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How a thread can see stale reference of safely initialized object

I have been trying to figure out that how immutable objects which are safely published could be observed with stale reference.
public final class Helper {
private final int n;
public Helper(int n) {
this.n = n;
}
}
class Foo {
private Helper helper;
public Helper getHelper() {
return helper;
}
public void setHelper(int num) {
helper = new Helper(num);
}
}
So far I could understand that Helper is immutable and can be safely published. A reading thread either reads null or fully initialized Helper object as it won't be available until fully constructed. The solution is to put volatile in Foo class which I don't understand.

The fact that you are publishing a reference to an immutable object is irrelevant here.
If you are reading the value of a reference from multiple threads, you need to ensure that the write happens before a read if you care about all threads using the most up-to-date value.
Happens before is a precisely-defined term in the language spec, specifically the part about the Java Memory Model, which allows threads to make optimisations for example by not always updating things in main memory (which is slow), instead holding them in their local cache (which is much faster, but can lead to threads holding different values for the "same" variable). Happens-before is a relation that helps you to reason about how multiple threads interact when using these optimisations.
Unless you actually create a happens-before relationship, there is no guarantee that you will see the most recent value. In the code you have shown, there is no such relationship between writes and reads of helper, so your threads are not guaranteed to see "new" values of helper. They might, but they likely won't.
The easiest way to make sure that the write happens before the read would be to make the helper member variable final: the writes to values of final fields are guaranteed to happen before the end of the constructor, so all threads always see the correct value of the field (provided this wasn't leaked in the constructor).
Making it final isn't an option here, apparently, because you have a setter. So you have to employ some other mechanism.
Taking the code at face value, the simplest option would be to use a (final) AtomicInteger instead of the Helper class: writes to AtomicInteger are guaranteed to happen before subsequent reads. But I guess your actual helper class is probably more complicated.
So, you have to create that happens-before relationship yourself. Three mechanisms for this are:
Using AtomicReference<Helper>: this has similar semantics to AtomicInteger, but allows you to store a reference-typed value. (Thanks for pointing this out, #Thilo).
Making the field volatile: this guarantees visibility of the most recently-written value, because it causes writes to flush to main memory (as opposed to reading from a thread's cache), and reads to read from main memory. It effectively stops the JVM making this particular optimization.
Accessing the field in a synchronized block. The easiest thing to do would be to make the getter and setter methods synchronized. Significantly, you should not synchronize on helper, since this field is being changed.

Cite from Volatile vs Static in Java
This means that if two threads update a variable of the same Object concurrently, and the variable is not declared volatile, there could be a case in which one of the thread has in cache an old value.
Given your code, the following can happen:
Thread 1 calls getHelper() and gets null
Thread 2 calls getHelper() and gets null
Thread 1 calls setHelper(42)
Thread 2 calls setHelper(24)
And in this case your trouble starts regarding which Helper object will be used in which thread. The keyword volatile will at least solve the caching problem.

The variable helper is being read by multiple threads simultaneously. At the least, you have to make it volatile or the compiler will begin caching it in registers local to threads and any updates to the variable may not reflect in the main memory. Using volatile, when a thread starts reading a shared variable, it will clear its cache and fetch a fresh value from the global memory. When it finishes reading it, it will flush the contents of its cache into the main memory so that other threads may get the updated value.

Do I have to extend class to ConcurrentHashMap or can I have variable ConcurrentHashMap for threadSafety

