Mike Gosling

Hello Ken,

As you may know, good chunk of us here are using Spring in our daily life. Can you explain me how does content of your book can help in understanding Spring's Mockito wrapper around core Mockito?

I have very thin understanding of what you exactly are stubs/spies etc, when to used them etc. Seems to me that Im struggling more how to understand connection Spring-Mockito rather then Mockito iteself.

In short, does your book can help us using Mockito with Spring?

AhH, I thought that OP who initiated thread well get a prize In any case, questions are somewhat relevant if anyone decides to buy a book, who knows..

Maybe Jeremy can reply back as separate answers.

Hello,

Would be super grateful if someone can explain me how wait/notify/notifyAll works and if is there better solution for the problem I am facing. Basically, we have to send a bunch of SMS messages. For sending messages an object called SMPPSession is used but in this example I'll just use superficial code. SMPPSession is supposed to send messages to SMSC server and to reestablish session in situations when connection breaks.  I would like to use multiple threads for sending multiple messages, and to have a separate single thread, some sort of "guardian"/ "watcher"/"notifier".  The role of that separate thread is to stop all other threads from executing their code, while it works on reestablishing session. Naturally, SMPPSession is shared among all these threads. Once that guardian finishes reconnecting, all other thread needs to continue with using the session and proceed with sending.

Now, I have some code and getting exception. Any help?

In reality we do send real SMS messages using jsmpp library and inside it there is SMPPSession object.

public class SMPPSession {    private boolean bind;    private static final Random idGenerator = new Random();    public int sendMessage(String msg){       try{           Thread.sleep(1000L);           System.out.println("Sending message: " + msg);           return Math.abs(idGenerator.nextInt());       } catch (InterruptedException e){           e.printStackTrace();       }        return -1;    }    public void reBind(){        try{            System.out.println("Rebinding...");            Thread.sleep(1000L);            this.bind = true;            System.out.println("Session established!");        } catch (InterruptedException e){            e.printStackTrace();        }    }    public boolean isBind(){        return this.bind;    } } public class Sender extends Thread{    private SMPPSession smppSession;    public Sender(String name, SMPPSession smppSession){        this.setName(name);        this.smppSession = smppSession;    }    @Override    public void run(){        while (!Client.messages.isEmpty()){            synchronized (Client.messages){                    if (smppSession.isBind()){                        final String msg = Client.messages.remove(0);                        final int msgId = smppSession.sendMessage(msg);                        System.out.println(Thread.currentThread().getName() + " sent msg and received msgId: " + msgId);                        Client.messages.notifyAll();                } else {                    try {                        Client.messages.wait();                    } catch (InterruptedException e) {                        throw new RuntimeException(e);                    }                }            }        }    } } public class SessionProducer extends Thread{    private SMPPSession smppSession;    public SessionProducer(String name, SMPPSession smppSession){        this.setName(name);        this.smppSession = smppSession;    }    @Override    public void run(){        while (!Client.messages.isEmpty()){            synchronized (Client.messages){                if (!smppSession.isBind()){                    smppSession.reBind();                    System.out.println(Thread.currentThread().getName() + " managed to reestablish SMPP session.");                    Client.messages.notifyAll();                } else{                    try {                        Client.messages.wait();                    } catch (InterruptedException e) {                        throw new RuntimeException(e);                    }                }            }        }    } } public class Client {    public static final List<String> messages = new CopyOnWriteArrayList<>();    public static void main(String[] args) {        //populate messages from db        messages.add("msg1"); messages.add("msg2"); messages.add("msg3");  messages.add("msg4"); messages.add("msg5"); messages.add("msg6");        SMPPSession smppSession = new SMPPSession();        SessionProducer sessionProducer = new SessionProducer("SessionProducer1", smppSession);        Sender sender1 = new Sender("Sender1", smppSession);        Sender sender2 = new Sender("Sender2", smppSession);        Sender sender3 = new Sender("Sender3", smppSession);        Sender sender4 = new Sender("Sender4", smppSession);        sessionProducer.start();        sender1.start();        sender2.start();        sender3.start();        sender4.start();    } } Naturally, I get exception and have no idea why. Somehow threads are not in sync. Rebinding... Session established! SessionProducer1 managed to reestablish SMPP session. Sending message: msg1 Sender4 sent msg and received msgId: 432995458 Sending message: msg2 Sender4 sent msg and received msgId: 113629699 Sending message: msg3 Sender4 sent msg and received msgId: 611735717 Sending message: msg4 Sender4 sent msg and received msgId: 1234995659 Sending message: msg5 Sender4 sent msg and received msgId: 922228968 Sending message: msg6 Sender4 sent msg and received msgId: 2097204472 Exception in thread "Sender2" Exception in thread "Sender1" Exception in thread "Sender3" java.lang.ArrayIndexOutOfBoundsException: Index 0 out of bounds for length 0 at java.base/java.util.concurrent.CopyOnWriteArrayList.elementAt(CopyOnWriteArrayList.java:385) at java.base/java.util.concurrent.CopyOnWriteArrayList.remove(CopyOnWriteArrayList.java:478) at demo.Sender.run(Sender.java:20) java.lang.ArrayIndexOutOfBoundsException: Index 0 out of bounds for length 0 at java.base/java.util.concurrent.CopyOnWriteArrayList.elementAt(CopyOnWriteArrayList.java:385) at java.base/java.util.concurrent.CopyOnWriteArrayList.remove(CopyOnWriteArrayList.java:478) at demo.Sender.run(Sender.java:20) java.lang.ArrayIndexOutOfBoundsException: Index 0 out of bounds for length 0 at java.base/java.util.concurrent.CopyOnWriteArrayList.elementAt(CopyOnWriteArrayList.java:385) at java.base/java.util.concurrent.CopyOnWriteArrayList.remove(CopyOnWriteArrayList.java:478) at demo.Sender.run(Sender.java:20)

