CODEWITHSUNDEEP
javaarraylist
kennycodes
kennycodes

Reputation: 536

ArrayList references to objects

I'm a little confused on what it means about ArrayLists holding references to objects. Can someone give me an example on how this is shown in code? Also, can arraylist have an element that is a reference to itself? Thanks!

Upvotes: 1

Views: 6045

Answers (2)

Sotirios Delimanolis
Sotirios Delimanolis

Reputation: 279910

You have to start making distinctions between variables, values, and objects.

Variables are containers. They contain values.

Values can be of two types, primitive and reference. Reference values are pointers to objects.

Objects are data structures which also have behavior.

So in

Object var = new Object();

var is a variable. new Object() is a new instance creation expression that evaluates to a value of type Object, that value is a reference to an object of type Object. That value is then assigned to var.

You can then use var to invoke a method

var.toString();

The runtime environment will evaluate var, which produces a reference value, retrieve the referenced object, and invoke the method. You can reassign the value stored in var by doing

var = someOtherValue;

Now var will hold a new value.

An ArrayList uses an array behind the scenes. An array is a special object where its elements (which you can think of as fields) are themselves variables.

So

Object[] arr = new Object[10];

is an array of type Object which contains ten variables. These variables contain values of type Object. Again, these values are references. You can evaluate them or you can reassign them.

arr[3].toString();
arr[7] = "maybe a string";
arr[9] = arr[9]; // possible, but makes no sense

Upvotes: 1

AJcodez
AJcodez

Reputation: 34156

It means instead of copying the object byte-for-byte, a reference to the location of memory where the object is stored is put in the list. 

List<Object> objects = new ArrayList<Object>();
Object myObject = new Object();
objects.add(myObject);
// objects contains one reference to myObject

objects.get(0).modify();
// myObject modified

Note that primitive values (int for example) will be copied.

Upvotes: 0

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