Avoid These 10 Java Mistakes – Beginner’s Guide to Better Coding

Top 10 Common Java Mistakes Beginners Make (and How to Avoid Them)

1. Forgetting to Close Curly Braces {}

Missing a closing brace can break your program or mess up your code structure.
Example:

public void foo() {
  if (x > 0) {
    System.out.println("Hello");
// missing closing brace here!

Fix: Always check your braces. Use your code editor’s auto-format or brace matching tools to keep track.

2. Using == Instead of .equals() for Strings

In Java, == compares memory locations, not text content. For strings, always use .equals().
Example:

String a = "hello";
String b = new String("hello");
if (a == b) {  // This will be false!
  // Wrong comparison
}

Fix: Use a.equals(b) to compare string values correctly.

3. Getting NullPointerException 

Calling a method on a null object crashes your program with a NullPointerException.
Example:

String s = null;
int length = s.length();  // Causes NPE

Fix: Always check if an object is null before using it:

if (s != null) {
  int length = s.length();
}

4. Mixing int and double Causing Wrong Results

Integer division drops decimals, which leads to wrong calculations.
Example:

int a = 5, b = 2;
double c = a / b;  // c becomes 2.0, not 2.5

Fix: Cast one number to double:

double c = (double) a / b;

5. Off-by-One Errors in Loops

Looping too far or not far enough can cause errors or miss data.
Example:

for (int i = 0; i <= arr.length; i++) {  // Incorrect loop condition
  System.out.println(arr[i]);
}

Fix: Use < instead of <= when looping arrays:

for (int i = 0; i < arr.length; i++) { ... }

6. Variable Shadowing and Scoping Issues

Using the same variable name inside nested blocks can cause confusion or errors.
Example:

int x = 5;
if (true) {
  int x = 3;  // Shadowing variable x
}

Fix: Use different variable names and keep scopes small and clear.

7. Forgetting break in Switch Statements

Without a break, Java executes all following cases (“fall-through”).
Example:

switch (n) {
  case 1: System.out.println("One");
  case 2: System.out.println("Two");  // Runs even if n == 1
}

Fix: Add break; at the end of each case unless fall-through is intentional.

8. Catching Generic Exceptions

Catching all exceptions hides the real cause and makes debugging hard.
Example:

try {
  // code here
} catch (Exception e) {
  e.printStackTrace();  // Too generic
}

Fix: Catch specific exceptions like IOException and handle them properly.

9. Not Using Collections Correctly (Array vs ArrayList)

Arrays have fixed size and can’t grow dynamically. Beginners often struggle with this.
Example:

int[] arr = new int[3];
// Want to add more? Not possible without creating a new array.

Fix: Use ArrayList for dynamic lists:

ArrayList<Integer> list = new ArrayList<>();
list.add(4);

10. Poor Variable Names and No Comments

Unclear names and missing comments make code hard to read and maintain.
Example:

int x;
double y;

Fix: Use meaningful names:

int studentCount;
double productPrice;

Extra Tips for Java Beginners

• Use an IDE like IntelliJ IDEA or Eclipse to catch errors early.

• Learn to read error messages and stack traces — they tell you what went wrong.

• Test your code in small chunks often instead of writing huge programs at once.

• Use version control (Git) to save and track your work.

• Practice daily with small coding problems to improve steadily.