Java - Custom class ArrayList has no add method

Question

ArrayList of custom class has no .add() method:

I can define an ArrayList of Object's:

ArrayList<Object> thing = new ArrayList<Object>();


thing.add(otherThing); // works

However, when I define a list of a custom class Thing:

ArrayList<Thing> thing = new ArrayList<Thing>();


thing.add(otherThing); // error


Canvas.java:33: cannot find symbol
symbol  : method add(java.lang.Object)
location: class java.util.ArrayList<Thing>
            thing.add(otherThing);
                   ^
1 error

Is this possible?

Thanks

How is otherThing declared?

Mark Peters
– Mark Peters

2013-04-23 03:39:48 +00:00
Commented Apr 23, 2013 at 3:39 — Mark Peters
– Mark Peters, Commented Apr 23, 2013 at 3:39

Rahul · Accepted Answer · 2013-04-23 03:39:36Z

7

Your otherThing must be of the type Thing. Currently its of the type Object, and that's why it worked for the first case, but failed in the second case.

In your first case, the ArrayList<Object> takes elements of type Object. Since the otherThing is of the type Object too, and so it works.

In the second case, the ArrayList<Thing> takes elements of type Thing. Since, your otherThing is still of the type Object, whereas it should have been of the type Thing, you get that error.

answered Apr 23, 2013 at 3:39

Rahul

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Comments

Alpesh Gediya · Accepted Answer · 2013-04-23 03:44:49Z

0

ArrayList<Thing> thing = new ArrayList<Thing>();

For this you can only add types of/instances of Thing only other than that is not allowed as its violates Java Generic guideline.

ArrayList<Object> thing = new ArrayList<Object>();

as here you are specifiying Object and as Object is super class it will work fine.

answered Apr 23, 2013 at 3:44

Alpesh Gediya

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Michael 'Maik' Ardan · Accepted Answer · 2013-04-23 03:50:14Z

0

otherThing is not declared as Thing but an Object.

answered Apr 23, 2013 at 3:50

Michael 'Maik' Ardan

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