java object reference assignment

This lesson discusses object reference variables. These variables refer to objects (as opposed to holding a primitive data value.)

The key topics for this lesson are:

A. Primitive Data Type Variables
B. Object Reference variables
C. Variable Versus Object Reference Assignment
D. Object References
E. The == operator with Variables and Object References
F. The Method.
G. The Value.

PRIMITIVE DATA
GARBAGE COLLECTION

OBJECT REFERENCE

1. Java has many data types built into it, and you (as a programmer) can define as many more as you need. Other than the primitive data types, all data types are classes. In other words, data is primitive data or object data. The only type of data a programmer can define is an object data type (a class).

2. Here is a tiny program that uses a primitive data type:
PrimitiveDataType { main(String[] args) { primitiveValue; primitiveValue = 95124; System.out.println(primitiveValue); } }

In this program, the variable value is the name for a 64 bit section of memory that is used to hold long integers. The statement

puts a particular bit pattern in that 64 bit section of memory.

3. With primitive data types, a variable is a section of memory reserved for a value of a particular style. For example by saying value, 64 bits of memory are reserved for an integer. By saying sum, 32 bits of memory are reserved an integer.

1. Since objects are big, complicated, and vary in size you do not automatically get an object when you declare an object reference variable. All you get is a name for a future object. For example:

2. Here is a tiny program that uses a String object (NOT a primitive data type):

3. An object contains data and methods (state and behavior). You can visualize the String object in the above program like this:

The data section of the object contains the characters. The methods section of the object contains many methods.

4. An object reference is information on how to find a particular object. The object is a chunk of main memory; a reference to the object is a way to get to that chunk of memory. The variable str does not actually contain the object, but contains information about where the object is.

5. Objects are created while a program is running. Each object has a unique object reference, which is used to find it. When an object reference is assigned to a variable, then that variable says how to find that object.

1. Notice that there is a difference between the two statements:

and

In the first statement, is a primitive type, so the assignment statement puts the data directly into it. In the second statement, is an object reference variable (the only other possibility) so a reference to the object is put into that variable.

2. A variable will contain an object. There are only primitive variables and object reference variables, and each contains a specific kind of information:

3. The two types of variable are distinguished by how they are declared. Unless it was declared to be of a primitive type, it is an object reference variable. A variable will not change its declared type.

D. Object References

2. Notice that:

a. Each time the new operator is used, a new object is created. b. Each time an object is created, there is a reference to it. c. This reference is saved in a variable. d. Later on, the reference in the variable is used to find the object. e. If another reference is saved in the variable, it replaces the previous reference. f. If no variables hold a reference to an object, there is no way to find it, and it becomes " garbage. "

3. The word "garbage" is the correct term from computer science to use for objects that have no references. This is a commonly occurring situation, not usually a mistake. As a program executes, a part of the Java system called the "garbage collector" reclaims the lost objects (the "garbage") so that their memory can be used again.

4. Multiple objects of the same class can be maintained by creating unique reference variables for each object.

5. Different reference variables that refer to the same object are called aliases . In effect, there are two names for the same object. For example:

When this program runs, only one object is created (by new). Information about how to find the object is put into strA. The assignment operator in the statement

strB = strA; // copy the reference to strB.

copies the information that is in strA to strB. It does not make a copy of the object.

E. The == operator with Variables and Object References.

1. The == operator is used to look at the contents of two reference variables. If the contents of both reference variables is the same, then the result is true . Otherwise the result is false .

2. In this program, there are two completely separate objects, each of which happens to contain character data equivalent to that in the other. Each object consists of a section of main memory completely separate from the memory that makes up the other object. The variable strA contains information on how to find the first object, and the variable strB contains information on how to find the second object

Since the information in strA is different from the information in strB, (strA == strB) is false, for the first comparison. Since there are two objects, made out of two separate sections of main memory, the reference stored in strA is different from the reference in strB. It doesn't matter that the data inside the objects looks the same.

3. The == operator does not look at objects. It only looks at references (information about where an object is located.)

4. For primitive types, the == operator looks only at the variables. For example:

In this code, only the contents of the variables x and y are examined. But with primitive types, the contents of a variable is the data, so with primitive types == looks at data.

