Generics in Java

From the Java cheat sheet · Generics · verified Jul 2026

Generic classes, methods, bounds, and wildcards.

java

// Quick Reference
class Box<T> { T value; }
<T> T first(List<T> list) { return list.get(0); }
List<? extends Number> nums;   // wildcard

💡 Generics give compile-time type safety and remove manual casts.

⚡ <T extends Number> bounds the type so you can call Number methods on T.

📌 Use ? extends for reading (producers), ? super for writing (consumers) - PECS.

🟢 Type parameters are erased at runtime (type erasure) - no new T() or T.class.

genericswildcards