Java: Pass by Value

Alright alright alright. So, here is the earth. (Round)

For those of you mingling in multiple languages it is hard to keep it all straight. Are parameters passed into functions pass-by-value? or pass-by-reference?

Let's get straight to the point. We are talking about Java here, and Java is pass by value. But I think that people are confused about what pass by value in java really means.

Let's look at an example.

public void passByValue(MyObject oParam){
    oParam.myAttribute = 10;
}

called by:

public static void main(String [] args)
{
    MyObject oMain = new MyObject();
    oMain.myAttribute = 5;
    passByValue(oMain);
    system.out.print(oMain.myAttribute)
}


What is the result of the system.out.print line above? If its pass by value the value should be 5, right? WRONG. 

Even though Java is pass by value, this common misconception gets a lot of people. Java passes object references by value, not the objects themselves. What do I mean by that? Well let's look at how a reference works conceptually.

This is what we get when we do "MyObject oMain = new MyObject();"

Now, when we pass in "oMain" as a parameter to the function "passByValue", the value of the reference is copied to the new parameter "oParam":

Now you can see that any modifications to the "oParam" reference, is acting on the same object that the "oMain" reference is pointing to. 

Things get interesting when we introduce the "new" operator into the discussion. The "new" operator allocates a new instance of an object, and sets the reference to point to that object. So say we rewrite our code above to:

public void passByValue(MyObject oParam){
    oParam = new MyObject();
    oParam.myAttribute = 10;
}

The result of the rewrite above will cause our original system.print line to print 5 instead of 10, since oParam changes the myAttribute of a new object instance other than the one passed in. Our diagram for this situation looks like this:

This leads into the discussion of "defensive copying". When we use the "new" operator we ensure that our method does not change the value of the object passed in. Of course, if you are passing in immutable objects, there is no need for this.

Defensive copying is often used when passing Lists.

public void passByValue(List<MyObject> list){
    list = new List<MyObject>(list); // clone list
    // do something with new list
}

POST vs PUT

Theodore came up to me the other day and said: "Hey John, I want to add this data to the database on the server, do I want to use HTTP POST? Or HTTP PUT?

Good question, Theo. Actually, either POST or PUT can be used to create or update data on the server, but they are different.

The details are elaberated on in the HTTP spec:

"The fundamental difference between the POST and PUT methods is
target resource in a POST request is intended to handle the enclosed
representation as a data-accepting process, such as for a gateway to
some other protocol or a document that accepts annotations. In
contrast, the target resource in a PUT request is intended to take
the enclosed representation as a new or replacement value."

So, to reiterate: POST means you are sending data to a resource that already exists. The resource (which you can look at as a java servlet or a script) processes the data and decides the implications to the online database. PUT means you want the data included with the request to be furthermore represented by the URL of the request. If there is data at the URL already, update it. If there is no data, create it.

"A successful PUT of a given representation would suggest that a subsequent GET on that same target resource will result in an equivalent representation being returned..."

Make sense?

A interesting inherited characteristic of PUT is that it is idiomatic. Which means that multiple requests to the same URL with the same data has no effect. This may come in handy when you lose your connection mid-request.

A hypothical example or using POST and PUT:

Let's say you want to add some data onto a message board website. You find an interesting forum topic, "Glaciers... why do they move so slow?" and want to post a comment. This will probably be done with http POST.

www.glacierforums.com/addCommentToforum?id=235434

Send a request to this URL with a message, and the resource located at "www.glacierforums.com/addCommentToforum" will take the input of "?id=235434" along with your message sent in the request and deal with making sure that this message is added on to the end of forum page.

Now for the same example, you decide that your profile alias "DarthGlacier" is dumb. Nobody references Star Wars nowadays anyway. So you update your user info by sending your new user information (via JSON or whatnot) with a http PUT to this URL.

www.glacierforums.com/users/352343

Your user information, which is located at the URL: www.glacierforums.com/users/352343, has now been updated. A HTTP GET with this URL will return the same user information that you sent to it.

This example assumes you have assumed all authentication privileges needed to write data on to the server.