Java

Advanced Topics

By Daniël Bos

Agenda

Enumerations

Looks like C++ ...

enum Animal { CAT, DOG, MOUSE }

Animal animal = Animal.CAT;
switch(animal) {
   case CAT:
      // do cat things
      break;
   case DOG:
      // do dog things
      break
   default:
      // do default things
}

... but very different

Enums are classes

public class Animal {
   public static final Animal CAT   = new Animal();
   public static final Animal DOG   = new Animal();
   public static final Animal MOUSE = new Animal();
}

Cool stuff

Animal.values() => Animal[];
Animal.valueOf(String s) => Animal;

Animal cat = Animal.CAT;
cat.name() => "CAT"

Cooler stuff

enum Animal {
   CAT("meow"), DOG("woof"), MOUSE("squeek");
	
   private String sound;
	
   Animal(String sound) {
      this.sound = sound;
   }
	
   String getSound() {
      return sound;
   }
}

Animal cat = Animal.CAT;
cat.getSound();

Generics

Similar to C++ templates

What's the problem?

List strings = new ArrayList();
strings.add("hello");
strings.add("world");
// no problem!
strings.add(42);

for (Object o : strings) {
   // ... RuntimeException here!
   String s = (String) o;
}

What's the solution?

List<String> strings = new ArrayList<String>();
strings.add("hello");
strings.add("world");
// Compile-time exception!
strings.add(42);

for (String s : strings) {
   // ... No need to cast!
}

This is going to look ugly ...

Creating generic code

public Pair<K, V> {
   private K key;
   private V value;
	
   public Pair(K key, V value) {
      this.key = key;
      this.value = value;
   }
}

public class Util {
   public static <K, V> boolean compare(Pair<K, V> p1, Pair<K, V> p2);
}

This is going to be confusing ...

Does this work?

public void print(List<Object> list) {
   for (Object elem : list)
      System.out.println(elem);
}

List<Integer> ii = new ArrayList<Integer>();
print(ii);

No! :-( Integer is not the same as Object, though it inherits from Object.

This does work

public void print(List<?> list) {
   for (Object elem : list)
      System.out.println(elem);
}

List<Integer> ii = new ArrayList<Integer>();
print(ii);

But now we throw out type-safety!If all objects that can be printed implement the "Printable" interface, we can do better.

Generic inheritance

public void print(List<? extends Printable> list) {
   for (Object elem : list)
      System.out.println(elem);
}

List<Integer> ii = new ArrayList<Integer>();
print(ii);

Confusing: extends is used for both classes and interfaces.

The other way around

Suppose your method accepts a List that you plan to add numbers to. How to make sure you can't use other lists?

public void addNumbers(List<? super Integer> list) {
   for (int i = 0; i < 10; i++)
      list.add(i);
}

Now you can supply a List of Integers, Numbers or Objects, but not a List of Strings or Widgets.

A word of warning

Erasure

Because Generics were added to the language (in SE 1.5/5.0) and byte-code compatibility is an important feature of Java, all Generics are erased during compilation, after type-checking.

This is usually okay, but it limits the information you can get using reflection.

Collections

Collections

Don't implement them yourself, almost any type already exist!

If you really need to implement one yourself, start from one of the abstractXxx collection types.

Algorithms

Type-safe

Map<Integer, String> map = new HashMap<Integer, String>();
map.put(1, "One");
map.put(2, "Two");
//map.put("Three", 3);

for (String value : map.values()) {
   // ...
}
for (Integer key : map.keySet()) {
   // ...
}
for (Map.Entry<Integer, String> entry : map.entrySet()) {
   entry.getKey();
   entry.getValue();
}

Comparable

class YourType {
   @Override
   public boolean equals(Object other);
	
   @Override
   public int hashCode();
}

class YourType implements Comparable<YourType> {
   @Override
   public int compareTo(YourType other) {
      // return -1, 0, +1
   }
}

