Java Tutorial · Advanced Java · Lesson 30 of 37

Java Generics

Write type-safe classes and methods that work with any type.

Why generics?

Generics let a class or method work with any type while keeping compile-time type checking. List<String> can only hold strings — no casts, no surprises at runtime.

Generic classes

Example 1

public class Main {
    public static void main(String[] args) {
        Box<String> name = new Box<>("Ada");
        Box<Integer> count = new Box<>(42);
        System.out.println(name.get().toUpperCase() + " " + (count.get() + 1));
        Pair<String, Double> price = new Pair<>("coffee", 3.5);
        System.out.println(price);
    }
}

class Box<T> {
    private final T value;
    Box(T value) { this.value = value; }
    T get() { return value; }
}

record Pair<A, B>(A first, B second) {}

Output

ADA 43
Pair[first=coffee, second=3.5]

Run this on your computer — see “Get Started” for setup.

Generic methods and bounds

A type parameter can be bounded: <T extends Comparable<T>> means "any T that can be compared":

Example 2

import java.util.List;

public class Main {
    public static void main(String[] args) {
        System.out.println(max(List.of(3, 9, 4)));
        System.out.println(max(List.of("pear", "apple", "zucchini")));
        System.out.println(sum(List.of(1, 2, 3)) + " " + sum(List.of(1.5, 2.5)));
    }

    static <T extends Comparable<T>> T max(List<T> items) {
        T best = items.get(0);
        for (T item : items) {
            if (item.compareTo(best) > 0) best = item;
        }
        return best;
    }

    static double sum(List<? extends Number> numbers) {
        double total = 0;
        for (Number n : numbers) total += n.doubleValue();
        return total;
    }
}

Output

9
zucchini
6.0 4.0

Run this on your computer — see “Get Started” for setup.

Wildcards: List<? extends Number> accepts lists of Integer, Double… for reading; List<? super Integer> for writing. Remember PECS — Producer Extends, Consumer Super.

Test yourself

What does <T extends Comparable<T>> mean?

T must be a subclass of Comparable
T must be a type that can be compared to itself
T can be any type
T must be a number

Show answer

Answer: T must be a type that can be compared to itself