String name = names.get(0);` becomes `String name = (String) names.get(0);`.

The loss of compile-time safety is terrifying. A single incorrect cast, previously impossible, now becomes a `ClassCastException` lurking at runtime. Debugging becomes an exercise in tracing object types through a sea of `Object` references.

Step 3: Deconstructing Lambdas and Streams into Anonymous Classes

This is where the beauty of modern Java shatters into a thousand pieces of boilerplate. Lambdas and Streams (Java 8) are syntactic sugar over anonymous classes and complex iterator logic.

A simple Java 23 stream operation like this:

List upperCaseNames = users.stream().filter(u -> u.isActive()).map(User::getName).collect(Collectors.toList());

...explodes into this Java 1.1 monstrosity:

MyList upperCaseNames = new MyArrayList();
MyIterator userIterator = users.iterator();
while (userIterator.hasNext()) {
User u = (User) userIterator.next();
if (new UserActivePredicate().test(u)) {
String name = new UserNameMapper().apply(u);
upperCaseNames.add(name);
}
}

This required creating explicit classes or, more commonly, anonymous inner classes for every single lambda, predicate, and function. The `collect` operation had to be rewritten as a manual loop that adds elements to a new collection. The code's intent is drowned in a flood of ceremony.

Step 4: Unraveling Virtual Threads into Raw `Thread` Objects

Virtual Threads (Project Loom) are lightweight, managed threads that allow for massive concurrency with minimal overhead. Java 1.1 has only one tool for concurrency: `java.lang.Thread`, which maps directly to a heavy OS thread.

Desugaring this concept is less about a direct translation and more about a crude, inefficient simulation. My approach was:

• Replace every `Executors.newVirtualThreadPerTaskExecutor()` with a cached thread pool implementation... oh wait, `Executors` doesn't exist.
• Okay, plan B: Manually implement a rudimentary thread pool using a `Vector` of `Thread` objects.
• Every task submitted to the virtual thread executor becomes a `Runnable` that I pass to my custom, rickety thread pool manager.

The result is a system that completely loses the "lightweight" benefit. Spawning a thousand tasks now means spawning a thousand heavyweight OS threads, which would instantly crash any machine from that era (and most from this one).

Step 5: The Final Boss - Rewriting Records and Pattern Matching

Finally, we tackle some of the latest syntactic sugar: Records for boilerplate-free data carriers and Pattern Matching for `instanceof` and `switch` for elegant type checks.

• Records to POJOs: Every `record User(String name, int age)` must be manually converted into a full-fledged Plain Old Java Object (POJO): a final class with private final fields, a constructor to initialize them, a getter for each field, and manually written `equals()`, `hashCode()`, and `toString()` methods. It's a verbose, error-prone process.
• Pattern Matching to `if-else` Chains: A clean Java 23 `switch` with type patterns becomes a hideous cascade of `if-else if` blocks.

For example, `if (obj instanceof String s)` becomes:

if (obj instanceof String) {
String s = (String) obj;
// ... use s here
}

This manual casting and reassignment for every single pattern match adds significant clutter and cognitive load, obscuring the core logic.

Conclusion: Reflections on a Ridiculous Journey

After weeks of this painstaking work, I had a JAR file. A JAR file that contained Java 23 logic, horrifically mangled and bloated, but technically capable of running on a Java 1.1 VM. Was it worth it? As a practical matter, absolutely not. The performance was atrocious, the code was unreadable, and the resulting binary was massive.

But as a learning experience, it was invaluable. This journey illuminated the sheer amount of complexity that modern Java and its compiler handle for us. Every new feature, from generics to virtual threads, isn't just a convenience; it's a paradigm shift that abstracts away mountains of boilerplate, enhances safety, and unlocks new possibilities. We stand on the shoulders of giants—the engineers who designed these features over decades. So next time you write a simple lambda or use a record, take a moment to appreciate the abyss of anonymous inner classes and manual getters you've been spared.