Step 1 — Identify the error type

Diagnose the Java error or stack trace I've provided and propose a fix. Be specific — do not give generic advice.

Step 1 — Identify the error type

Compile errors (javac / Maven / Gradle output):

• cannot find symbol → missing import, misspelled name, or wrong scope
• incompatible types → type mismatch; check generics, autoboxing, widening
• method X is not applicable → wrong number or types of arguments
• variable X might not have been initialized → missing initialisation on all code paths
• reached end of file while parsing → unmatched { or }
• class X is public, should be declared in a file named X.java → filename mismatch

Runtime exceptions (stack traces):

• NullPointerException → identify the line; determine which reference is null; suggest a null check or Optional
• ClassCastException → show the actual vs expected type; fix the cast or use instanceof first
• ArrayIndexOutOfBoundsException → check loop bounds and array length
• StackOverflowError → identify the recursive call; check base case
• ConcurrentModificationException → iterating and modifying a collection simultaneously; suggest iterator.remove() or a copy
• IllegalArgumentException / IllegalStateException → read the message; check the API contract

Step 2 — Find the root cause

Read the full error message carefully. The root cause is usually the last "Caused by:" in a stack trace. Show the user:

1. The exact line causing the error (if visible in the stack trace)
2. Why that line fails

Step 3 — Propose the fix

Show:

1. The problematic code (before)
2. The corrected code (after)
3. One sentence explaining what was wrong

If the fix requires reading a file not currently shown, ask for it: "Can you share the contents of ClassName.java? I need to see [specific thing]."

Step 4 — Prevent recurrence

Add one short note on how to avoid this class of error in the future (e.g., "Use Objects.requireNonNull() at method entry to catch nulls early").

Next Steps

After applying the fix:

• Suggest running the build to verify: mvn compile -q or ./gradlew build -q
• If the fix changed business logic → suggest running /java-test to verify behaviour is preserved
• If the root cause was a design problem → suggest running /java-review on the surrounding code