I am creating Socket based Server-Client reservation service, and have problem about class which will be accessed by multiple threads, does it need to Extend ConcurrentHashMap or is it enough to create variable ConcurrentHashMap to be thread safe?
I have two ideas but I am not sure if first one will work, so the first one would be creating class which only implements Serializable has variable date and then variable ConcurrentHashMap on which threads want to operate, second idea is to have class which extends Concurrent Hash Map and just is CHP but with addiontal variable to make sure it is distinguishable from others
public class Day implements Serializable {
private LocalDate date;
private ConcurrentHashMap<String, Boolean> schedule;
public Day(LocalDate date){
this.date = date;
this.schedule = new ConcurrentHashMap<>();
IntStream.range(10, 18).forEachOrdered(
n -> this.schedule.put(LocalTime.of(n, 0).toString(), TRUE));
}
public void changeaval(String key,Boolean status) {
this.schedule.replace(key,status);
}
public boolean aval(String key){
return this.schedule.get(key);
}
public LocalDate getDate(){return this.date;}
public ConcurrentHashMap getSchedule(){return this.schedule;}
}
I just want to have Class/Object which can be accessed by multiple threads and can be distinguishable from others/comparable and has ConcurrentHashMap which maps Int -> Boolean
This is the first time I am using Stack and It is my first project in Java so I don't know much sorry if something is not right.

There are basically two things to look out for when dealing with objects accessed by multiple threads:
Race condition - Due to thread scheduling by the operating system and instruction reordering optimizations by the compiler, the instructions are executed in a order not intended by the programmer causing bugs
Memory visibility - In a multi processor system, changes made by one processor is not always immediately visible to other processors. Processors keep things in their local registers and caches for performance reasons and therefore not visible to threads being executed by other processors.
Luckily we can handle both these situation using proper synchronizations.
Let's talk about this particular program.
Localdate by itself is an immutable and thread safe class. If we look at the source code of this class, we'd see that all the fields of this class are final. This means that as soon as the constructor of Localdate finishes initializing the object, the object itself will be visible across threads. But when it is assigned to a reference variable in a different object, whether the assignment (in other words, the content of the reference variable) would be visible to other threads or not is what we need to look out for.
Given the constructor in your case, we can ensure the visibility of the field date across threads provided date is either final or volatile. Since you are not modifying the date field in your class, you can very well make it final and that ensures safe initialization. If you later decide to have a setter method for this field (depending on your business logic and your design), you should make the field volatile instead of final. volatile creates a happens-before relationship which means that any instruction that is executed in the particular thread before writing to the volatile variable would be immediately visible to the other threads as soon as they read the same volatile variable.
Same goes for ConcurrentHashMap. You should make the field schedule final. Since ConcurrentHashMap by itself has all the necessary synchronizations in it, any value you set against a key would be visible to the other threads when they try to read it.
Note, however, that if you had some mutable objects as ConcurrentHashMap values instead of Boolean, you would have to design it in the same way as mentioned above.
Also, it may be good to know that there is a concept called piggy-backing which means that if one thread writes to all its fields and then writes to a volatile variable, everything written by the thread before writing to the volatile variable would be visible to the other threads, provided the other threads first read value of the volatile variable after it is written by the first thread. But when you do this you have to ensure very carefully the sequence of reading and writing and it is error prone. So, this is done when you want to squeeze out the last drop of performance from the piece of code which is rare. Favor safety, maintainability, readability before performance.
Finally, there is no race condition in the code. The only write that is happening is on the ConcurrentHashMap which is thread safe by itself.

Basically, both approaches are equivalent. From architectural point of view, making a variable inside dedicated class is preferred because of better control of which methods are accessible to the user. When extending, a user can access many methods of underlying ConcurrentHashMap and misuse them.

Best practices or principles for sharing objects between threads in Java

I tried to search but couldn't find exact answer I was looking for hence putting up a new question.
If you wish to share any mutable object(s) between multiple threads, are there any best practices/principles/guidelines to do it ?
Or will it simply vary case by case ?

Sharing mutable objects between threads is risky.
The safest way is to make the objects immutable, you can then share them freely.
If they must be mutable then each of the objects each needs to ensure their own thread safety using the usual methods to do so. (synchronized, AtomicX classes, etc).
The ways to protect the individual objects will vary a lot though depending on how you are using them and what you are using them for.

