4 · Data & Collections · lesson 22 of 41

Generics

Type parameters for reusable, type-safe containers.

Java

class Box<T> {
    private T value;
    public void set(T v) { value = v; }
    public T get() { return value; }
}

Box<Integer> b = new Box<>();
b.set(42);

// Bounded type parameter
static <T extends Comparable<T>> T max(T a, T b) {
    return a.compareTo(b) > 0 ? a : b;
}

List<? extends Number> readOnly;   // upper-bounded wildcard

Key takeaways

✓Generics are erased at runtime — type checks happen at compile time.
✓Use bounds (T extends X) to constrain what T can be.

← Back to Java playground