5. With primitive and reference types, == looks at the contents of the variables. However, with reference types, the variables contain object references and with primitive types, the variables contain the actual data values.

F. The equals() Method.

2. The equals(String) method does look at the contents of objects. It detects "equivalence." The == operator detects "identity". For example,

3. In this example, there are two objects. Since each object has its own identity, == reports false. Each object contains equivalent data so equals() reports true.

G. The null Value

1. In most programs, objects are created and objects are destroyed, depending on the data and on what is being computed. A reference variable sometimes does and sometimes does not refer to an object. You need a way to say that a variable does not now refer to an object. You do this by assigning null to the variable.

2. The value null is a special value that means "no object." A reference variable is set to null when it is not referring to any object.

3. Variables a and c are initialized to object references. Variable b is initialized to null. Note that variable c is initialized to a reference to a String object containing no characters. This is a different value than null.

SUMMARY/ REVIEW:

Last Minute Java Object Assignment and Object Passing By Value or Reference Explained Tutorial

In Java, we create Classes. We create objects of the Classes. Let us know if it is possible to assign a Java Object  from one variable to another variable by value or reference in this Last Minute Java Tutorial. We also try to know whether it is possible to pass Java Objects to another method by Value or Reference in this tutorial.

You may also read about Introduction to Java Class Structure .

Java Object Assignment by Value or Reference Explained

In Java, objects are always passed by Reference. Reference is nothing but the starting address of a memory location where the actual object lies. You can create any number of references to just One Object. Let us go by an example.

We create a Class "Cloud" with a single Property called "color". It is a String object. We create 3 objects of the Cloud type.

Notice that all three references obj1, obj2 and obj3 are pointing to the same Cloud object. Thus the above example outputs the same color-property value " RED ". Later, we made the references obj2 and obj3 to point to the null location. Still the original object pointed or referenced by obj1 holds the value. So the final PRINT statement outputs "RED" as usual.

Note : Reference is also called a Variable whether Primitive or Object.

This concludes that a Java object assignment happens by Reference . But the actual value of memory location pointed by References are copied by Value only. So we are actually copying memory locations from one Reference variable to another Reference variable by Value . Interviewers ask this famous Java question just to test your knowledge. If someone asks in C language point of view , say that it is by Reference . If they ask in Java point of view , say that it is by Value. 

Java Object Passing by Value or Reference to a Method Explained

You already learnt that Object assignment in Java happens by Reference. Let us try to pass Objects to another method as a parameter or argument. We try to know whether Objects are passed by Value or Reference to another Method using the below example.

In the above example, we have created an object "obj1" of type Tractor. We assigned value of 100 to the property "height". We printed the value before passing the object to another method. Now, we passed the object to another method called "change" from the calling-method " main ". We modified the height property to 200 in the called-method . Now, we tried to print the value of the property in the Calling method. It shows that the Object is modified in the called-method.

Similarly, we can pass Java objects to the Constructor using Pass by Reference or simply Reference . 

This concludes that Java Objects are passed to methods by Reference . Here also, we pass the memory location pointed by one Reference variable to another Reference variable of a Called method as a Value only. So, you are actually passing Values (memory locations represented by integer or long data type). But the actual passed-object can only be referenced by a Reference variable. Interviewers take advantage of this tricky logic to confuse you. If someone asks in C language point of view , say that it is by Reference . If they ask in Java point of view , say that it is by Value .

Let us know more about Java Classes with different Constructors and Methods in coming chapters.

Share this Last Minute Java  Tutorial explaining Java Object Assignment using Reference and Java Object Passing with Pass by Reference with your friends and colleagues to encourage authors.

Java Class Structure Tutorial

Java Method Signature Rules Tutorial

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Does Java pass by reference or pass by value?

You might know that java passes by value, but it helps to understand why. here's what happens when you pass mutable and immutable object references in java..

