Java Community | Help. Code. Learn.•9mo ago

Understanding the three steps of object declaration, creation, and assignment (easy)

I am trying to understand the three steps of object declaration, creation and assignment, and want to know if what I wrote about the picture is correct (from Head First Java, but edited text below to use my own words and see if I can explain it properly): 1) Declare a reference variable Dog myDog Here we tell JVM to make space for a reference variable. This reference can only EVER be a type Dog from now on. myDog is the name we give the reference variable, to make it more readable and usable for us. Beneath myDog is actually a bunch of bits representing a way to get to the actual object on the heap (e.g. @Dog1234 fx). NOT SURE if this is true 2) Create an object new Dog(); Here we tell JVM to make space for a new Object instance on the Heap. Its the actual Dog object that is stored somewhere in the heap by a certain address (e.g. @Dog1234). 3) Link the object and reference = Last part is to assign the new Dog() we created on the heap to the reference variable myDog. In other words, we make sure that the reference variable we call myDog is connected to the actual new Dog() we created, and therefore are able to control the object on the heap through this reference variable myDog.

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JavaBot•9mo ago

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Unknown User•9mo ago

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SteadhavenOP•9mo ago

This is also a thing I want to ask about sometimes I see something like List<String> animals = new ArrayList<>(); (or something like that, if my syntax is wrong) I don't understand what it exactly means. Is the left side more important than the right? Is it a List, or is it an ArrayList? I heard a guy say something like "Oh we don't need to write ArrayList<String> animals = new ArrayList<>(); but just List<String> animals = new ArrayList<>(); since we don't need something from the ArrayList... Something like that. I don't understand this topic at all.

Unknown User•9mo ago

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SteadhavenOP•9mo ago

Yea apparently @Dog1234 is just the toString sorry

Unknown User•9mo ago

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SteadhavenOP•9mo ago

Ok thanks

List<String> animals = new ArrayList<>();
//             ^type List
animals = new LinkedList<>(); //not sure this is how you create LinkedList
// ^still type List

List<String> animals = new ArrayList<>();
//             ^type List
animals = new LinkedList<>(); //not sure this is how you create LinkedList
// ^still type List

We cannot use any method/field that is exclusive to either ArrayList or LinkedList in the above example. However, the benefit has to be that whatever method/field they have in common with List itself, will be different in their implementation because they can override it. So by changing the instantiation from ArrayList to LinkedList on the Right Hand Side, we can still only use the same methods/fields as before, but now they might behave a bit differently. Is this correct? Or does it not change anything?

Unknown User•9mo ago

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SteadhavenOP•9mo ago

So then later writing animals = new LinkedList<>(); does practically nothing?

Unknown User•9mo ago

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SteadhavenOP•9mo ago

Aha, so the thing that confuses me here, is that I don't see what the difference is between:

//difference between
List<String> animals1 = new ArrayList<>();
List<String> animals2  = new LinkedList<>();

//simarly does this even do anything practically new:
List<String> animals = new ArrayList<>();
animals  = new LinkedList<>();

//difference between
List<String> animals1 = new ArrayList<>();
List<String> animals2  = new LinkedList<>();

//simarly does this even do anything practically new:
List<String> animals = new ArrayList<>();
animals  = new LinkedList<>();

Since its the left hand side that decides the type, and we said we want the behavior of a list so we made our type List. What difference then does the right hand side do?

Unknown User•9mo ago

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SteadhavenOP•9mo ago

yea but they are identitical right? SInce I said both are type List on the left hand side there should be no difference in their speed, or the method/fields available since its the left that decided all that they are both type List

Unknown User•9mo ago

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SteadhavenOP•9mo ago

No sorry, I miswrote

//difference between
List<String> animals1 = new ArrayList<>();
List<String> animals2  = new LinkedList<>();

//difference between
List<String> animals1 = new ArrayList<>();
List<String> animals2  = new LinkedList<>();

only speaking about these two they seem identical, since both are declared as List type on left hand side they are two distinct List of course, but I mean their type is identical and therefore they have similar kind of speed/big O, and method/field available regardless of the right side (new ArrayList or new LinkedList)

Unknown User•9mo ago

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SteadhavenOP•9mo ago

My confusion is just what the difference is between the left hand side type and the right hand side type.

Car myCar1         = new Lamborghini();
Lamborghini myCar2 = new Lamborghini();

Car myCar3         = new Prius();
Prius myCar4       = new Prius();

Car myCar5         = new Lamborghini();
myCar5             = new Prius();

Car myCar6         = new Prius();
myCar6             = new Lamborghini();

Car myCar1         = new Lamborghini();
Lamborghini myCar2 = new Lamborghini();

Car myCar3         = new Prius();
Prius myCar4       = new Prius();

Car myCar5         = new Lamborghini();
myCar5             = new Prius();

Car myCar6         = new Prius();
myCar6             = new Lamborghini();

Unknown User•9mo ago

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SteadhavenOP•9mo ago

Ok thanks for the clarification. Hopefully I finally understood the difference.

JavaBot•9mo ago

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Understanding the three steps of object declaration, creation, and assignment (easy)

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