Comparator

public class Person {
   public static final Comparator<Person> ORDER_FIRSTNAME
      = new Comparator<Person>() {
         @Override
         public int compare(Person p1, Person p2) {
            return p1.firstname.compareTo(p2.firstname);
         }
   };
   public static final Comparator<Person> ORDER_AGE
      = new Comparator<Person>() {
         @Override
         public int compare(Person p1, Person p2) {
            return p1.age - p2.age;
         }
   };
}

Collections.sort(persons, Person.ORDER_FIRSTNAME);

Reflection

Meta-class

Every class has an associated meta-class that can be used to get all sorts of information about the class and to interact with the class.

Object.class;

Object o = new Object();
o.getClass();

Besides that, there's a Class utility class (note the capitalization):

Class c = Class.forName("...");

Instanciating

try {
   Class c = Class.forName("com.example.Test");
   Test t = (Test) c.newInstance();
} catch (ClassNotFoundException e) {
   // ...
} catch (InstantiationException e) {
   // ...
} catch (IllegalAccessException e) {
   // ...
}

Many many possible exception!

Inspecting a class

// Get all the constructors:
Constructor[] constructors = clazz.getDeclaredConstructors();
// Get the constructor matching a specific signature:
Constructor constructor = clazz.getDeclaredConstructor(String.class);
// Instantiate an object:
Example example = (Example) constructor.newInstance("String");

// Get all the methods (public and private):
Method[] methods = clazz.getDeclaredMethods();
// Get all visible methods (including from super-classes)
Method[] methods = clazz.getMethods();
// Get the method for a specific signature:
Method method = clazz.getDeclaredMethod("setExample", String.class);
// Invoke the method on an object:
method.invoke(example, "New String");

Annotations

You've probably seen

@Override
public void someMethod() {}

@SuppressWarnings
public void badMethod() {}

@Deprecated
public void oldMethod() {}

Annotations provide information to the compiler, some can be made available at runtime through reflection.

You can create them

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD})
public @interface Shared {
   boolean restricted() default false;
}

public class Users {
   @Shared
   public int getCount();
	
   @Shared(restricted=true)
   public User getUserById(int id);
   
   public void hidden();
}

And use them

// Iterate over all the methods in the class
for (Method m : Users.class.getMethods()) {
   Shared shared = m.getAnnotation(Shared.class);
   // If the annotation is present
   if (shared != null) {
      if (shared.restricted()) {
         // share privately
      } else {
         // share publicly
      }
   }
}

Concurrency

Threads

public class MyRunnable implements Runnable {
   @Override
   public void run() {
      // ...
   }
}
Runnable r = new MyRunnable();
new Thread(r).start()

public class MyThread extends Thread {
   @Override
   public void run() {
      // ...
   }
}
new MyThread().start()

Sleeping/Waiting

try {
   Thread.sleep(4000);
} catch (InterruptedException e) {
   // ...
}

while (true) {
   doHeavyWork();
   if (Thread.interrupted()) {
      // ...
   }
}

try {
   AnotherThread.join()
} catch (InterruptedException e) {
   // ...
}

Synchronization (1)

The problem:

public class Counter {
   private int c = 0;
   public void inc() { c = c + 1; }
   public void dec() { c = c - 1; }
}

public class Counter {
   private int c = 0;
   public synchronized void inc() { c = c + 1; }
   public synchronized void dec() { c = c - 1; }
}

Atomic variables

public class Counter {
   private AtomicInteger c = new AtomicInteger();
   
   public void inc() { c.incrementAndGet(); }
   public void dec() { c.decrementAndGet(); }
}

Synchronization (2)

public void inc() {
   synchronized(this) {
      c = c + 1;
   }
}

private Object lock = new Object();

public void inc() {
   synchronized(lock) {
      c = c + 1;
   }
}   

Deadlock

public class Friend {
   public synchronized void greet(Friend f) {
      System.out.println("Hi!");
      f.greetBack(this);
   }
   
   public synchronized void greetBack(Friend f) {
      System.out.println("Hi too!");
   }
}