Many programming languages allow passing objects by reference or by value. In Java, we can only pass object parameters by value. This imposes limits and also raises questions. For instance, if the parameter value is changed in the method, what happens to the value following method execution? You may also wonder how Java manages object values in the memory heap. This article helps you resolve these and other common questions about object references in Java.

Passing object references in Java

In this article you’ll learn the difference between passing by reference and passing by value in Java and how to pass Java object references:

‘Pass by reference’ and ‘pass by value’ defined

Java object references are passed by value, passing primitive types in java, passing immutable object references in java, passing mutable object references in java.

• What to avoid when passing object references
• What to remember about passing object references

Pass by reference  means we pass the location in memory where the variable’s value is stored and pass by value means we pass a copy of the variable’s actual value. It’s a bit more complicated than that, of course, but this definition is a good starting point.

• Passing by value means we pass a copy of the value.
• Passing by reference means we pass the real reference to the variable in memory.

All object references in Java are passed by value. This means that a copy of the value will be passed to a method. The tricky part is that passing a copy of the value changes the real value of the object. This example should help you understand why:

What do you think the simpson.name will be after the transformIntoHomer method is executed?

In this case, it will be Homer! The reason is that Java object variables are simply references that point to real objects in the memory heap. Therefore, even though Java passes parameters to methods by value, if the variable points to an object reference, the real object will also be changed.

If you’re still not sure how this works, consider the following diagram:

Like object types, primitive types are also passed by value. Can you guess what will happen to the primitive types in the following code example?

If you determined that the value would change to 30, you are correct. It’s 30 because (again) Java passes object parameters by value. The number 30 is just a copy of the value, not the real value. Primitive types are allocated in the stack memory, so only the local value will be changed. In this case, there is no object reference.

What if we did the same test with an immutable String object? Notice what happens when we change the value of a String :

What do you think the output will be? If you guessed “” then congratulations! That happens because a String object is immutable , which means that the fields inside the String are final and can’t be changed.

Making the String class immutable gives us better control over one of Java’s most used objects. If the value of a String could be changed, it would create many bugs. Note, also, that we are not changing an attribute of the String class; instead, we’re simply assigning a new String value to it. In this case, the “Homer” value will be passed to name in the changeToHomer method. The String “Homer” will be eligible to be garbage collected as soon as the changeToHomer method completes execution. Even though the object can’t be changed, the local variable will be.

Unlike String , most objects in the JDK are mutable, like the StringBuilder class. The example below is similar to the previous one, but features StringBuilder rather than String :

Can you guess the output for this example? In this case, because we’re working with a mutable object, the output will be “Homer Simpson.” You could expect the same behavior from any other mutable object in Java.

Summary of what you’ve learned

You’ve learned that Java objects are passed by value, meaning that a copy of the value is passed . Just remember that the copied value points to a real object in the Java memory heap. Also, remember that passing by value changes the value of the real object.

What to avoid when passing Java object references

• Don’t try to change an immutable value by reference.
• Don’t try to change a primitive variable by reference.
• Don’t expect the real object won’t change when you change a mutable object parameter in a method (it will change).

What to remember about passing Java object references

• Java always passes parameter variables by value.
• Object variables in Java always point to the real object in the memory heap.
• A mutable object’s value can be changed when it is passed to a method.
• An immutable object’s value cannot be changed, even if it is passed a new value.

Test what you’ve learned about object references

Now, let’s test what you’ve learned about object references. In the code example below, you see the immutable String and the mutable StringBuilder class. Each is being passed as a parameter to a method. Knowing that Java only passes by value, what do you believe will be the output once the main method from this class is executed?

Here are the options, check the end of the article for the answer.

A : Warrior=null Weapon=null B : Warrior=Dragon Weapon=Dragon C : Warrior=Dragon Knight Weapon=Dragon Sword D : Warrior=Dragon Knight Weapon=Sword

Solving the challenge

The first parameter in the above example is the warriorProfession variable, which is a mutable object. The second parameter, weapon, is an immutable String :

At the first line of this method, we append the Knight value to the warriorProfession variable. Remember that warriorProfession is a mutable object; therefore the real object will be changed, and the value from it will be “Dragon Knight.”