In java, you should synchronize any method that changes/reads the state of shared object, it is the easiest way.
other strategies are:
make use of thread safe classes (ConcurrentHashMap) for example
use of locks
use of volatile keyword, to avoid stale objects (sometimes could be used as lightweight synchronizer)
they key is sync your updates/reads to guarantee consistent state, the way you do it, could vary a lot.

The problems with sharing objects between threads are caused by having the two threads access the same data structure at the same time, with one mutating the structure while the other depends on the structure to be complete, correct or stable. Which of these cause the problem is important and should be considered when choosing the strategy.
These are the strategies I use.
Use immutable objects as much as possible.
This removes the issue of changing the data structure altogether. There are however a lot of useful patterns that can not be written using this approach. Also unless you are using a language/api which promotes immutability it can be inefficient. Adding a entry to a Scala list is much faster than making a copy of a Java list and adding a entry to the copy.
Use the synchronize keyword.
This ensures that only one thread at a time is allowed to change the object. It is important to choose which object to synchronize on. Changing a part of a structure might put the hole structure in an illegal state until another change is made. Also synchronize removes many of the benefits of going multithreaded in the first place.
The Actor model.
The actor model organizes the world in actors sending immutable messages to each other. Each actor only has one thread at once. The actor can contain the mutability.
There are platforms, like Akka, which provide the fundamentals for this approach.
Use the atomic classes. (java.util.concurrent.atomic)
These gems have methods like incrementAndGet. They can be used
to achieve many of the effects of synchronized without the overhead.
Use concurrent data structures.
The Java api contains concurrent data structures created for this purpose.
Risk doing stuff twice.
When designing a cache it is often a good idea to risk doing the work twice instead of using synchronize. Say you have a cache of compiled expressions from a dsl. If an expression is compiled twice that is ok as long as it eventually ends up in the cache. By allowing doing some extra work during initialization you may not need to use the synchronize keyword during cache access.

There is example. StringBuilder is not thread safe, so without synchronized (builder) blocks - result will be broken. Try and see.
Some objects are thread safe (for example StringBuffer), so no need to use synchronized blocks with them.
public static void main(String[] args) throws InterruptedException {
StringBuilder builder = new StringBuilder("");
Thread one = new Thread() {
public void run() {
for (int i = 0; i < 1000; i++) {
//synchronized (builder) {
builder.append("thread one\n");
//}
}
}
};
Thread two = new Thread() {
public void run() {
for (int i = 0; i < 1000; i++) {
//synchronized (builder) {
builder.append("thread two\n");
//}
}
}
};
one.start();
two.start();
one.join();
two.join();
System.out.println(builder);
}

Although there are some good answers already posted, but here is what I found while reading Java Concurrency in Practice Chapter 3 - Sharing Objects.
Quote from the book.
The publication requirements for an object depend on its mutability:
Mutable objects can be published through any mechanism;
Effectively immutable objects (whose state will not be modified after publication) must be safely published;
Mutable objects must be safely published, and must be either threadsafe or guarded by a lock.
Book states ways to safely publish mutable objects:
To publish an object safely, both the reference to the object and the object's state must be made visible to other threads at the same time. A properly constructed object can be safely published by:
Initializing an object reference from a static initializer;
Storing a reference to it into a volatile field or AtomicReference;
Storing a reference to it into a final field of a properly constructed object; or
Storing a reference to it into a field that is properly guarded by a lock.
The last point refers to using various mechanisms like using concurrent data structures and/or using synchronize keyword.

Immutable objects are thread safe, but why?

Lets say for example, a thread is creating and populating the reference variable of an immutable class by creating its object and another thread kicks in before the first one completes and creates another object of the immutable class, won't the immutable class usage be thread unsafe?
Creating an immutable object also means that all fields has to be marked as final.
it may be necessary to ensure correct behavior if a reference to
a newly created instance is passed from one thread to another without
synchronization
Are they trying to say that the other thread may re-point the reference variable to some other object of the immutable class and that way the threads will be pointing to different objects leaving the state inconsistent?

