Does ArrayList.get() return the same thing as array[]?

Question

In Processing, I have an ArrayList of objects built from a custom class. When I use the .get() function to return one of the objects, it seems to return the object all right -- but I can't access any of the object's variables or methods. I get the error message "[variable] cannot be resolved or is not a field." Is this a bug, or am I doing something wrong?

Here's a sample. Notice the values returned from the setup() function.

// regular array
Thing[] thinglist1 = new Thing[1];

// ArrayList array
ArrayList thinglist2 = new ArrayList<Thing>(1);

// instantiate the class
Thing thing = new Thing(12345);

// class definition
class Thing {
  int var;

  Thing(int i){
    var = i;
    thinglist1[0] = this;
    thinglist2.add(this);
  };
};

// run it!
void setup(){
  println(thinglist1[0] == thinglist2.get(0));
  // true

  println(thinglist1[0].var);
  // 12345

  println(thinglist2.get(0).var);
  // ERROR: "var cannot be resolved or is not a field"
};

related: What is a raw type and why shouldn't we use it?

Paul Bellora
– Paul Bellora

2013-06-23 04:37:22 +00:00
Commented Jun 23, 2013 at 4:37 — Paul Bellora
– Paul Bellora, Commented Jun 23, 2013 at 4:37

Brian Roach · Accepted Answer · 2013-06-23 04:31:52Z

9

You've kinda messed up your generics.

Change

ArrayList thinglist2 = new ArrayList<Thing>(1);

to:

ArrayList<Thing> thinglist2 = new ArrayList<Thing>(1);

Because you didn't specify a type, what you really had was:

ArrayList<? extends Object> thinglist2 = new ArrayList<Thing>(1);

So when you retrieved an item from it using get, it was typed as Object rather than your Thing

Edit to add: The reason for this is legacy; when generics were introduced things were put in place for backward compatibility. Unfortunately that creates situations like this which is confusing to someone new to Java.

You'd prob expect a compiler warning or error, but Java silently changes that non-generic-typed ArrayList into "Arraylist that contains something that extends Object" ... which is anything (except a primitive), because all objects implicitly extend Object

edited Jun 23, 2013 at 4:31

answered Jun 23, 2013 at 4:08

Brian Roach

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4 Comments

Paul Bellora Over a year ago

I would just say "what you really had was: ArrayList<Object> ..." - since it's more accurate and adding wildcards to the mix could be confusing.

Brian Roach Over a year ago

@PaulBellora - already fixed that in an edit - refresh ;) I think stating what's really happening is more accurate, and then explaining why.

Paul Bellora Over a year ago

Right, I'm saying I would change it back.

Brian Roach Over a year ago

I understand your point, I just disagree that posting something incorrect is "more accurate". "Easier to understand" perhaps, but certainly not accurate.

Vahid · Accepted Answer · 2013-06-23 04:17:39Z

-2

In the last line you must cast return value from get function to the array of object.

System.out.println(((Thing[])thinglist2.get(0))[0].var);

answered Jun 23, 2013 at 4:17

Vahid

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