In the second instruction, the immutable local String variable will be changed to “Dragon Sword.” The real object will never be changed, however, since String is immutable and its attributes are final:

Finally, we pass null to the variables here, but not to the objects. The objects will remain the same as long as they are still accessible externally—in this case through the main method. And, although the local variables will be null, nothing will happen to the objects:

From this we can conclude that the final values from our mutable StringBuilder and immutable String will be:

The only value that changed in the changeWarriorClass method was warriorProfession , because it’s a mutable StringBuilder object. Note that warriorWeapon did not change because it’s an immutable String object.

The correct output from our Challenger code would be:

D : Warrior=Dragon Knight Weapon=Sword .

Video challenge! Debugging object references in Java

Debugging is one of the easiest ways to fully absorb programming concepts while also improving your code. In this video, you can follow along while I debug and explain object references in Java.

Learn more about Java

• Get more quick code tips: Read all of Rafael’s articles in the InfoWorld Java Challengers series.
• Check out more videos in Rafael’s Java Challengers video playlist .
• Find even more Java Challengers on Rafael’s Java Challengers blog and in his book, with more than 70 code challenges .

Related content

Thread behavior in the jvm, polymorphism and inheritance in java, java inheritance vs. composition: how to choose, sorting java objects with comparable and comparator.

Rafael del Nero is a Java Champion and Oracle Ace, creator of the Java Challengers initiative, and a quiz master in the Oracle Dev Gym. Rafael is the author of "Java Challengers" and "Golden Lessons." He believes there are many techniques involved in creating high-quality software that developers are often unaware of. His purpose is to help Java developers use better programming practices to code quality software for stress-free projects with fewer bugs.

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Java is an object-oriented programming language. The core concept of the object-oriented approach is to break complex problems into smaller objects.

An object is any entity that has a state and behavior . For example, a bicycle is an object. It has

• States : idle, first gear, etc
• Behaviors : braking, accelerating, etc.

Before we learn about objects, let's first know about classes in Java.

A class is a blueprint for the object. Before we create an object, we first need to define the class.

We can think of the class as a sketch (prototype) of a house. It contains all the details about the floors, doors, windows, etc. Based on these descriptions we build the house. House is the object.

Since many houses can be made from the same description, we can create many objects from a class.

• Create a class in Java

We can create a class in Java using the class keyword. For example,

Here, fields ( variables ) and methods represent the state and behavior of the object respectively.

• fields are used to store data
• methods are used to perform some operations

For our bicycle object, we can create the class as

In the above example, we have created a class named Bicycle . It contains a field named gear and a method named braking() .

Here, Bicycle is a prototype. Now, we can create any number of bicycles using the prototype. And, all the bicycles will share the fields and methods of the prototype.

Note : We have used keywords private and public . These are known as access modifiers. To learn more, visit Java access modifiers .

• Java Objects

An object is called an instance of a class. For example, suppose Bicycle is a class then MountainBicycle , SportsBicycle , TouringBicycle , etc can be considered as objects of the class.

Creating an Object in Java

Here is how we can create an object of a class.

We have used the new keyword along with the constructor of the class to create an object. Constructors are similar to methods and have the same name as the class. For example, Bicycle() is the constructor of the Bicycle class. To learn more, visit Java Constructors .

Here, sportsBicycle and touringBicycle are the names of objects. We can use them to access fields and methods of the class.

As you can see, we have created two objects of the class. We can create multiple objects of a single class in Java.

Note : Fields and methods of a class are also called members of the class.

• Access Members of a Class

We can use the name of objects along with the . operator to access members of a class. For example,

In the above example, we have created a class named Bicycle . It includes a field named gear and a method named braking() . Notice the statement,

Here, we have created an object of Bicycle named sportsBicycle . We then use the object to access the field and method of the class.

• sportsBicycle.gear - access the field gear
• sportsBicycle.braking() - access the method braking()

We have mentioned the word method quite a few times. You will learn about Java methods in detail in the next chapter.

Now that we understand what is class and object. Let's see a fully working example.

• Example: Java Class and Objects

In the above program, we have created a class named Lamp . It contains a variable: isOn and two methods: turnOn() and turnOff() .