Actually immutable objects are always thread-safe, but its references may not be.
Confused?? you shouldn't be:-
Going back to basic:
Thread-safe simply means that two or more threads must work in coordination on the shared resource or object. They shouldn't over-ride the changes done by any other thread.
Now String is an immutable class, whenever a thread tries to change it, it simply end up creating a new object. So simply even the same thread can't make any changes to the original object & talking about the other thread would be like going to Sun but the catch here is that generally we use the same old reference to point that newly created object.
When we do code, we evaluate any change in object with the reference only.
Statement 1:
String str = "123"; // initially string shared to two threads
Statement 2:
str = str+"FirstThread"; // to be executed by thread one
Statement 3:
str=str+"SecondThread"; // to be executed by thread two
Now since there is no synchronize, volatile or final keywords to tell compiler to skip using its intelligence for optimization (any reordering or caching things), this code can be run in following manner.
Load Statement2, so str = "123"+"FirstThread"
Load Statement3, so str = "123"+"SecondThread"
Store Statement3, so str = "123SecondThread"
Store Statement2, so str = "123FirstThread"
and finally the value in reference str="123FirstThread" and for sometime if we assume that luckily our GC thread is sleeping, that our immutable objects still exist untouched in our string pool.
So, Immutable objects are always thread-safe, but their references may not be. To make their references thread-safe, we may need to access them from synchronized blocks/methods.

In addition to other answers posted already, immutable objects once created, they cannot be modified further. Hence they are essentially read-only.
And as we all know, read-only things are always thread-safe. Even in databases, multiple queries can read same rows simultaneously, but if you want to modify something, you need exclusive lock for that.

Immutable objects are thread safe, but why?
An immutable object is an object that is no longer modified once it has been constructed. If in addition, the immutable object is only made accessible to other thread after it has been constructed, and this is done using proper synchronization, all threads will see the same valid state of the object.
If one thread is creating populating the reference variable of the immutable class by creating its object and at the second time the other thread kicks in before the first thread completes and creates another object of the immutable class, won't the immutable class usage be thread unsafe?
No. What makes you think so? An object's thread safety is completely unaffected by what you do to other objects of the same class.
Are they trying to say that the other thread may re-point the reference variable to some other object of the immutable class and that way the threads will be pointing to different objects leaving the state inconsistent?
They are trying to say that whenever you pass something from one thread to another, even if it is just a reference to an immutable object, you need to synchronize the threads. (For instance, if you pass the reference from one thread to another by storing it in an object or a static field, that object or field is accessed by several threads, and must be thread-safe)

Thread safety is data sharing safety, And because in your code you make decisions based on the data your objects hold, the integrity and deterministic behaviour of it is vital. i.e
Imagine we have a shared boolean instance variable across two threads that are about to execute a method with the following logic
If flag is false, then I print "false" and then I set the flag back to true.
If flag is true, then I print "true" and then I set the flag back to false.
If you run continuously in a single thread loop, you will have a deterministic output which will look like:
false - true - false - true - false - true - false ...
But, if you ran the same code with two threads, then, the output of your output is not deterministic anymore, the reason is that the thread A can wake up, read the flag, see that is false, but before it can do anything, thread B wakes up and reads the flag, which is also false!! So both will print false... And this is only one problematic scenario I can think of... As you can see, this is bad.
If you take out the updates of the equation the problem is gone, just because you are eliminating all the risks associated with data sync. that's why we say that immutable objects are thread safe.
It is important to note though, that immutable objects are not always the solution, you may have a case of data that you need to share among different threads, in this cases there are many techniques that go beyond the plain synchronization and that can make a whole lot of difference in the performance of your application, but this is a complete different subject.
Immutable objects are important to guarantee that the areas of the application that we are sure that don't need to be updated, are not updated, so we know for sure that we are not going to have multithreading issues
You probably might be interested in taking a look at a couple of books:
This is the most popular: http://www.amazon.co.uk/Java-Concurrency-Practice-Brian-Goetz/dp/0321349601/ref=sr_1_1?ie=UTF8&qid=1329352696&sr=8-1
But I personally prefer this one: http://www.amazon.co.uk/Concurrency-State-Models-Java-Programs/dp/0470093552/ref=sr_1_3?ie=UTF8&qid=1329352696&sr=8-3
Be aware that multithreading is probably the trickiest aspect of any application!

