Systematic code review for Java with null safety, exception handling, concurrency, and performance checks. Use when user says "review code", "check this PR", "code review", or before merging changes.

Java Code Review Skill Systematic code review checklist for Java projects. When to Use User says "review this code" / "check this PR" / "code review" Before merging a PR After implementing a feature Review Strategy 1. Quick scan Understand intent, identify scope 2. Checklist pass Go through each category below 3. Summary List findings by severity (Critical → Minor) Output Format Review Checklist 1. Null Safety Check for: Flags: Chained method calls without null checks Missing @Nullable / @NonNull annotations on public APIs Optional.get() without isPresent() check Returning null from methods that could return Optional or empty collection Suggest: Use Optional for return types that may be absent Use Objects.requireNonNull() for constructor/method params Return empty collections instead of null: Collections.emptyList() 2. Exception Handling Check for: Flags: Empty catch blocks Catching Exception or Throwable broadly Losing original exception (not chaining) Using exceptions for flow control Checked exceptions leaking through API boundaries Suggest: Log with context AND stack trace Use specific exception types Chain exceptions with cause Consider custom exceptions for domain errors 3. Collections & Streams Check for: Flags: Modifying collections during iteration Overusing streams for simple operations Assuming Collectors.toList() returns mutable list Not using List.of() , Set.of() , Map.of() for immutable collections Parallel streams without understanding implications Suggest: List.copyOf() for defensive copies removeIf() instead of iterator removal Streams for transformations, loops for side effects 4. Concurrency Check for: Flags: Shared mutable state without synchronization Check then act patterns without atomicity Missing volatile on shared variables Synchronized on non final objects Thread unsafe lazy initialization Suggest: Prefer immutable objects Use java.util.concurrent classes AtomicReference , AtomicInteger for simple cases Consider @ThreadSafe / @NotThreadSafe annotations 5. Java Idioms equals/hashCode: toString: Builders: Flags: equals without hashCode Mutable fields in hashCode Missing toString on domain objects Constructors with 3 4 parameters (suggest builder) Not using instanceof pattern matching (Java 16+) 6. Resource Management Check for: Flags: Not using try with resources for Closeable / AutoCloseable Resources opened but not in try with resources Database connections/statements not properly closed 7. API Design Check for: Flags: Boolean parameters (prefer enums) Methods with 3 parameters (consider parameter object) Inconsistent null handling across similar methods Missing validation on public API inputs 8. Performance Considerations Check for: Flags: String concatenation in loops Regex compilation in loops N+1 query patterns Creating objects in tight loops that could be reused Not using primitive streams ( IntStream , LongStream ) 9. Testing Hints Suggest tests for: Null inputs Empty collections Boundary values Exception cases Concurrent access (if applicable) Severity Guidelines Severity Criteria Critical Security vulnerability, data loss risk, production crash High Bug likely, significant performance issue, breaks API contract Medium Code smell, maintainability issue, missing best practice Low Style, minor optimization, suggestion Token Optimization Focus on changed lines (use git diff ) Don't repeat obvious issues group similar findings Reference line numbers, not full code quotes Skip files that are auto generated or test fixtures Quick Reference Card Category Key Checks Null Safety Chained calls, Optional misuse, null returns Exceptions Empty catch, broad catch, lost stack trace Collections Modification during iteration, stream vs loop Concurrency Shared mutable state, check then act Idioms equals/hashCode pair, toString, builders Resources try with resources, connection leaks API Boolean params, null handling, validation Performance String concat, regex in loop, N+1

java-code-review