In order to make it visual:

Hello,

Are there any practical exercise/assignments in your book after each chapter? Or it is project based perhaps?

Also, which programming language is used to explain concepts? Which book you would recommend after reading yours?

How much does your style of writing about DS is differ from say Head First approach series?

Hope you have a success with your book

Hi,

The title of your books really does sound templating.

Would like to know your thoughts why do average Java developer would need to understand byte code manipulation? I have no idea what it is.. I will take a look later today.. I mean, majority of Java developers are using Spring (Boot) and how does knowledge from your book can be used within the context of Spring?

Second thing, I see that Part 3, 4 and 5 are somewhat unrelated? Are they? Or in your book they should be read one after the other if they are related? If they are, can you explain how do you make connection between maven and functional programming?

Regarding concurrency, I myself haven't actually studied concurrent programming at school, I did however employ CompletableFutures at work for doing (commutative) operation upon some large dto (to examine different sections of it in parallel). What will be your advice to learn concurrency, for someone who doesn't have a lot of strong CS core background, didn't learn assembler for instance. Can your book help on learning concurrency by example?

Don't get me wrong. I am not trying to say bad about your work or anything along those lines..

Thanks Mike for a quick one!

I am studying for OCP17 exam. Currently starting with Streams chapter.

Stream<Double> x2 = Stream.iterate(23, previousd -> previousd - 1);

When I manully compile, I get:

A.java:29: error: incompatible types: inference variable T has incompatible bounds        Stream<Double> x2 = Stream.iterate(23, previousd -> previousd - 1);                                          ^    equality constraints: Double    lower bounds: Integer  where T is a type-variable:    T extends Object declared in method <T>iterate(T,UnaryOperator<T>) 1 error error: compilation failed

Why doesn't numeric type promotion from int to double doesn't apply here?

If I would have to guess (hopefully some will explain me this):

Long x = 15; //DOES NOT COMPILE

Java is capable to do boxing for one type of primitives to its corresponding boxed class, but not between various boxed types.

Hopefully this would help.

class A {} class B extends A {} class C extends B {} public class Demo4 {    public static void main(String[] args) {        Demo4 demo4 = new Demo4();        List<? super B> listOfBs1 = demo4.getListOfBsAsBObjects();        List<? super B> listOfBs2 = demo4.getListOfBsAsAObjects();        List<? super B> listOfBs3 = demo4.getListOfBsAsObjects();        //Getting        final Object object = listOfBs1.get(0);  //DOES COMPILE;        final Object object1 = listOfBs2.get(0); //DOES COMPILE;        final Object object2 = listOfBs3.get(0); //DOES COMPILE;        B b = listOfBs1.get(0); //DOES NOT COMPILE;        A a = listOfBs2.get(0); //DOES NOT COMPILE;        Object o = listOfBs3.get(0); //DOES COMPILE;        //Adding        listOfBs2.add(new A()); //DOES NOT COMPILE;        listOfBs2.add(new B()); //DOES COMPILE;        listOfBs2.add(new C()); //DOES COMPILE;        /*          I try to think of it, as Liutauras nicely explained, that Java cannot know at which object listOfBsX is pointing to.          Let's say you called getListOfBsAsBObjects(). You would have a list of subclass objects (since B extends A) in which you try to add a superclass object (A). Cannot do that in Java.          That is not "is-a" relationship. "A is not B" but "B is A". You might even call getListOfBsAsBObjects() or getListOfBsAsObjects() and attempted to add new A() in one of those lists. Which seems          okay at first. But Java doesn't know which object List<? super B> it is, so it defend itself by allowing to add only B and its covariant (subclasses) and getting only Object.         */    }    public List<? super B> getListOfBsAsBObjects() {        return new ArrayList<B>(); //DOES COMPILE    }    public List<? super B> getListOfBsAsAObjects() {        return new ArrayList<A>(); //DOES COMPILE    }    public List<? super B> getListOfBsAsObjects() {        return new ArrayList<>(); //DOES COMPILE    } }

