Write the test, then let a Java AI code generator try to pass it
Test a Java AI Code Generator with real Java tasks, compile generated code, run your own harness, compare multiple runs, and catch edge-case failures.
```java public static List<String> parse(String line)
eq("06 lone comma is two empty fields", ",", List.of("", "")); ```The first line is the contract for a CSV field splitter. A Java AI Code Generator can produce the implementation quickly, but the generated code still needs to prove that it follows the contract. The second is one of twelve checks in a harness I wrote before asking any model. It caught the only bad run of nine: one generation returned four empty fields for a lone comma.
I do not trust generated Java. I compile it, run my own tests, and keep whichever run passes.
Write the harness first
No framework: one main per task, a pass counter, a FAIL line for every miss. Three tasks: CsvLine.parse on the standard CSV quoting rules, twelve tests; a generic LruCache with constant-time get and put, eleven; DateRanges.merge for inclusive ranges that overlap or touch, fourteen. Every test passed against my own reference implementations first, so a later failure points at the generated class.
That makes the harness the baseline for evaluating each Java AI Code Generator run rather than relying on the generated code looking correct.
Pin the contract in the prompt
Class name, exact signature, the edge cases spelled out, and the same closing line every time: “Java 17, no external libraries, no package declaration.” Pinned that tightly, all nine generations compiled with zero edits. Two LRU runs drew an unchecked-operations note.
Open the generator
Rewind lists the code generator on its no-sign-up page under Our Free Popular Tools: click Show all categories and a Code tile appears. The generator’s FAQ says “No. You can use AI Code Generator right away without signing up.” In a fresh browser, the page asked for no sign-in before sending the request. I could not confirm a Java run finishing without an account, so treat that line as the site’s promise, not my result. Set Language to Java, one of 20 options. Paste, leave Advanced options empty, Generate. The label beside the button says Qwen 3 Coder, and the request asks for exactly that model.
Java in the Language box, the end of the CSV prompt with its Java 17 line, and Qwen 3 Coder named beside the Generate Java Code button.
| Setting | Value |
|---|---|
| Language | Java |
| Prompt | class name + signature + edge cases + “Java 17, no external libraries, no package declaration” |
| Advanced options | empty |
| Runs per task | 3 |
| Output | one fenced java block; Download .java |
| Check | javac + the harness |
Generate more than once
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Same prompt, three runs per task, interleaved. Running the Java AI Code Generator three times also makes it easier to see whether a passing result is repeatable or simply a lucky generation. Every response was one fenced java block, no prose. The first CSV run passed 10/12; the second and third passed 12/12. All three LRU runs passed 11/11, each hand-writing a HashMap plus doubly linked list rather than LinkedHashMap. The date runs sorted a copy and did their arithmetic on LocalDate, so year-end and leap day passed, 14/14 three times.
Copy, Download .java and Try again sit above the result, and the class opens with two imports and a public static parse method.
Compile, run the harness, and read the failure
Resist Try again. Read the class. The first CSV run compiled clean and looked tidy; the bug was a special case bolted onto a loop that already handled the lone comma:
``java // Special case: line with only commas should have appropriate number of empty fields if (line.equals(",")) { fields.add(""); fields.add(""); } ``
`` FAIL 06 lone comma is two empty fields input=[,] expected=[<> | <>] size=2 got=[<> | <> | <> | <>] size=4 FAIL 10 unclosed quote input=["x,y] expected IllegalArgumentException, got=[<x,y>] size=1 ``The second failure: the quoted-field loop ran off the end of the line without noticing the quote never closed. Two edits, a closed flag that throws and the special case deleted, and the harness reports 12/12. The other two runs got both cases right. Let the harness pick the run, never your eye.
Know the budget
The site’s own ledger showed 94 to 325 tokens a run, median 208. The catch came before the first token: on one Java prompt the site refused to start until 1,996 tokens were free, several times what any run cost. The published allowances are 2,500 tokens a day without an account and 5,000 with a free one. On that arithmetic, a no-account day has room for a few starts and a free account for roughly fifteen at the median.
| Task | Runs | Compiled as generated | Passed every test | Tokens per run |
|---|---|---|---|---|
| CsvLine.parse | 3 | 3 | 2 | 231, 150, 237 |
| LruCache<K,V> | 3 | 3 | 3 | 325, 294, 208 |
| DateRanges.merge | 3 | 3 | 3 | 106, 165, 94 |
Where the others fit
CodeConvert has a Run code button beside the output. That distinction matters when choosing a Java AI Code Generator, because being able to execute generated code can shorten the gap between generation and verification. Developers can also explore JavaScript libraries when working across JavaScript-based projects and generated code. Rewind hands you Copy, Download .java and Try again; you compile it yourself, so if your question is “does it run”, that button is a real advantage. Two generations a day without an account, per its FAQ and the counter under Generate; 5 free credits with a free account; paid plans from $10 a month billed yearly. Two is a plain number to plan around; without an account, Rewind is not clearly more generous. I did not press the button; its FAQ lists code execution among the reasons to sign in, so I claim the button, nothing more.
ZZZ Code AI asks for nothing. No sign-up, no counter, a 10,000-character box, and an option for three candidates in one click, which is the run-it-again step done for you. It mentions a daily limit without a number. Before pasting employer code, read its Legal page: by submitting, you grant “a perpetual, irrevocable, worldwide, royalty-free, non-exclusive license to use, reproduce, modify, adapt, publish” the input. Rewind’s FAQ says input “isn’t stored permanently unless you choose to save it. We don’t sell or share it.” Each site’s own words; weigh them yourself.
The harness stays in the repo next to whichever generated class passed. Generators swap models; next year I will paste this contract into whatever is free that week. The twelve CSV tests do not change. That is the part I trust.
