Generics let you write classes, interfaces, and methods that work with a type parameter specified by the caller — providing compile-time type safety and eliminating casts. They power the Collections Framework and reusable libraries.
();
list.add();
list.add();
(String) list.get();
List<String> list = <>();
list.add();
list.add();
list.get();
Generics move type errors from runtime (ClassCastException) to compile time, and remove the need for manual casts — making code safer and cleaner.
// a generic class — T is a type parameter filled in by the user
public class Box<T> {
private T value;
public void set(T value) { this.value = value; }
public T get() { return value; }
}
Box<String> b = new Box<>(); // T = String
b.set("hi");
String x = b.get(); // typed, no cast
// a generic method
public static <T> T firstElement(List<T> list) {
return list.get(0);
}
// T must be Number (or a subclass) — lets you call Number methods
public static <T extends Number> double sum(List<T> list) {
double total = 0;
for (T n : list) total += n.doubleValue(); // safe — T is a Number
return total;
}
<T extends X> constrains the type parameter, enabling you to use that type's members safely.
List<? extends Number> nums; // "some subtype of Number" — read (producer)
List<? super Integer> sink; // "Integer or a supertype" — write (consumer)
// PECS: Producer Extends, Consumer Super
Wildcards add flexibility for method parameters that should accept ranges of types.
// generics are a COMPILE-TIME feature — erased at runtime (for backward compatibility)
List<String> a = new ArrayList<>();
List<Integer> b = new ArrayList<>();
a.getClass() == b.getClass(); // true — both are just ArrayList at runtime
// consequence: can't do `new T()` or `instanceof List<String>`
Java implements generics via type erasure — type parameters exist at compile time but are removed at runtime, which has some limitations.
Generics are essential to modern Java — they provide compile-time type safety (catching type errors early instead of as runtime ClassCastExceptions), eliminate verbose and error-prone casts, and enable reusable, type-flexible code.
They're the foundation of the entire Collections Framework (List<String>, Map<K,V>) and countless libraries.
Understanding generic classes/methods, bounded parameters, wildcards (PECS), and the type-erasure caveat lets you both use generic APIs correctly and design your own reusable, type-safe abstractions — a core skill for writing robust, maintainable Java.