Friend f1 = new Friend();
Friend f2 = new Friend();

new Thread() {
   public void run() { f1.greet(f2); }
}.start();
new Thread() {
   public void run() { f2.greet(f1); }
}.start();

Starvation

public class Example {
   private boolean busy = false;
   public synchronized boolean isBusy() {
      return busy;
   }
   public synchronized void doHeavyWork() {
      busy = true; heavyWork(); busy = false;
   }
}

Example e = new Example();
new Thread() {
   public void run() {
      while(true) { e.doHeavyWork(); }
   }
}.start();

while(true) {
   Thread.sleep(1000);
   System.out.println("busy? " + e.isBusy());
}

Livelock

public class Person {
   private boolean left;
   
   public void pass(Person p) {
      while (true) {
         if (p.canPass(left)) return;
            else left = !left;
         Thread.yield();
      }
   }
   public boolean canPass(boolean other) {
      if (left == other) {
         left = !left;
         return false;
      } else return true;
   }
}

Wait/Notify

public class Drop {
   private boolean empty = true;
   private String message;
   
   public synchronized String take() {
      while (empty) wait();
      empty = true;
      notifyAll();
      return message;
   }
   public synchronized void put(String message) {
      while (!empty) wait();
      empty = false;
      this.message = message;
      notifyAll();
   }
}

Executors

ExecutorService pool = Executors.newFixedThreadPool(10);

while (!pool.isShutdown()) {
    pool.execute(new Handler(serverSocket.accept()));
}

Fork/Join

public class MyAction extends RecursiveAction {
   protected void compute() {
      int size = list.size();
      if (size < 100) {
         computeDirectly();
      } else {
         invokeAll(new MyAction(list.subList(0, size / 2)),
                   new MyAction(list.subList(size / 2 + 1, size)));
   }
   protected void computeDirectly() {
      // ...
   }
}

ForkJoinPool pool = new ForkJoinPool();
pool.invoke(new MyAction(bigList));

Futures

ExecutorService pool = Executors.newFixedThreadPool(10);

public Future<Document> download(String url) {
   return pool.submit(new Callable<Document>() {
      @Override
      public Document call() {
         return doDownload(url);
      }
   }
}

Future<Document> future = download("http://example.com/robots.txt");

future.isDone();
future.cancel(mayInterrupt);
future.isCancelled();
Document future.get(timeout);
Document future.get();

Syntactic Sugar

• Variadic arguments
• For-each syntax
• Diamond operator
• Lambda's
• Method references
• Optionals

Variadic arguments

void example(String ... args);

example("one");
example("one", "two");

void example(String[] args);

For-each syntax

for (int i = 0; i < list.size(); i++) {
   String item = list.get(i);
   // ...
}

List list;
for (String item : list) {
    // ...
}

Diamond operator

Map<Integer, Pair<String, List<Double>>> map
   = new HashMap<Integer, Pair<String, List<Double>>>()

Map<Integer, Pair<String, List<Double>>> map = new HashMap<>()

Lambda's

List<Person> persons;
Collections.sort(persons, new Comparator<Person>() {
   @Override
   public int compare(Person p1, Person p2) {
      return p1.age - p2.age;
   }
});

Collections.sort(persons, (p1, p2) -> p1.age - p2.age);

Method references

public class Person {
   public static int CompareByAge(Person p1, Person p2) {
      return p1.age - p2.age;
   }
}

Collections.sort(persons, Person::CompareByAge);

Optionals

for (Person p : db.getPeople()) {
   if (p.age > 20) {
      Address a = p.getAddress();
      if (a != null) {
         System.out.println(a.getCity());
      }
   }
}

db.streamPeople()
   .filter(p -> p > 20)
   .flatMap(p -> p.getAddress())
   .flatMap(Address::getCity)
   .forEach(System.out::println);