Java ArrayList - can't read back properties of objects

Question

I want to be able to make an array list of objects, and then be able to edit and read back some simple properties of each object. In this case, a String (colour) and integer (X). I can't seem to make the simple code below work. Note that I am aware people sometimes use the <> notation with array lists, but I have read that it should be possible without, and at the moment I am very new to Java and wish to keep things as simple as possible.

At the moment I get an error on the line which is commented out (cannot find symbol X)

import java.util.ArrayList;

public class ArrayList_of_Objects {

 public static void main(String[] args)
    {

        ArrayList al = new ArrayList();

        for(int i =0;i<5;i++)
        {
            MyObj ob1 = new MyObj();
            ob1.X = i + 5;
            al.add(ob1);
            //System.out.println("X: "+al.get(i).X);
        }
        for(int j=0;j<5;j++)
        {System.out.println("X: "+al.get(j).X);}

        al.get(3).X=4;
        al.get(3).colour="orange";
        System.out.println(al.get(3).X);
        System.out.println(al.get(3).colour);
  } 

}

class MyObj
{
    int X;
    String colour;
}

Jon Skeet · Accepted Answer · 2013-04-06 07:12:01Z

1

Yes, you need to use generics to say what kind of object will be in your list:

ArrayList<MyObj> al = new ArrayList<MyObj>();

Otherwise the compiler doesn't know that al.get(3) is meant to return a MyObj rather than just anything. With this code, it will not only fix the get calls - it will stop you from accidentally adding objects of inappropriate types to the list.

Also, it's usually preferred to declare variables as an appropriate interface type, such as:

List<MyObj> al = new ArrayList<MyObj>();

(There are times where you really need it to be the concrete type, but generally prefer programming to interfaces.)

Generics is a massive topic, but the tutorial linked at the start of this answer should get you going. The Java Generics FAQ is a great resource for finding out more.

answered Apr 6, 2013 at 7:12

Jon Skeet

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3 Comments

user2094585 Over a year ago

Many thanks for your answer. Does this mean then that it is not possible to achieve my goal without the generics <>? I guess I will have to read up on them, but if there is a simpler way in the mean time, then I would be interested to know !

Jon Skeet Over a year ago

@user2094585: Well you could cast, or use an array instead - but it would be best to learn generics. They're a core part of modern Java.

user2094585 Over a year ago

OK Jon, I will have a look at both. Thanks.

NPE · Accepted Answer · 2013-04-06 07:12:03Z

0

Change

    ArrayList al = new ArrayList();

to

    ArrayList<MyObj> al = new ArrayList<MyObj>();

What you have right now is an array list of Object rather than an array list of MyObj, and Object has no knowledge of MyObj.X.

answered Apr 6, 2013 at 7:12

NPE

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Comments

BlackJoker · Accepted Answer · 2013-04-06 07:13:11Z

0

Use ArrayList<MyObj> al = new ArrayList<MyObj>(); instead

answered Apr 6, 2013 at 7:13

BlackJoker

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1 Comment

Cyril Deba Over a year ago

use List<MyObj> al = new ArrayList<MyObj>(); instead

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