```ts
export class LruCache<K, V> {
    private capacity: number;
    private cache: Map<K, V>;
    private order: K[];

    constructor(capacity: number) {
        if (!Number.isInteger(capacity) || capacity <= 0) {
            throw new Error("Capacity must be a positive integer");
        }
        this.capacity = capacity;
        this.cache = new Map<K, V>();
        this.order = [];
    }

    get(key: K): V | undefined {
        if (this.cache.has(key)) {
            const index = this.order.indexOf(key);
            if (index !== -1) {
                this.order.splice(index, 1);
                this.order.push(key);
            }
            return this.cache.get(key)!;
        }
        return undefined;
    }

    set(key: K, value: V): void {
        if (this.cache.has(key)) {
            const index = this.order.indexOf(key);
            if (index !== -1) {
                this.order.splice(index, 1);
            } else if (this.cache.size === this.capacity) {
                const oldestKey = this.order.shift()!;
                this.cache.delete(oldestKey);
            }
        }
        this.cache.set(key, value);
        this.order.push(key);
    }

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