Careeroza

Generic Classes

Writing a Generic Class

Example

Box<T> works for any type. Pair<K, V> has two type parameters.

public class Main {
    public static void main(String[] args) {
        Box<String> nameBox = new Box<>("Laptop");
        Box<Integer> qtyBox = new Box<>(3);
        String name = nameBox.get();          // no cast
        int qty = qtyBox.get();
        System.out.println(name + " x " + qty);

        Pair<String, Double> price = new Pair<>("Laptop", 54999.0);
        System.out.println(price);
        Pair<Integer, Boolean> status = new Pair<>(404, false);
        System.out.println(status.key() + " ok? " + status.value());
    }
}

class Box<T> {
    private final T value;
    Box(T value) { this.value = value; }
    T get() { return value; }
}

class Pair<K, V> {
    private final K key;
    private final V value;
    Pair(K key, V value) { this.key = key; this.value = value; }
    K key() { return key; }
    V value() { return value; }
    @Override public String toString() { return "(" + key + ", " + value + ")"; }
}

Output

Laptop x 3
(Laptop, 54999.0)
404 ok? false

Generic Interfaces

interface Repository<T, ID> {
    void save(T entity);
    T findById(ID id);
    List<T> findAll();
}

class ProductRepository implements Repository<Product, Long> {
    // T = Product, ID = Long
    ...
}

Note: Spring Data's JpaRepository<Product, Long> is exactly this idea.