Generics

A type parameter lets one algorithm preserve the caller's element type without unsafe casts.

Build a Complete Mental Model

Generics provide compile-time type safety and reusable algorithms. Java generics are invariant; use producer extends and consumer super (PECS), and account for runtime limitations caused by type erasure.

Apply It in Real Java

Write a generic `first` method and a copy method using bounds and wildcards, then verify compiler diagnostics for raw types and invalid assignments.

After This Lesson

• Explain the contract and runtime behavior of Generics
• Compile and run an original example
• Automate a boundary or failure test

import java.util.List;

class Main {
    static <T> T first(List<T> values) {
        if (values.isEmpty()) throw new IllegalArgumentException("empty list");
        return values.getFirst();
    }

    public static void main(String[] args) {
        String name = first(List.of("Ada", "Linus"));
        System.out.println(name);
    }
}

Ada

Java Language Specification, Java SE 25 — Oracle