Pass by value and pass by reference in java program

The output shows that the field is changed in the calling method too. Here pv is the variable that stores the reference to that created object. When you call method in this statement — pv. Both pv and obj share the same reference. As shown in the image both variables pv and obj share the same memory reference so the change made to the field num using object obj is visible when num field is retrieved using pv object.

Here you can see that a new object is created and assigned to the obj parameter in the displayValue method. Related Posts. If something is missing or you have something to share about the topic please write a comment. Your email address will not be published. If we made changes to the parameter's instance member, it would affect the original value.

Java does not support pass by reference concept. The fundamental concept for passing the parameters in modern programming languages is passing by value and passing by reference. But, in Java, the pass by reference concept is degraded.

It supports only the pass by value concept. The primitive variables hold the actual values, whereas the non-primitive variables hold the reference variable. However, both variables use stack memory to store the values.

See more about data types in Java. In Java, during the method invokation, a copy of each argument is created then passed to the method. In the case of primitive data types, it copies the value inside stack memory then pass it to the method.

In the case of non-primitive data types, it points a reference in stack memory to actual data, which occurs in a heap. When we pass an object, it will copy a reference from the stack memory and pass it to the callee method. From the above output, we can see that the swap method did not work.

It did not work because Java is pass by value, and, here, we are passing the reference of the object. So it is clear that Java does not support pass by reference.

Explanation: In the above program, when we create an instance of the class Bike using the new operator, the instance of the class is created, and the variable holds the reference of the memory where the object is saved. While calling the swap method, we have created two new variables o1 and o2, which are pointing to the memory location of the apache and pulsar variable. Below is the swap method implementation in the above program:.

As we can see from the above code snippet, the values of o1 and o2 are changed. They are copies of the apache and pulsar reference locations.

So, it did not change the values of apache and pulsar in the output. Passing the parameters by values does not affect the original variable. Below is the example of Passing by Value:. As we can see from the above output, the original values is not affected by the pass by value mechanism.

JavaTpoint offers too many high quality services. Mail us on [email protected] , to get more information about given services. Strict pass-by-value is also useless, it would mean that a Mbyte array should have to be copied every time we call a method with the array as argument, therefore Java cannot be stricly pass-by-value.

Every language would pass a reference to this huge array as a value and either employs copy-on-write mechanism if that array can be changed locally inside the method or allows the method as Java does to modify the array globally from the caller's view and a few languages allows to modify the Value of the reference itself. So in short and in Java's own terminology, Java is pass-by-value where value can be: either a real value or a value that is a representation of a reference.

As far as I know, Java only knows call by value. This means for primitive datatypes you will work with an copy and for objects you will work with an copy of the reference to the objects. However I think there are some pitfalls; for example, this will not work:. This will populate Hello World and not World Hello because in the swap function you use copys which have no impact on the references in the main. But if your objects are not immutable you can change it for example:. This will populate Hello World on the command line.

For example:. However you could make a wrapper for String like this which would make it able to use it with Strings:. Java arguments are all passed by value the value or reference is copied when used by the method :.

In the case of primitive types, Java behaviour is simple: The value is copied in another instance of the primitive type. The behaviour can appear different from primitive types: Because the copied object-variable contains the same address to the same Object.

In effect, using a method, you will never be able, to update the value of a String passed as argument:. A String Object, holds characters by an array declared final that can't be modified. Only the address of the Object might be replaced by another using "new". Using "new" to update the variable, will not let the Object be accessed from outside, since the variable was initially passed by value and copied. Let me try to explain my understanding with the help of four examples.

Java is pass-by-value, and not pass-by-reference. In Java, all parameters are passed by value, i. Some people say primitive types and 'String' are 'pass by value' and objects are 'pass by reference'. But from this example, we can understand that it is infact pass by value only, keeping in mind that here we are passing the reference as the value. That's why are able to change and still it holds true after the local scope.

But we cannot change the actual reference outside the original scope. Note: I am not pasting the code for private class Student. The class definition for Student is same as Example3. You can never pass by reference in Java, and one of the ways that is obvious is when you want to return more than one value from a method call. Sometimes you want to use the same pattern in Java, but you can't; at least not directly.

Instead you could do something like this:. As was explained in previous answers, in Java you're passing a pointer to the array as a value into getValues. That is enough, because the method then modifies the array element, and by convention you're expecting element 0 to contain the return value.

Obviously you can do this in other ways, such as structuring your code so this isn't necessary, or constructing a class that can contain the return value or allow it to be set.

First, What's the difference between passing by reference vs. Passing by reference means the called functions' parameter will be the same as the callers' passed argument not the value, but the identity.

Pass by value means the called functions' parameter will be a copy of the callers' passed argument. Or from wikipedia, on the subject of pass-by-reference.

In call-by-reference evaluation also referred to as pass-by-reference , a function receives an implicit reference to a variable used as argument, rather than a copy of its value.

This typically means that the function can modify i. And on the subject of pass-by-value. In call-by-value, the argument expression is evaluated, and the resulting value is bound to the corresponding variable in the function [ If the function or procedure is able to assign values to its parameters, only its local copy is assigned [ Second, we need to know what Java uses in its method invocations.

The Java Language Specification states. Let's consider reference types, the Java Virtual Machine Specification states. There are three kinds of reference types : class types, array types, and interface types.

Their values are references to dynamically created class instances, arrays, or class instances or arrays that implement interfaces, respectively. The Java Language Specification also states. The reference values often just references are pointers to these objects , and a special null reference, which refers to no object. The value of an argument of some reference type is a pointer to an object.

Note that a variable, an invocation of a method with a reference type return type, and an instance creation expression new This is exactly what the definition of pass-by-value describes. As such, Java is pass-by-value. The fact that you can follow the reference to invoke a method or access a field of the referenced object is completely irrelevant to the conversation.

