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

## Code Review: [file/feature name]

### Critical
- [Issue description + line reference + suggestion]

### Improvements
- [Suggestion + rationale]

### Minor/Style
- [Nitpicks, optional improvements]

### Good Practices Observed
- [Positive feedback - important for morale]

---

Review Checklist

1. Null Safety

Check for:

// ❌ NPE risk
String name = user.getName().toUpperCase();

// ✅ Safe
String name = Optional.ofNullable(user.getName())
    .map(String::toUpperCase)
    .orElse("");

// ✅ Also safe (early return)
if (user.getName() == null) {
    return "";
}
return user.getName().toUpperCase();

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:

// ❌ Swallowing exceptions
try {
    process();
} catch (Exception e) {
    // silently ignored
}

// ❌ Catching too broad
catch (Exception e) { }
catch (Throwable t) { }

// ❌ Losing stack trace
catch (IOException e) {
    throw new RuntimeException(e.getMessage());
}

// ✅ Proper handling
catch (IOException e) {
    log.error("Failed to process file: {}", filename, e);
    throw new ProcessingException("File processing failed", e);
}

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:

// ❌ Modifying while iterating
for (Item item : items) {
    if (item.isExpired()) {
        items.remove(item);  // ConcurrentModificationException
    }
}

// ✅ Use removeIf
items.removeIf(Item::isExpired);

// ❌ Stream for simple operations
list.stream().forEach(System.out::println);

// ✅ Simple loop is cleaner
for (Item item : list) {
    System.out.println(item);
}

// ❌ Collecting to modify
List<String> names = users.stream()
    .map(User::getName)
    .collect(Collectors.toList());
names.add("extra");  // Might be immutable!

// ✅ Explicit mutable list
List<String> names = users.stream()
    .map(User::getName)
    .collect(Collectors.toCollection(ArrayList::new));

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:

// ❌ Not thread-safe
private Map<String, User> cache = new HashMap<>();

// ✅ Thread-safe
private Map<String, User> cache = new ConcurrentHashMap<>();

// ❌ Check-then-act race condition
if (!map.containsKey(key)) {
    map.put(key, computeValue());
}

// ✅ Atomic operation
map.computeIfAbsent(key, k -> computeValue());

// ❌ Double-checked locking (broken without volatile)
if (instance == null) {
    synchronized(this) {
        if (instance == null) {
            instance = new Instance();
        }
    }
}

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:

// ❌ Only equals without hashCode
@Override
public boolean equals(Object o) { ... }
// Missing hashCode!

// ❌ Mutable fields in hashCode
@Override
public int hashCode() {
    return Objects.hash(id, mutableField);  // Breaks HashMap
}

// ✅ Use immutable fields, implement both
@Override
public boolean equals(Object o) {
    if (this == o) return true;
    if (!(o instanceof User user)) return false;
    return Objects.equals(id, user.id);
}

@Override
public int hashCode() {
    return Objects.hash(id);
}

toString:

// ❌ Missing - hard to debug
// No toString()

// ❌ Including sensitive data
return "User{password='" + password + "'}";

// ✅ Useful for debugging
@Override
public String toString() {
    return "User{id=" + id + ", name='" + name + "'}";
}

Builders:

// ✅ For classes with many optional parameters
User user = User.builder()
    .name("John")
    .email("john@example.com")
    .build();

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:

// ❌ Resource leak
FileInputStream fis = new FileInputStream(file);
// ... might throw before close

// ✅ Try-with-resources
try (FileInputStream fis = new FileInputStream(file)) {
    // ...
}

// ❌ Multiple resources, wrong order
try (BufferedWriter writer = new BufferedWriter(new FileWriter(file))) {
    // FileWriter might not be closed if BufferedWriter fails
}

// ✅ Separate declarations
try (FileWriter fw = new FileWriter(file);
     BufferedWriter writer = new BufferedWriter(fw)) {
    // Both properly closed
}

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:

// ❌ Boolean parameters
process(data, true, false);  // What do these mean?

// ✅ Use enums or builder
process(data, ProcessMode.ASYNC, ErrorHandling.STRICT);

// ❌ Returning null for "not found"
public User findById(Long id) {
    return users.get(id);  // null if not found
}

// ✅ Return Optional
public Optional<User> findById(Long id) {
    return Optional.ofNullable(users.get(id));
}

// ❌ Accepting null collections
public void process(List<Item> items) {
    if (items == null) items = Collections.emptyList();
}

// ✅ Require non-null, accept empty
public void process(List<Item> items) {
    Objects.requireNonNull(items, "items must not be null");
}

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:

// ❌ String concatenation in loop
String result = "";
for (String s : strings) {
    result += s;  // Creates new String each iteration
}

// ✅ StringBuilder
StringBuilder sb = new StringBuilder();
for (String s : strings) {
    sb.append(s);
}

// ❌ Regex compilation in loop
for (String line : lines) {
    if (line.matches("pattern.*")) { }  // Compiles regex each time
}

// ✅ Pre-compiled pattern
private static final Pattern PATTERN = Pattern.compile("pattern.*");
for (String line : lines) {
    if (PATTERN.matcher(line).matches()) { }
}

// ❌ N+1 in loops
for (User user : users) {
    List<Order> orders = orderRepo.findByUserId(user.getId());
}

// ✅ Batch fetch
Map<Long, List<Order>> ordersByUser = orderRepo.findByUserIds(userIds);

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