Summary - Generics

Combine generic class and method to write a Box<T>.

Summary - Generics

In this group you learned generics: generic class and method with <T>, bounded type with <T extends Number>, wildcards ? extends T and ? super T, and finally type erasure. Generics give compile-time type safety; at runtime the type is erased.

In this summary we will combine everything into a Box<T> class and a Box.of generic factory method. You will see generic class features, generic method type inference and the diamond operator together.

Exercise Box<T> class and Box.of method

Define class Box<T> with a T value field, a Box(T value) constructor and a static <T> Box<T> of(T value) generic factory method whose body is return new Box<>(value);. In main, instantiate with Box<String> b = Box.of("Notebook"); and print it with System.out.println(b.value);.

Show example solution

class Box<T> {
    T value;
    Box(T value) { this.value = value; }
    static <T> Box<T> of(T value) {
        return new Box<>(value);
    }
}

public class Main {
    public static void main(String[] args) {
        Box<String> b = Box.of("Notebook");
        System.out.println(b.value);
    }
}

Test yourself

What is the core benefit of generics?
- Compile-time type safety
- Keeping type info at runtime
- Running code faster
- Removing reflection
Show answer

Compile-time type safety — Generics give compile-time type safety; at runtime the type is erased.

Complete the generic factory method

static <T> Box____ of(T value) {
    return new Box<>(value);
}

Show answer

static <T> Box<T> of(T value) {
    return new Box<>(value);
}

Match the generics summary

<T>

Type parameter
<T extends Number>

Bounded type
? extends T

Producer wildcard
Box.of

Generic factory method

Next lesson: List<E>

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