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Writing code in comment? I will use example from this site :. Creating two different Point object with two different reference associated.
References pnt1 and pnt2 are passed by value to the tricky method, which means that now yours references pnt1 and pnt2 have their copies named arg1 and arg2. So pnt1 and arg1 points to the same object. Same for the pnt2 and arg2. Here, you first create new temp Point reference which will point on same place like arg1 reference. Then you move reference arg1 to point to the same place like arg2 reference.
Finally arg2 will point to the same place like temp. From here scope of tricky method is gone and you don't have access any more to the references: arg1 , arg2 , temp. But important note is that everything you do with these references when they are 'in life' will permanently affect object on which they are point to. So after executing method tricky , when you return to main , you have this situation:.
Pass by value means that you are making a copy in memory of the actual parameter's value that is passed in. This is a copy of the contents of the actual parameter. Pass by reference also called pass by address means that a copy of the address of the actual parameter is stored.
Sometimes Java can give the illusion of pass by reference. Let's see how it works by using the example below:. As we all know it will create an object in the heap and return the reference value back to t. For example, suppose the value of t is 0x we don't know the actual JVM internal value, this is just an example. When passing reference t to the function it will not directly pass the actual reference value of object test, but it will create a copy of t and then pass it to the function.
Since it is passing by value , it passes a copy of the variable rather than the actual reference of it.
Since we said the value of t was 0x , both t and f will have the same value and hence they will point to the same object. If you change anything in the function using reference f it will modify the existing contents of the object. That is why we got the output changevalue , which is updated in the function. Will this throw a NullPointerException? No, because it only passes a copy of the reference. In the case of passing by reference, it could have thrown a NullPointerException , as seen below:.
There are already great answers that cover this. I'm posting the same example in pascal because I think pass-by-reference looks cleaner in pascal, but I could be wrong. I might just be confusing people more; I hope not. In pascal, parameters passed-by-reference are called "var parameters". In the procedure setToNil below, please note the keyword 'var' which precedes the parameter 'ptr'.
When a pointer is passed to this procedure, it will be passed by reference. Note the behavior: when this procedure sets ptr to nil that's pascal speak for NULL , it will set the argument to nil--you can't do that in Java.
All parameters to methods are passed "by value". In other words, values of parameter variables in a method are copies of the invoker specified as arguments. You should note that when the parameter is an object reference, it is the object reference-not the object itself-that is passed "by value". And towards the end of the same section he makes a broader statement about java being only pass by value and never pass by reference.
The Java programming language does not pass objects by reference; it passes object references by value. Because two copies of the same reference refer to the same actual object, changes made through one reference variable are visible through the other.
There is exactly one parameter passing mode- pass by value -and that helps keep things simple. This section of the book has a great explanation of parameter passing in Java and of the distinction between pass-by-reference and pass-by-value and it's by the creator of Java. I would encourage anyone to read it, especially if you're still not convinced. I think the difference between the two models is very subtle and unless you've done programming where you actually used pass-by-reference, it's easy to miss where two models differ.
I might be a little obsessed with this post. Probably because I feel that the makers of Java inadvertently spread misinformation. If instead of using the word "reference" for pointers they had used something else, say dingleberry, there would've been no problem. You could say, "Java passes dingleberries by value and not by reference", and nobody would be confused. That's the reason only Java developers have issue with this. They look at the word "reference" and think they know exactly what that means, so they don't even bother to consider the opposing argument.
Anyway, I noticed a comment in an older post, which made a balloon analogy which I really liked. So much so that I decided to glue together some clip-art to make a set of cartoons to illustrate the point. Passing a reference by value --Changes to the reference are not reflected in the caller's scope, but the changes to the object are. This is because the reference is copied, but the both the original and the copy refer to the same object.
Pass by reference --There is no copy of the reference. Single reference is shared by both the caller and the function being called. Any changes to the reference or the Object's data are reflected in the caller's scope. I have seen posts on this topic which describe the low level implementation of parameter passing in Java, which I think is great and very helpful because it makes an abstract idea concrete.
However, to me the question is more about the behavior described in the language specification than about the technical implementation of the behavior.
This is an exerpt from the Java Language Specification, section 8. The Identifier that appears in the DeclaratorId may be used as a simple name in the body of the method or constructor to refer to the formal parameter. Which means, java creates a copy of the passed parameters before executing a method. It has a good description of "Call-by-value" and "Call-by-Reference" in Chapter 1. The Call-by-value description matches up with Java Specs exactly. Back when I studied compilers-in the 90's, I used the first edition of the book from which pre-dated Java by about 9 or 10 years.
However, I just ran across a copy of the 2nd Eddition from which actually mentions Java! Section 1. Here is an excerpt under the heading "Call-by-value" which mentions Java:. In call-by-value, the actual parameter is evaluated if it is an expression or copied if it is a variable. The value is placed in the location belonging to the corresponding formal parameter of the called procedure. Primitive data types itself and object references are stored in the stack.
Objects themselves are stored in the heap. If it's a primitive data type then these copied bits contain the value of the primitive data type itself, That's why when we change the value of argument inside the method then it does not reflect the changes outside. So for better understanding make shortcut file and feel. Getting an outside of the box view, let's look at Assembly or some low level memory management.
At the CPU level a reference to anything immediately becomes a value if it gets written to memory or to one of the CPU registers. That is why pointer is a good definition. It is a value, which has a purpose at the same time. Data in memory has a Location and at that location there is a value byte,word, whatever.
In Assembly we have a convenient solution to give a Name to certain Location aka variable , but when compiling the code, the assembler simply replaces Name with the designated location just like your browser replaces domain names with IP addresses. Down to the core it is technically impossible to pass a reference to anything in any language without representing it when it immediately becomes a value. Lets say we have a variable Foo, its Location is at the 47th byte in memory and its Value is 5.
We have another variable Ref2Foo which is at rd byte in memory, and its value will be This Ref2Foo might be a technical variable, not explicitly created by the program. If you just look at 5 and 47 without any other information, you will see just two Values.
If you use them as references then to reach to 5 we have to travel:. In every cases above a value - a copy of an existing value - has been created, it is now upto the receiving method to handle it. This is hidden from the developer until she circumvents the dereferencing process.
So a reference is a value when represented, because a reference is a value that has to be processed at language level. Nitpicking on insignificant details, even languages that do pass-by-reference will pass values to functions, but those functions know that they have to use it for dereferencing purposes.
This pass-the-reference-as-value is just hidden from the programmer because it is practically useless and the terminology is only pass-by-reference. 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:. 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. In this case, modifying the value of the parameter will change the value. So, why does it appear that Java is mostly pass-by-reference, but books say that Java is always pass-by-value? It is in the nuance of conceptualizing what Java does and how best to understand that at this stage in our discussions.
We will then look at an adoption method that changes the name of a given cat:.
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