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Basically i want to add students to a class list. Assuming i have the following code

public class ClassList {

    //Constructor methods...


    private Student [] studList = new Student [20];

    public boolean addStudent (Student newStudent)
    {

        studList[14] = newStudent;
    }
}

Does studList[14] add a reference to newStudent object or copies that object into the studList[14] student object?

As far as i understand newStudent object will get deleted when the method addStudent() is called a second time. So studList[14] will point to null then? What if i want studList[14] to persist throughout the code execution?

Sorry if it is hard to understand, i do not know how to explain my query easier...

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  • What you have there is an array of students for which you'd need to know the index of the element that you're currently adding. If you want a list you should check the specific List implementations: docs.oracle.com/javase/7/docs/api/java/util/List.html Commented Oct 31, 2013 at 8:19

3 Answers 3

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There's one fundamental rule in Java that you have to wrap your head around:

The only way you can access an object is via its reference. And the only values that variables can hold are references(*). That's true for local variables, parameters, instance fields and static fields: they all are the same in this respect.

The object itself is never "contained" in a variable.

This directly leads to the answer to your first question:

The reference is copied into the array (as an array can only hold references(*), never objects).

This also mean that "newStudent being deleted" is not actually a big deal: it's just another reference to the same object, and if it goes away nothing much happens.

Now, if all references to a given object are removed (or no longer reachable), then the object itself becomes eligible for garbage collection, but that's not a bad thing, because you could not access it anymore anyway.

(*) ... or primitive values, but we'll ignore those for this dicussion.

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2 Comments

"it's just another reference to the same object, and if it goes away nothing much happens." So is it correct to say, the more and more i call addStudent, the more and more memory gets allocated (provided i keep adding new references to the data)?Since the actual data doesnt get deleted...
@kype: no. Read the last paragraph. If there is no other reference to the Student object it will eventually be garbage collected. And since you store all students in the same slot of the array, the next one will just overwrite the reference to the current one. In general: memory usage increases if you create new objects and hold on to them with some reachable reference. Just creating objects and discarding references to them can slightly increase memory usage in the short run (i.e. until GC cleans that mess up), but will not do so in the long run.
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The assignment doesn't copy the object. It just adds a reference to the object into the array.

An object gets deleted by the garbage collector after there are no more references to it anywhere. You don't need to worry too much about this process, because it's kind of invisible most of the time; and once there are no references to an object, you couldn't have used it anyway.

Perhaps you want to be able to pass in an int to your method, to tell it which entry in the array to set, instead of always setting entry number 14?

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So when i do this studList[14] = newStudent; im just adding a new reference, and thus the data will be preserved beyond the method call because i have this new reference that doesnt get deleted?
Yes, that's right. Unless you go and overwrite the reference later, of course.
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You should have a look at the JLS about types, values and variables:

The values of a reference type are references to objects.

If there is no remaining reference (aside weak ones) to an object it will be garbage-collected out of the heap.

If you call two times: addStudent(new Student()); the first Student object created is qualified for GC since you have no other reference variable "pointing" to the object.

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