Immutability doesn't imply thread safety.In the sense, the reference to an immutable object can be altered, even after it is created.
//No setters provided
class ImmutableValue
{
private final int value = 0;
public ImmutableValue(int value)
{
this.value = value;
}
public int getValue()
{
return value;
}
}
public class ImmutableValueUser{
private ImmutableValue currentValue = null;//currentValue reference can be changed even after the referred underlying ImmutableValue object has been constructed.
public ImmutableValue getValue(){
return currentValue;
}
public void setValue(ImmutableValue newValue){
this.currentValue = newValue;
}
}

Two threads will not be creating the same object, so no problem there.
With regards to 'it may be necessary to ensure...', what they are saying is that if you DON'T make all fields final, you will have to ensure correct behavior yourself.

Are non-synchronised static methods thread safe if they don't modify static class variables?

I was wondering if you have a static method that is not synchronised, but does not modify any static variables is it thread-safe? What about if the method creates local variables inside it? For example, is the following code thread-safe?
public static String[] makeStringArray( String a, String b ){
return new String[]{ a, b };
}
So if I have two threads calling ths method continously and concurrently, one with dogs (say "great dane" and "bull dog") and the other with cats (say "persian" and "siamese") will I ever get cats and dogs in the same array? Or will the cats and dogs never be inside the same invocation of the method at the same time?

This method is 100% thread safe, it would be even if it wasn't static. The problem with thread-safety arises when you need to share data between threads - you must take care of atomicity, visibility, etc.
This method only operates on parameters, which reside on stack and references to immutable objects on heap. Stack is inherently local to the thread, so no sharing of data occurs, ever.
Immutable objects (String in this case) are also thread-safe because once created they can't be changed and all threads see the same value. On the other hand if the method was accepting (mutable) Date you could have had a problem. Two threads can simultaneously modify that same object instance, causing race conditions and visibility problems.

A method can only be thread-unsafe when it changes some shared state. Whether it's static or not is irrelevant.

The function is perfectly thread safe.
If you think about it... assume what would happen if this were different. Every usual function would have threading problems if not synchronized, so all API functions in the JDK would have to be synchronized, because they could potentially be called by multiple threads. And since most time the app is using some API, multithreaded apps would effectively be impossible.
This is too ridiculous to think about it, so just for you: Methods are not threadsafe if there is a clear reason why there could be problems. Try to always think about what if there were multiple threads in my function, and what if you had a step-debugger and would one step after another advance the first... then the second thread... maybe the second again... would there be problems? If you find one, its not thread safe.
Please be also aware, that most of the Java 1.5 Collection classes are not threadsafe, except those where stated, like ConcurrentHashMap.
And if you really want to dive into this, have a close look at the volatile keyword and ALL its side effects. Have a look at the Semaphore() and Lock() class, and their friends in java.util.Concurrent. Read all the API docs around the classes. It is worth to learn and satisfying, too.
Sorry for this overly elaborate answer.

Use the static keyword with synchronized static methods to modify static data shared among threads. With the static keyword all the threads created will contend for a single version of the method.
Use the volatile keyword along with synchronized instance methods will guarantee that each thread has its own copy of the shared data and no read/ writes will leak out between threads.

String objects being immutable is another reason for thread-safe scenario above. Instead if mutable objects are used (say makeMutableArray..) then surely thread-safety will break.

Since the complete method was pushed onto the stack, any variable creation that takes place lives within the stack (again exceptions being static variables) and only accessible to one thread. So all the methods are thread safe until they change the state of some static variable.
See also:
Is static method is thread safe in Java?