Inside the Main class, we have created two objects: led and halogen of the Lamp class. We then used the objects to call the methods of the class.

• led.turnOn() - It sets the isOn variable to true and prints the output.
• halogen.turnOff() - It sets the isOn variable to false and prints the output.

The variable isOn defined inside the class is also called an instance variable. It is because when we create an object of the class, it is called an instance of the class. And, each instance will have its own copy of the variable.

That is, led and halogen objects will have their own copy of the isOn variable.

Example: Create objects inside the same class

Note that in the previous example, we have created objects inside another class and accessed the members from that class.

However, we can also create objects inside the same class.

Here, we are creating the object inside the main() method of the same class.

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Operators constitute the basic building block of any programming language. Java too provides many types of operators which can be used according to the need to perform various calculations and functions, be it logical, arithmetic, relational, etc. They are classified based on the functionality they provide.

Types of Operators: 

• Arithmetic Operators
• Unary Operators
• Assignment Operator
• Relational Operators
• Logical Operators
• Ternary Operator
• Bitwise Operators
• Shift Operators

This article explains all that one needs to know regarding Assignment Operators. 

Assignment Operators

These operators are used to assign values to a variable. The left side operand of the assignment operator is a variable, and the right side operand of the assignment operator is a value. The value on the right side must be of the same data type of the operand on the left side. Otherwise, the compiler will raise an error. This means that the assignment operators have right to left associativity, i.e., the value given on the right-hand side of the operator is assigned to the variable on the left. Therefore, the right-hand side value must be declared before using it or should be a constant. The general format of the assignment operator is, 

Types of Assignment Operators in Java

The Assignment Operator is generally of two types. They are:

1. Simple Assignment Operator: The Simple Assignment Operator is used with the “=” sign where the left side consists of the operand and the right side consists of a value. The value of the right side must be of the same data type that has been defined on the left side.

2. Compound Assignment Operator: The Compound Operator is used where +,-,*, and / is used along with the = operator.

Let’s look at each of the assignment operators and how they operate: 

1. (=) operator: 

This is the most straightforward assignment operator, which is used to assign the value on the right to the variable on the left. This is the basic definition of an assignment operator and how it functions. 

Syntax:  

Example:  

2. (+=) operator: 

This operator is a compound of ‘+’ and ‘=’ operators. It operates by adding the current value of the variable on the left to the value on the right and then assigning the result to the operand on the left. 

Note: The compound assignment operator in Java performs implicit type casting. Let’s consider a scenario where x is an int variable with a value of 5. int x = 5; If you want to add the double value 4.5 to the integer variable x and print its value, there are two methods to achieve this: Method 1: x = x + 4.5 Method 2: x += 4.5 As per the previous example, you might think both of them are equal. But in reality, Method 1 will throw a runtime error stating the “i ncompatible types: possible lossy conversion from double to int “, Method 2 will run without any error and prints 9 as output.

Reason for the Above Calculation

Method 1 will result in a runtime error stating “incompatible types: possible lossy conversion from double to int.” The reason is that the addition of an int and a double results in a double value. Assigning this double value back to the int variable x requires an explicit type casting because it may result in a loss of precision. Without the explicit cast, the compiler throws an error. Method 2 will run without any error and print the value 9 as output. The compound assignment operator += performs an implicit type conversion, also known as an automatic narrowing primitive conversion from double to int . It is equivalent to x = (int) (x + 4.5) , where the result of the addition is explicitly cast to an int . The fractional part of the double value is truncated, and the resulting int value is assigned back to x . It is advisable to use Method 2 ( x += 4.5 ) to avoid runtime errors and to obtain the desired output.

Same automatic narrowing primitive conversion is applicable for other compound assignment operators as well, including -= , *= , /= , and %= .

3. (-=) operator: 

This operator is a compound of ‘-‘ and ‘=’ operators. It operates by subtracting the variable’s value on the right from the current value of the variable on the left and then assigning the result to the operand on the left. 

