Summary - Generics
- Java course
- Intermediate
- Lesson 87/116
- 5 min
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.
Practice 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);.
Starter code
// Define the Box<T> generic class (T value field, constructor)
// Add the Box.of generic method
// main: instantiate via Box.of and print with printlnSample 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);
}
}Expected output
Notebook
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What is the core benefit of generics?
- Compile-time type safety Correct answer
- Keeping type info at runtime
- Running code faster
- Removing reflection
Explanation: Generics give compile-time type safety; at runtime the type is erased.
2 Complete the generic factory method
static <T> Box____ of(T value) {
return new Box<>(value);
}static <T> Box<T> of(T value) {
return new Box<>(value);
}Explanation: <T> gives the Box class its type parameter.
3 Match the generics summary
| Term | Meaning |
|---|---|
<T> |
Type parameter |
<T extends Number> |
Bounded type |
? extends T |
Producer wildcard |
Box.of |
Generic factory method |