public class Box {
 private String object;
 public void set(String object) { this.object = object; }
 public String get() { return object; }
}

public class Box<T> {
 // T stands for "Type"
 private T t;
 public void set(T t) { this.t = t; }
 public T get() { return t; }
}

Box<Integer> integerBox = new Box<Integer>();
Box<Double> doubleBox = new Box<Double>();
Box<String> stringBox = new Box<String>();

public class Util {
 public static <K, V> boolean compare(Pair<K, V> p1, Pair<K, V> p2) {
  return p1.getKey().equals(p2.getKey()) &&
    p1.getValue().equals(p2.getValue());
 }
}
public class Pair<K, V> {
 private K key;
 private V value;
 public Pair(K key, V value) {
  this.key = key;
  this.value = value;
 }
 public void setKey(K key) { this.key = key; }
 public void setValue(V value) { this.value = value; }
 public K getKey() { return key; }
 public V getValue() { return value; }
}

Pair<Integer, String> p1 = new Pair<>(1, "apple");
Pair<Integer, String> p2 = new Pair<>(2, "pear");
boolean same = Util.<Integer, String>compare(p1, p2);

Pair<Integer, String> p1 = new Pair<>(1, "apple");
Pair<Integer, String> p2 = new Pair<>(2, "pear");
boolean same = Util.compare(p1, p2);

public static <T> int countGreaterThan(T[] anArray, T elem) {
 int count = 0;
 for (T e : anArray)
  if (e > elem) // compiler error
   ++count;
 return count;
}

public interface Comparable<T> {
 public int compareTo(T o);
}

public static <T extends Comparable<T>> int countGreaterThan(T[] anArray, T elem) {
 int count = 0;
 for (T e : anArray)
  if (e.compareTo(elem) > 0)
   ++count;
 return count;
}

public void boxTest(Box<Number> n) { /* ... */ }

class Fruit {}
class Apple extends Fruit {}
class Orange extends Fruit {}

public class GenericReading {
 static List<Apple> apples = Arrays.asList(new Apple());
 static List<Fruit> fruit = Arrays.asList(new Fruit());
 static class Reader<T> {
  T readExact(List<T> list) {
   return list.get(0);
  }
 }
 static void f1() {
  Reader<Fruit> fruitReader = new Reader<Fruit>();
  // Errors: List<Fruit> cannot be applied to List<Apple>.
  // Fruit f = fruitReader.readExact(apples);
 }
 public static void main(String[] args) {
  f1();
 }
}

static class CovariantReader<T> {
 T readCovariant(List<? extends T> list) {
  return list.get(0);
 }
}
static void f2() {
 CovariantReader<Fruit> fruitReader = new CovariantReader<Fruit>();
 Fruit f = fruitReader.readCovariant(fruit);
 Fruit a = fruitReader.readCovariant(apples);
}
public static void main(String[] args) {
 f2();
}

public class GenericsAndCovariance {
 public static void main(String[] args) {
  // Wildcards allow covariance:
  List<? extends Fruit> flist = new ArrayList<Apple>();
  // Compile Error: can't add any type of object:
  // flist.add(new Apple())
  // flist.add(new Orange())
  // flist.add(new Fruit())
  // flist.add(new Object())
  flist.add(null); // Legal but uninteresting
  // We Know that it returns at least Fruit:
  Fruit f = flist.get(0);
 }
}

List<? extends Fruit> flist = new ArrayList<Fruit>();
List<? extends Fruit> flist = new ArrayList<Apple>();
List<? extends Fruit> flist = new ArrayList<Orange>();

public class GenericWriting {
 static List<Apple> apples = new ArrayList<Apple>();
 static List<Fruit> fruit = new ArrayList<Fruit>();
 static <T> void writeExact(List<T> list, T item) {
  list.add(item);
 }
 static void f1() {
  writeExact(apples, new Apple());
  writeExact(fruit, new Apple());
 }
 static <T> void writeWithWildcard(List<? super T> list, T item) {
  list.add(item)
 }
 static void f2() {
  writeWithWildcard(apples, new Apple());
  writeWithWildcard(fruit, new Apple());
 }
 public static void main(String[] args) {
  f1(); f2();
 }
}

