Java Common Coding Mistakes - Cheat Sheet

1. NullPointerException (NPE)

• Mistake: Calling methods on null objects.
• Fix:

  if (obj != null) {  
      obj.method();  
  }  
  // Or use Optional:  
  Optional.ofNullable(obj).ifPresent(o -> o.method());  
  

2. == vs. equals()

• Mistake: Using == for object comparison.
• Fix:

  String s1 = "hello", s2 = new String("hello");  
  System.out.println(s1.equals(s2)); // true (content comparison)  
  

3. Immutable Strings

• Mistake: Concatenating in loops (creates many objects).
• Fix: Use StringBuilder:

  StringBuilder sb = new StringBuilder();  
  for (int i = 0; i < 10; i++) sb.append(i);  
  String result = sb.toString();  
  

4. Forgetting break in Switch

• Mistake: Missing break causes fall-through.
• Fix:

  switch (x) {  
      case 1: System.out.println("One"); break;  
      case 2: System.out.println("Two"); break;  
      default: System.out.println("Unknown");  
  }  
  

5. Resource Leaks

• Mistake: Not closing streams/files.
• Fix: Use try-with-resources:

  try (FileInputStream fis = new FileInputStream("file.txt")) {  
      // Read file  
  } // Automatically closes  
  

6. Misusing ArrayList vs. LinkedList

• Mistake: Using LinkedList for frequent random access.
• Fix:
  • ArrayList → Fast access (O(1)).
  • LinkedList → Fast insertions/deletions (O(1)).

7. Ignoring ConcurrentModificationException

• Mistake: Modifying a collection while iterating.
• Fix: Use Iterator.remove() or CopyOnWriteArrayList:

  Iterator<Integer> it = list.iterator();  
  while (it.hasNext()) {  
      if (it.next() == 2) it.remove(); // Safe removal  
  }  
  

8. Overriding equals() Without hashCode()

• Mistake: Breaking the hashCode contract.
• Fix: Override both:

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

9. Using Raw Types

• Mistake: Not using generics.
• Fix: Always specify type:

  List<String> list = new ArrayList<>(); // Not `List list`  
  

10. Integer Division

• Mistake: Forgetting floating-point division.
• Fix: Cast to double:

  double result = (double) a / b;  
  

11. Static vs. Instance Methods/Variables

• Mistake: Accessing instance members from static methods.
• Fix: Make variable static or use an instance.

12. Unchecked Exceptions

• Mistake: Catching Exception instead of specific ones.
• Fix: Catch precise exceptions:

  try { ... }  
  catch (IOException e) { ... } // Not just `Exception`  
  

13. Mutable Date Objects

• Mistake: Using outdated java.util.Date.
• Fix: Use java.time (Java 8+):

  LocalDate date = LocalDate.now(); // Immutable  
  

14. Infinite Loops

• Mistake: Missing loop termination.
• Fix: Ensure exit condition:

  while (true) {  
      if (condition) break;  
  }  
  

15. Incorrect equals() Implementation

• Mistake: Not handling null or wrong types.
• Fix: Follow the contract:

  @Override  
  public boolean equals(Object o) {  
      if (this == o) return true;  
      if (o == null || getClass() != o.getClass()) return false;  
      MyClass obj = (MyClass) o;  
      return Objects.equals(field, obj.field);  
  }  
  

---

Best Practices:
✔ Use final for constants.
✔ Prefer interface types (e.g., List<String> list = new ArrayList<>()).
✔ Avoid float for precise calculations (use BigDecimal).
✔ Always override toString() for debugging.

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