Java Generics Cheat Sheet

public class Box<T> {    private T value;    public void set(T value) { this.value = value; }    public T get() { return value; }}// UsageBox<String> box = new Box<>();box.set("hello");String s = box.get();

// Upper bound: T must extend Numberpublic class NumberBox<T extends Number> {    private T value;    public double doubleValue() { return value.doubleValue(); }}// Multiple bounds (one class, multiple interfaces)public <T extends Comparable<T> & Cloneable> T max(T a, T b) {    return a.compareTo(b) >= 0 ? a : b;}

// Upper bounded wildcard: read-only, covariantpublic void printAll(List<? extends Number> list) {    for (Number n : list) System.out.println(n);}// Lower bounded wildcard: write-only, contravariantpublic void addIntegers(List<? super Integer> list) {    list.add(1);    list.add(2);}// Unbounded wildcardpublic void printSize(List<?> list) {    System.out.println(list.size());}

public static <T> List<T> listOf(T... items) {    return Arrays.asList(items);}public static <K, V> void printEntry(Map.Entry<K, V> entry) {    System.out.println(entry.getKey() + "=" + entry.getValue());}// Explicit type witness when inference failsList<String> empty = Collections.<String>emptyList();