4. (*=) operator:

 This operator is a compound of ‘*’ and ‘=’ operators. It operates by multiplying the current value of the variable on the left to the value on the right and then assigning the result to the operand on the left. 

5. (/=) operator: 

This operator is a compound of ‘/’ and ‘=’ operators. It operates by dividing the current value of the variable on the left by the value on the right and then assigning the quotient to the operand on the left. 

6. (%=) operator: 

This operator is a compound of ‘%’ and ‘=’ operators. It operates by dividing the current value of the variable on the left by the value on the right and then assigning the remainder to the operand on the left. 

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JavaScript ( JS ) is a lightweight interpreted (or just-in-time compiled) programming language with first-class functions . While it is most well-known as the scripting language for Web pages, many non-browser environments also use it, such as Node.js , Apache CouchDB and Adobe Acrobat . JavaScript is a prototype-based , multi-paradigm, single-threaded , dynamic language, supporting object-oriented, imperative, and declarative (e.g. functional programming) styles.

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Why is reference assignment atomic in Java?

As far as I know reference assignment is atomic in a 64 bit JVM. Now, I assume the jvm doesn't use atomic pointers internally to model this, since otherwise there would be no need for Atomic References. So my questions are:

Is atomic reference assignment in the "specs" of java/Scala and guaranteed to happen or is it just a happy coincidence that it is that way most times ?

Is atomic reference assignment implied for any language that compiles to the JVM's bytecode (e.g. clojure, Groovy, JRuby, JPython...etc) ?

How can reference assignment be atomic without using an atomic pointer internally ?

• concurrency

• @JornVernee I meant assignment not swapping, thanks for the correction –  George Commented May 31, 2017 at 11:21
• This might give a first clue: stackoverflow.com/questions/23232242/… –  GhostCat Commented May 31, 2017 at 11:35
• I did... however I've been searching all over the internet for how getfield, putfiled and iastore are implemented and I haven't found an explaination that doesn't involve digging through the openjvm code, which I'm not proficient enough to do :( –  George Commented May 31, 2017 at 12:23

3 Answers 3

First of all, reference assignment is atomic because the specification says so. Besides that, there is no obstacle for JVM implementors to fulfill this constraint, as 64 Bit references are usually only used on 64 Bit architectures, where atomic 64 Bit assignment comes for free.

Your main confusion stems from the assumption that the additional “Atomic References” feature means exactly that, due to its name. But the AtomicReference class offers far more, as it encapsulates a volatile reference, which has stronger memory visibility guarantees in a multi-threaded execution.

Having an atomic reference update does not necessarily imply that a thread reading the reference will also see consistent values regarding the fields of the object reachable through that reference. All it guarantees is that you will read either the null reference or a valid reference to an existing object that actually was stored by some thread. If you want more guarantees, you need constructs like synchronization, volatile references, or an AtomicReference .

AtomicReference also offers atomic update operations like compareAndSet or getAndSet . These are not possible with ordinary reference variables using built-in language constructs (but only with special classes like AtomicReferenceFieldUpdater or VarHandle ).

• 2 An “atomic pointer” isn’t necessarily an “atomic reference”, in the sense of AtomicReference . Hence, it was necessary to point out that references (i.e. the underlying pointers) are atomic, still don’t match the functionality of an AtomicReference . –  Holger Commented May 31, 2017 at 15:54
• 3 “Publishing” means assigning to a variable that is visible to other threads. And no, there are no guaranties regarding visibility of previous updates when publishing though normal references. If you write point=new Point(42,100) and point is neither final nor volatile , other threads may see the new reference, while still seeing the default value ( 0 ) for either, x or y , or both (in the absence of other synchronization). –  Holger Commented May 31, 2017 at 15:57
• 3 This is not about an older instance; the reference might have been null before the assignment, that doesn’t matter. The issue is, new Point(42,100) is not an atomic operation. It creates a new Point instance, all fields being at their default value ( 0 for int ), then, the constructor will be executed, which will assign the values 42 to x and 100 to y , then the reference is assigned to point , from a single thread’s point of view . A different thread may see these actions out of order, see the reference to the new Point instance without seeing the effect of the assignments. –  Holger Commented May 31, 2017 at 16:36
• 2 This doesn’t apply to immutable objects. If you declare x and y as final , they’re immune to such data races. Alternatively, you may declare point as final , but this implies that the whole action is part of a constructor or initializer. Or, well, declaring point as volatile ensures that other threads see all previous updates ( x=42; y=100; ) when seeing the new Point instance through the point reference. –  Holger Commented May 31, 2017 at 16:37
• 2 @Powet any reference assignment that doesn’t care about the previous reference, is atomic. The update operations provided by AtomicReference include compareAndSet (“only assign the new reference if the old reference has the expected value”) and getAndSet (“give me the old reference after assigning the new one”). If you try to implement the same logic without AtomicReference , it will inevitably consist of more than one access to the field, so it’ll be obvious why this wouldn’t be atomic (without additional measures). –  Holger Commented Apr 16 at 8:16

