Avoid These Java Collections Mistakes and Follow Best Practices

Ramesh Fadatare

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4 min read

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Mar 17, 2025

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Java Collections Framework is powerful, but many developers unknowingly make mistakes that can lead to memory leaks, poor performance, and unexpected behavior. 😱

In this article, we will explore the top 10 mistakes developers make when using Java Collections and how to fix them with best practices. Let’s dive in! 🚀

🔴 1️⃣ Using the Wrong Collection Type

Mistake: Choosing the wrong collection type can reduce performance or cause unexpected behavior.

❌ Bad Practice: Using ArrayList Instead of LinkedList for Frequent Insertions

List<Integer> list = new ArrayList<>();
for (int i = 0; i < 100000; i++) {
    list.add(0, i); // ❌ Inefficient! Shifts all elements on each insert
}

👉 Issue: ArrayList is bad for frequent insertions because it shifts all elements, leading to O(n) complexity.

✅ Best Practice: Use LinkedList for Frequent Insertions

List<Integer> list = new LinkedList<>();
for (int i = 0; i < 100000; i++) {
    list.add(0, i); // ✅ Efficient! O(1) insertions at the head
}

🔹 Fix: Use LinkedList when you need frequent insertions/removals at both ends.

🔴 2️⃣ Not Using Generics (Type Safety Issue)

Mistake: Using raw types instead of generics, leading to type safety issues.

❌ Bad Practice: Using a Raw Type List

List list = new ArrayList();
list.add("Java");
list.add(100); // ❌ No type safety! May cause `ClassCastException`

✅ Best Practice: Use Generics for Type Safety

List<String> list = new ArrayList<>();
list.add("Java"); // ✅ Compile-time safety
// list.add(100); ❌ Compilation error - prevents accidental type mismatches

🔹 Fix: Always use Generics to avoid runtime exceptions.

🔴 3️⃣ Ignoring hashCode() and equals() in Hash-based Collections