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Module 12 - Generics

What You Will Learn

Concept	Summary
Generic classes & methods	class Pair<A,B>, <T> T pick(T[] arr)
Bounded type parameters	<T extends Comparable<T>>, <T extends Number & Cloneable>
Wildcards	?, ? extends T (upper-bounded), ? super T (lower-bounded)
PECS	Producer Extends, Consumer Super - when to use which wildcard
Type erasure	What the JVM really sees; why new T() is illegal
Generic patterns	Result, generic cache, generic pipeline, type-safe heterogeneous container

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Why Generics?

Without generics                  With generics
──────────────────                ──────────────
List list = new ArrayList();      List<String> list = new ArrayList<>();
list.add("hello");                list.add("hello");
String s = (String) list.get(0);  String s = list.get(0);  // no cast
// ClassCastException at runtime  // type error caught at compile time

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Bounded Type Parameters

// Upper bound - T must be Comparable to itself
<T extends Comparable<T>> T max(T a, T b) {
    return a.compareTo(b) >= 0 ? a : b;
}

// Multiple bounds (class must come first)
<T extends Number & Comparable<T>> double sum(List<T> list) { ... }

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Wildcards - PECS Rule

PECS: Producer Extends, Consumer Super

void copy(List<? extends Number> src,    // PRODUCER - you only READ from it
          List<? super   Number> dst) {  // CONSUMER - you only WRITE to it
    for (Number n : src) dst.add(n);
}

Unbounded  ?            - read as Object; cannot write (except null)
Upper      ? extends T  - safe to READ as T; cannot write
Lower      ? super T    - safe to WRITE T; can only read as Object

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Type Erasure

Source code                       After erasure (what JVM sees)
───────────────────               ─────────────────────────────
List<String>                  →   List
Pair<Integer, String>         →   Pair
<T extends Comparable<T>>     →   Comparable  (leftmost bound)

Consequences:
  • Cannot do: new T(), new T[n], instanceof List<String>
  • Can do:    instanceof List<?>,  (T) value  (unchecked cast)
  • Generic type info IS preserved in class/method signatures (reflection)

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Source Files

File	What it Demonstrates
GenericClasses.java	Generic classes, generic methods, bounded type params, multiple bounds
Wildcards.java	All three wildcard forms, PECS in action, unbounded wildcards
TypeErasure.java	Erasure effects, bridge methods, @SuppressWarnings("unchecked"), Class tokens
GenericPatterns.java	Result, generic LRU cache, type-safe heterogeneous container, generic pipeline

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Running

cd module-12-generics
mvn test