Java Generics — Free Java Tutorial

TL;DR: Learn Java Generics in Java with a free, beginner-friendly tutorial, examples and practice for Indian students on Syllab.in.

Java Generics in Java

Generics allow you to write code that works with any type, specified at compile time. They provide type safety without casting: List<String> vs raw List.

Generic class: class Box<T> { T value; }. Generic method: <T> T getFirst(List<T> list). Use extends for bounds: <T extends Number> means T must be a Number or subclass.

Wildcards: List<?> accepts any type. List<? extends Number> accepts any Number subtype (read-only). List<? super Integer> accepts Integer or any parent (write-only).

Generics are erased at runtime (type erasure) — they exist only at compile time for type checking. At runtime, a List<String> is just a List.

// Generic class
class Pair<A, B> {
    private A first;
    private B second;
    Pair(A first, B second) { this.first = first; this.second = second; }
}

// Generic method
public static <T extends Comparable<T>> T max(T a, T b) {
    return a.compareTo(b) >= 0 ? a : b;
}

// Bounded wildcard
public static double sum(List<? extends Number> list) {
    return list.stream().mapToDouble(Number::doubleValue).sum();
}

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