List<? super Apple> list = new ArrayList<Apple>();
List<? super Apple> list = new ArrayList<Fruit>();
List<? super Apple> list = new ArrayList<Object>();

public class Collections {
 public static <T> void copy(List<? super T> dest, List<? extends T> src) {
  for (int i=0; i<src.size(); i++)
   dest.set(i, src.get(i));
 }
}

public class Node<T> {
 private T data;
 private Node<T> next;
 public Node(T data, Node<T> next) }
  this.data = data;
  this.next = next;
 }
 public T getData() { return data; }
 // ...
}

public class Node {
 private Object data;
 private Node next;
 public Node(Object data, Node next) {
  this.data = data;
  this.next = next;
 }
 public Object getData() { return data; }
 // ...
}

public class Node<T extends Comparable<T>> {
 private T data;
 private Node<T> next;
 public Node(T data, Node<T> next) {
  this.data = data;
  this.next = next;
 }
 public T getData() { return data; }
 // ...
}

public class Node {
 private Comparable data;
 private Node next;
 public Node(Comparable data, Node next) {
  this.data = data;
  this.next = next;
 }
 public Comparable getData() { return data; }
 // ...
}

List<Integer>[] arrayOfLists = new List<Integer>[2]; // compile-time error

Object[] strings = new String[2];
strings[0] = "hi"; // OK
strings[1] = 100; // An ArrayStoreException is thrown.

Object[] stringLists = new List<String>[]; // compiler error, but pretend it's allowed
stringLists[0] = new ArrayList<String>(); // OK
// An ArrayStoreException should be thrown, but the runtime can't detect it.
stringLists[1] = new ArrayList<Integer>();

public class ErasedTypeEquivalence {
 public static void main(String[] args) {
  Class c1 = new ArrayList<String>().getClass();
  Class c2 = new ArrayList<Integer>().getClass();
  System.out.println(c1 == c2); // true
 }
}

public class Node<T> {
 public T data;
 public Node(T data) { this.data = data; }
 public void setData(T data) {
  System.out.println("Node.setData");
  this.data = data;
 }
}
public class MyNode extends Node<Integer> {
 public MyNode(Integer data) { super(data); }
 public void setData(Integer data) {
  System.out.println("MyNode.setData");
  super.setData(data);
 }
}

public class Node {
 public Object data;
 public Node(Object data) { this.data = data; }
 public void setData(Object data) {
  System.out.println("Node.setData");
  this.data = data;
 }
}
public class MyNode extends Node {
 public MyNode(Integer data) { super(data); }
 public void setData(Integer data) {
  System.out.println("MyNode.setData");
  super.setData(data);
 }
}

MyNode mn = new MyNode(5);
Node n = mn; // A raw type - compiler throws an unchecked warning
n.setData("Hello"); // Causes a ClassCastException to be thrown.
// Integer x = mn.data;

class MyNode extends Node {
 // Bridge method generated by the compiler
 public void setData(Object data) {
  setData((Integer) data);
 }
 public void setData(Integer data) {
  System.out.println("MyNode.setData");
  super.setData(data);
 }
 // ...
}

public static <E> void append(List<E> list) {
 E elem = new E(); // compile-time error
 list.add(elem);
}

public static <E> void append(List<E> list, Class<E> cls) throws Exception {
 E elem = cls.newInstance(); // OK
 list.add(elem);
}

List<String> ls = new ArrayList<>();
append(ls, String.class);

public static <E> void rtti(List<E> list) {
 if (list instanceof ArrayList<Integer>) { // compile-time error
  // ...
 }
}
=> { ArrayList<Integer>, ArrayList<String>, LinkedList<Character>, ... }

public static void rtti(List<?> list) {
 if (list instanceof ArrayList<?>) { // OK; instanceof requires a reifiable type
  // ...
 }
}