Common Pitfalls in Java Arrays

Arrays are powerful, but beginners often encounter mistakes when using them. Knowing these common pitfalls will help you write bug-free and efficient code.

1. ArrayIndexOutOfBoundsException

Occurs when you access an index that doesn’t exist in the array.

public class OutOfBoundsExample {
    public static void main(String[] args) {
        int[] numbers = {1, 2, 3};
        System.out.println(numbers[3]); //  Error
    }
}

Error : Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: Index 3 out of bounds for length 3

2. Null Arrays

If an array reference points to null, any operation will throw a NullPointerException.

public class NullArrayExample {
    public static void main(String[] args) {
        int[] numbers = null;
        System.out.println(numbers.length); //  Error
    }
}

int[] numbers = new int[5]; // Correct

3. Choosing the Wrong Array Size

Arrays in Java have fixed sizes. Declaring a size too small can cause errors, too large wastes memory.

int[] scores = new int[3]; // Can only store 3 elements
scores[3] = 50; //  OutOfBounds

4. Arrays vs Collections

Feature	Array	Collection (e.g., ArrayList)
Size	Fixed	Dynamic (can grow/shrink)
Type	Can store primitives	Stores objects only
Performance	Faster (less overhead)	Slightly slower (more flexible)
Methods	Limited (length, indexing)	Many utility methods (add, remove, sort, contains)

Common mistake: Using arrays when a dynamic structure is needed, or using ArrayList when performance-critical.

5. Mixing Types

Arrays can only store a single data type. Trying to mix types leads to compilation errors.

Object[] arr = {1, "Java", true}; //  Possible with Object array
int[] numbers = {1, "2"};        //  Error

Tips to Avoid Common Pitfalls

Always check array bounds.
Initialize arrays before use.
Use numbers.length to control loops.
Choose arrays for fixed-size, primitive-heavy data.
Prefer Collections for dynamic and object-heavy data.
Avoid mixing incompatible types.

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