Generics

Box<T> is a generic class — T is a placeholder filled in with a real type wherever Box is used, so Box<Integer> and Box<String> are both valid, type-safe instantiations of the exact same class, with the compiler catching any type mismatch at the call site.

Main.java

Try it yourself →

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());
    }
}

Output

42
hello

Generic methods

A method can be generic on its own, independent of its class — <T> before the return type declares first(T a, T b) works for any single type T, with the compiler inferring T from the arguments at the call site, no explicit type needed.

Main.java

Try it yourself →

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));
    }
}

Output

Next: the Collections Framework — HashMap and HashSet, two of Java's standard collection types.

Practice exercise

Define a generic class Box<T> with a value field and a getValue() method. Create a Box<String> holding "hello", then print its value.

← Exception Handling The Collections Framework →