Generics

Box<T> ek generic class hai — T ek placeholder hai jo jahan bhi Box use hota hai wahan ek real type se fill ho jaata hai, isliye Box<Integer> aur Box<String> dono bilkul usi class ke valid, type-safe instantiations hain, compiler call site par koi bhi type mismatch pakadte hue.

public class Main {
    static class Box<T> {
        private T value;

        Box(T value) {
            this.value = value;
        }

        T getValue() {
            return value;
        }
    }

    public static void main(String[] args) {
        Box<Integer> intBox = new Box<>(42);
        Box<String> strBox = new Box<>("hello");
        System.out.println(intBox.getValue());
        System.out.println(strBox.getValue());
    }
}

42
hello

Generic methods

Ek method apne aap generic ho sakta hai, apni class se independent — return type se pehle <T>, first(T a, T b) declare karta hai jo kisi bhi single type T ke liye kaam karta hai, compiler call site par arguments se T infer karte hue, koi explicit type ki zaroorat nahi.

public class Main {
    static <T> T first(T a, T b) {
        return a;
    }

    public static void main(String[] args) {
        System.out.println(first(10, 20));
    }
}

10

Aage: Collections Framework — HashMap aur HashSet, Java ke standard collection types mein se do.