Hvad er generics, og hvorfor skal man bruge dem?

// ❌ before generics (or with raw types) — no type safety, requires casts
List list = new ArrayList();
list.add("hello");
list.add(42);                       // oops — anything can go in
String s = (String) list.get(1);    // ClassCastException at RUNTIME! 💥

// ✅ with generics — the type is enforced at COMPILE time
List<String> list = new ArrayList<>();
list.add("hello");
list.add(42);                       // ❌ compile error — caught immediately
String s = list.get(0);             // no cast needed — known to be String

// a generic class — T is a type parameter filled in by the user
public class Box<T> {
    private T value;
    public void set(T value) { this.value = value; }
    public T get() { return value; }
}
Box<String> b = new Box<>();   // T = String
b.set("hi");
String x = b.get();             // typed, no cast

// a generic method
public static <T> T firstElement(List<T> list) {
    return list.get(0);
}

// T must be Number (or a subclass) — lets you call Number methods
public static <T extends Number> double sum(List<T> list) {
    double total = 0;
    for (T n : list) total += n.doubleValue();   // safe — T is a Number
    return total;
}

List<? extends Number> nums;   // "some subtype of Number" — read (producer)
List<? super Integer> sink;    // "Integer or a supertype" — write (consumer)
// PECS: Producer Extends, Consumer Super

// generics are a COMPILE-TIME feature — erased at runtime (for backward compatibility)
List<String> a = new ArrayList<>();
List<Integer> b = new ArrayList<>();
a.getClass() == b.getClass();   // true — both are just ArrayList at runtime
// consequence: can't do `new T()` or `instanceof List<String>`