Please go through this tutorial (https://docs.oracle.com/javase/tutorial/java/generics/index.html) and combined with this thread it should make sense. It helped me I am know seeing that List<? super T> is more convenient for addition and that List<? extends T> is more handy for getting things.

Hey Liutauras, thanks for excellent post. After reading Oracle's tutorial on generics it became a bit clearer, but now with your post, these wildcards are becoming far more clearer now. I will just follow your strategy for determining what can be added/removed in generics with bounded wildcards.

Good thing you dismantled my example  

While explaining Lower-Bounded Wildcards in table 9.14, authors wrote that unbounded generics are immutable. This may not be entirely correct since it is allowed to add null values.

public class Demo3 {    static void addSound(List<?> list) {        someList2.add("");        list.add("quack"); //DOES NOT COMPILE        list.add(null); //DOES COMPILE    }    public static void main(String[] args) {        //addSound        List<String> strings = new ArrayList<>();        strings.add("tweet");        List<Object> objects = new ArrayList<Object>(strings);        addSound(strings);          addSound(objects);   } }

Oracle docs: https://docs.oracle.com/javase/tutorial/java/generics/wildcardGuidelines.html

Thanks Stephan, I must make sure to memorize this.

So from the OCP Java17 they give an example on what method call will compile, "line a" or "line b"?

import java.util.ArrayList; import java.util.List; interface Flyer { void fly(); } class HangGlider implements Flyer { public void fly() {} } class Goose implements Flyer { public void fly() {} } public class Demo {    private void anyFlyer(List<Flyer> flyer) {}    private void groupOfFlyers(List<? extends Flyer> flyer) {}    void m(){        List<Goose> x = null;        this.anyFlyer(x); //Line a        this.groupOfFlyers(x);//Line b    }

I wanted to check with you guys if my way of "reasoning" is correct:

First I notice I'm dealing with two classes which have common interface, also I notice I'm dealing with parameterized collections. So I envision three groups of hierarchies like so:

//1. List<?> /|\  |  (is)  | List<? extends HangGlider> /|\  |  (is)  | List<HangGlider> //2. List<?> /|\  |  (is)  | List<? extends Goose> /|\  |  (is)  | List<Goose> //3. List<?> /|\  |  (is)  | List<? extends Flyer> /|\  |  (is)  | List<Flyer>

First lets check anyFlyer.

Since formal parameter of anyFlyer is List<Flyer> the object it points to, if we were to write it somewhere in the code, it could only:

List<Flyer> flyer = new ArrayList<Flyer> //not counting LinkedList

It couldn't be:

List<Flyer> flyer = new ArrayList<Goose>

This is because of parameterized types (I'm not sure are there any more reasons or is this correct):

public class ArrayList<E> extends AbstractList<E>        implements List<E> {

So flyer can only be pointing to appropriate parameterized collection.

Having that  in mind, passing variable x, to anyFlyer would cause compilation error.

Lets check groupOfFlyers:

If we take a look at third group of hierarchies, we can see 'List<? extends Flyer>'. These can be type for (I don't know proper term) four objects (options ?), that would compile:

       List<? extends Flyer> f = new ArrayList<>();        List<? extends Flyer> f1 = new ArrayList<Goose>();        List<? extends Flyer> f2 = new ArrayList<HangGlider>();        List<? extends Flyer> f3 = new ArrayList<Flyer>();

And since they all compile and x is one of them, f1, then we can safely say "Line b" is the one which does compile.


I'm interested to hear is this good way to reason about?

I am reading tutorial from Oracle on Generics (https://docs.oracle.com/javase/tutorial/java/generics/upperBounded.html)

I don't understand why the last piece of code compiles>

   static void generic(List<? extends String> objects){        //do smtg...    }        public static void main(String[] args) {        var x = new ArrayList<>();        generic(x); //DOES NOT COMPILE        var y = new ArrayList<Object>();        generic(y);  //DOES NOT COMPILE        generic(new ArrayList<>()); //How type inference knows that this is "ArrayList<String> strList = new ArrayList<>()"

Is there any kind of java library, that can determine whether some class satisfies criterias of immutability?

Something like a simple method call:

boolean immutable = ImmutableChecker.isImmutable(Student.class):