The definition of pass-by-reference was. In Java, modifying the variable means reassigning it. In Java, if you reassigned the variable within the method, it would go unnoticed to the caller. Modifying the object referenced by the variable is a different concept entirely. Primitive values are also defined in the Java Virtual Machine Specification, here. The value of the type is the corresponding integral or floating point value, encoded appropriately 8, 16, 32, 64, etc.

The distinction, or perhaps just the way I remember as I used to be under the same impression as the original poster is this: Java is always pass by value. All objects in Java, anything except for primitives in Java are references. These references are passed by value. As many people mentioned it before, Java is always pass-by-value. Here is another example that will help you understand the difference the classic swap example :. This happens because iA and iB are new local reference variables that have the same value of the passed references they point to a and b respectively.

So, trying to change the references of iA or iB will only change in the local scope and not outside of this method. I always think of it as "pass by copy". It is a copy of the value be it primitive or reference. If it is a primitive it is a copy of the bits that are the value and if it is an Object it is a copy of the reference.

Unlike some other languages, Java does not allow you to choose between pass-by-value and pass-by-reference—all arguments are passed by value. A method call can pass two types of values to a method—copies of primitive values e. When a method modifies a primitive-type parameter, changes to the parameter have no effect on the original argument value in the calling method.

When it comes to objects, objects themselves cannot be passed to methods. So we pass the reference address of the object. We can manipulate the original object using this reference. Let's analyze the following statement. The constructor to the right of keyword new which creates the object is called implicitly by the keyword new.

Address of the created object result of right value, which is an expression called "class instance creation expression" is assigned to the left value which is a reference variable with a name and a type specified using the assign operator. Since the reference stored in the parameter is a copy of the reference that was passed as an argument, the parameter in the called method and the argument in the calling method refer to the same object in memory.

Passing references to arrays, instead of the array objects themselves, makes sense for performance reasons. Because everything in Java is passed by value, if array objects were passed, a copy of each element would be passed. For large arrays, this would waste time and consume considerable storage for the copies of the elements. Primitive and reference variables are kept in stack memory left side in images below.

If we pass the value of array1 reference variable as an argument to the reverseArray method, a reference variable is created in the method and that reference variable starts pointing to the same array a. We have another reference variable in reverseArray method array2 that points to an array c. If we were to say. If we return value of reference variable array2 as the return value of method reverseArray and assign this value to reference variable array1 in main method, array1 in main will start pointing to array c.

And now that reverseArray method is over, its reference variables array1 and array2 are gone. Which means we now only have the two reference variables in main method array1 and array2 which point to c and b arrays respectively.

No reference variable is pointing to object array a. So it is eligible for garbage collection. To make a long story short, Java objects have some very peculiar properties. In general, Java has primitive types int , bool , char , double , etc that are passed directly by value.

Then Java has objects everything that derives from java. Objects are actually always handled through a reference a reference being a pointer that you can't touch. That means that in effect, objects are passed by reference, as the references are normally not interesting. It does however mean that you cannot change which object is pointed to as the reference itself is passed by value. Does this sound strange and confusing?

Let's consider how C implements pass by reference and pass by value. In C, the default convention is pass by value. I have created a thread devoted to these kind of questions for any programming languages here. Java is also mentioned. Here is the short summary:. Stack Overflow for Teams — Collaborate and share knowledge with a private group.

Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. Is Java "pass-by-reference" or "pass-by-value"? Ask Question. Asked 13 years, 4 months ago. Active 7 days ago. Viewed 2. I always thought Java uses pass-by-reference. I don't think I understand the distinction they're making. What is the explanation? Improve this question. We would more commonly say that a variable "passed-by-reference" can be mutated. The term appears in textbooks because language theorists needed a way to distinguish how you treat primitive data types int, bool, byte from complex and structured objects array, streams, class -- that is to say, those of possibly unbounded memory allocation.

I want to note that you do not have to think about this in most cases. Until this point in time i had no clue what pass-by-reference and pass-by-value are. The intuitive solution always worked for me, which is why java is one of the best languages for beginners.

So if you currently are worried, if your function needs a reference or a value, just pass it as it is and you will be fine. Java pass the reference by value. However, passing is always be value. So for all non-primitive types reference is passed by its value. All primitive types are also passed by value.

Consider, there is also an object! Show 4 more comments. Active Oldest Votes. Improve this answer. Does it cease to exist, was it never created, or does it exist in the heap but without a reference variable in the stack? To me, saying that an object's reference is passed by value is the same as saying that the object is passed by reference. I'm a Java novice, but I presume that in contrast primitive data is pass by value. Did you work through the example with Fifi and look carefully through the results?

Check that indeed foo aDog ; did not change aDog despite foo overwriting the value of d , showing that indeed all inputs to a function are passed by value.

To pass an object by reference would mean that if the function modifies the variable then it modifies the object itself. Show 28 more comments. The key to understanding this is that something like Dog myDog; is not a Dog; it's actually a pointer to a Dog. Let's say he's at address 74 we assign the parameter someDog to 74 at line "CCC" someDog is followed to the Dog it points to the Dog object at address 74 that Dog the one at address 74 is asked to change his name to Rowlf then, we return Now let's think about what happens outside the method: Did myDog change?

There's the key. The only pointer to the new dog at 74 I assume you meant 74 rather than 72 is the parameter to the foo function. When foo returns, all of its parameters are popped off the stack, so nothing is left pointing to 72 and it can be garbage collected. I say "mostly" as there is no "revert" happening; the pointer myDog in the caller was pointing to 42 all along and never changed, no matter what happened in the function, hence, no "revert".