Atomic reference assignment is in the specs.

Writes to and reads of references are always atomic, regardless of whether they are implemented as 32 or 64 bit values.

Quoted from JSR-133: Java(TM) Memory Model and Thread Specification , section 12 Non-atomic Treatment of double and long , http://www.cs.umd.edu/~pugh/java/memoryModel/jsr133.pdf .

• Ok, that answers the first part but not the other 2. To be honest the thing that I'm most curios about here is how this behavior happens if references are not atomic pointers and that's related to how one would implement a JVM rather than the specifications for java and/or the jvm. –  George Commented May 31, 2017 at 12:17
• True. I on purpose did not pretend to be a wiseguy on matters where I’m not sure. The way I understand it, though, this is a spec of the JVM, so I would expect it to cover all languages running on the JVM. And the way I read it, in requires internal pointer operations to happen atomically (at least as seen from outside the JVM). –  Anonymous Commented May 31, 2017 at 12:25
• Well yes, but I doubt those are actual atomic pointer operations since otherwise things like compare and swap would also be atomic using standard references. –  George Commented May 31, 2017 at 12:29
• 1 Nice answer - good find; my vote for that. And thanks for leaving out that small part that left room for my answer ;-) –  GhostCat Commented May 31, 2017 at 12:29
• 3 The linked PDF describes the memory model for Java 5 proposed by JSR133. To prove that it indeed made it into the Java 5 JLS and is still there today, it’s better to link to JLS §17.7 . –  Holger Commented May 31, 2017 at 14:38

As the other answer outlines, the Java Memory Model states that references read/writes are atomic.

But of course, that is the Java language memory model. On the other hand: no matter if we talk Java or Scala or Kotlin or ... in the end everything gets compiled into bytecode .

There are no special bytecode instructions for Java. Scala in the end uses the very same instructions.

Leading to: the properties of that memory model must be implemented inside the VM platform. Thus they must apply to other languages running on the platform as well.

• Since the name was ' JavaTM Memory Model' I was unsure if it referred to the JVM as a whole and as such I assumed there might be instructions to non atomically asign available to other JVM languages –  George Commented May 31, 2017 at 12:40
• Back then when that memory model was written, there was only the Java language on the Java VM platform. Methinks. –  GhostCat Commented May 31, 2017 at 12:41
• Even now, I don't think there are JVM instructions that aren't used by Java itself. –  Jasper-M Commented May 31, 2017 at 12:44
• 5 @Jasper-M: there are. E.g., swap is never used by javac generated code. In Java 7, the invokedynamic instruction was not used by Java itself, which is quite a big feature of that JVM version… –  Holger Commented May 31, 2017 at 14:26
• 3 One thing to keep in mind when drawing this conclusion, is, that this only applies if the concepts of that other language (i.e. references) are compiled 1:1 to the equivalent byte code artifacts. E.g. a reference of a particular language might be so far away from what Java’s object references do, that it doesn’t match a sole byte code reference variable, but, e.g. a compound data structure. For Java, it is impossible to encounter an object reference having a not-yet-initialized vtable, but that doesn’t have to apply to another language, having an entirely different method dispatch algorithm. –  Holger Commented Jun 1, 2017 at 8:28

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