What is the difference between object-oriented programming and functional programming?

OOP focuses on objects, their state, and behavior. Functional programming emphasizes functions and data transformation.

Main differences:

• State: In OOP, objects have mutable state. In FP, functions generally do not have mutable state and work with immutable data (immutability).
• Side effects: In OOP, methods can have side effects (modify object state). In FP, functions aim to be pure (no side effects, return the same result for the same inputs).
• Paradigm: OOP is based on classes, objects, inheritance, polymorphism, encapsulation. FP is based on functions as first-class entities, lambda calculus, recursion, immutability.
• Code structure: In OOP, code is organized around objects. In FP, code is organized around functions that transform data.
• Concurrency: FP usually simplifies parallel execution due to the absence of mutable state and side effects. OOP requires careful management of state in parallelism.

Example (simplified):

OOP:

class Counter {
    private int count = 0; // Object state

    public void increment() {
        count++; // State change (side effect)
    }

    public int getCount() {
        return count;
    }
}

FP:

// Function for incrementing. Does not change external state.
// Returns a new value.
public static int increment(int value) {
    return value + 1;
}

In real systems, elements of both paradigms are often used.