Design pattern vs algorithm — concept vs computation

Direct Answer

An algorithm is a defined procedure for computing a result or performing a task, such as binary search or a graph traversal. A design pattern is a named, reusable arrangement of responsibilities and collaborations for a recurring design problem. Algorithms can be deterministic or randomized and have measurable complexity; patterns are adapted to a codebase and judged by design trade-offs.

Code Example

// ALGORITHM: a concrete, deterministic procedure (binary search)
static int binarySearch(int[] a, int key) {
    int lo = 0, hi = a.length - 1;
    while (lo <= hi) {
        int mid = (lo + hi) >>> 1;          // exact steps, same every time
        if (a[mid] == key) return mid;
        if (a[mid] < key)  lo = mid + 1;
        else               hi = mid - 1;
    }
    return -1;
}

// DESIGN PATTERN: a structural template you adapt (Strategy holds algorithms)
interface SearchStrategy { int search(int[] a, int key); }
class BinarySearchStrategy implements SearchStrategy {
    public int search(int[] a, int key) { return binarySearch(a, key); }
}
// The algorithm is the method body; the pattern is the class design around it.

Common Mistakes & Pitfalls

• Calling Strategy an algorithm: Freshers say 'Strategy pattern is an algorithm for swapping behaviors' — it's a pattern for structuring interchangeable algorithms, not an algorithm itself.
• Thinking algorithms are patterns: Saying 'I used the binary search pattern' when you mean 'I implemented the binary search algorithm' — patterns structure classes, algorithms compute results.
• Missing the abstraction level: Algorithms are concrete and procedural; patterns are abstract and organisational. Confusing the two shows you don't understand that code has both computational logic (algorithms) and structural design (patterns).