```ts
export class LruCache<K, V> {
    #capacity: number;
    #cache = new Map<K, V>();
    #usageOrder: K[] = [];

    constructor(capacity: number) {
        if (!Number.isInteger(capacity) || capacity <= 0) {
            throw new Error('Capacity must be a positive integer');
        }
        this.#capacity = capacity;
    }

    get(key: K): V | undefined {
        const value = this.#cache.get(key);
        if (value !== undefined) {
            this.#usageOrder = this.#usageOrder.filter(k => k !== key).concat(key);
        }
        return value;
    }

    set(key: K, value: V): void {
        if (this.#cache.has(key)) {
            this.#usageOrder = this.#usageOrder.filter(k => k !== key).concat(key);
        } else if (this.#cache.size >= this.#capacity) {
            const leastRecentlyUsedKey = this.#usageOrder.shift();
            if (leastRecentlyUsedKey) {
                this.#cache.delete(leastRecentlyUsedKey);
            }
        }
        this.#cache.set(key, value);
        this.#usageOrder.push(key);
    }

    get size(): number {
        return this.#cache.size;
    }
}
```