2D array of multiple different objects

Question

For my project, I need a 2D array that can hold multiple different object types. The problem with java is that it doesn't let you do that. You can only have an array of a particular object type.

Now don't get me wrong, but I did some research, and one proposed solution is to use an Object array, since every class in java extends the object class by default. For example:

Object[][] someArray = new Object[5][5];
someArray[1][2] = new Cat();
someArray[3][4] = new Dog();

The problem with this is that the object class, being a superclass, cannot access the methods and fields of the subclasses that extend it. So if I have a public eat() method in my cat or dog class, the elements in someArray wont be able to access that method.

So I'm at a dead-end here. Any help is appreciated. Thanks in advance.

if object is not a solutionthen use inheritance and interfaces.... — ΦXocę 웃 Пepeúpa ツ
– ΦXocę 웃 Пepeúpa ツ, Commented Jun 14, 2017 at 3:58
Use instanceof and casting to check the type of a given Object and and to convert it to that type accordingly. — xunatai
– xunatai, Commented Jun 14, 2017 at 4:00
Why do you want a 2D array filled with different objects? You will need to cast to get to their specific methods. Maybe you should look for a common interface besides Object. — matt
– matt, Commented Jun 14, 2017 at 5:58

Rapid Readers · Accepted Answer · 2017-06-14 04:32:38Z

1

[ please don't judge me for answering my own question ]

One possible solution is using 2D ArrayLists. The way a 2D Array works is by literally having a 1D array consisting of 1D arrays.

Maybe the same concept can be applied to ArrayLists, which can in fact store any object types without having accessibility issues. A 5x5 ArrayList can be created as follows:

ArrayList<ArrayList> someArray = new ArrayList<ArrayList>();
for(int x=0 ; x<5 ; x++)
{
    someArray.add(new ArrayList());
    for(int y=0 ; y<5 ; y++)
        someArray.get(x).add(null);
}

and to set row 1 col 2 to a cat object:

someArray.get(1).set(2, new Cat());
someArray.get(3).set(4, new Dog());

So 2D ArrayLists, though a bit confusing might solve this issue pretty efficiently.

answered Jun 14, 2017 at 4:32

Rapid Readers

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2 Comments

user7627726 Over a year ago

You're not being judged. in fact, it is encouraged to answer your question if you find out what the problem is. I'm glad you figured it out. +1 :)

matt Over a year ago

While it is better to use List this doesn't really solve your problem.

kuro · Accepted Answer · 2017-06-14 05:29:04Z

1

to call the eat() method of the Dog class and `Cat`` class, You can cast the array reference to a subclass reference.((Dog)someArray[x][y]).eat();

public class Dog{

    public Dog(){
        super();
    }

    public String eat(){
      String str = "yum";
      return str;
    }

    public static void main (String[]args){
        Object [][] arr = new Object [2] [2];
        for(int i = 0; i < arr.length; i++){
            for(int j = 0; j < arr[i].length; j++){

               arr[i][j] = new Dog(); 
            }
        }
       System.out.println(((Dog)arr[1][1]).eat());
    }
}

edited Jun 14, 2017 at 5:29

kuro

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answered Jun 14, 2017 at 4:39

Caojinjin008

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1 Comment

Rapid Readers Over a year ago

ClassCastException is thrown. You can't cast superclass object to its subclass.

Rafael Coelho · Accepted Answer · 2017-06-14 14:03:16Z

0

The ideal use in your case is to create a superclass called Animal that is extended by dog and cat. Your ArrayList or array will be of Animal type.

answered Jun 14, 2017 at 14:03

Rafael Coelho

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Comments

CJ Burkey · Accepted Answer · 2017-06-14 04:04:57Z

How different do these objects need to be? If they're your own classes, then just create some sort of superclass that they all extend and use that as the array type. If you don't want to do that, I guess you could set something up like this:

private Object[][] array = new Object[5][5];

public void setValue(int x, int y, Object val) {
    if(x > 0 && x < array.length && y > 0 && y < array[0].length) {
        array[x][y] = val;
    }
}

public Object getValue(int x, int y) {
    if(x > 0 && x < array.length && y > 0 && y < array[0].length) {
        return array[x][y];
    }
    return null;
}

Using it would be a little tedious, but you could cast each call to getValue() to the type you needed it to be. With Object Oriented programming, having a simple solution to this can be kind of difficult. I could write some more advanced code that would return the object casted to a certain class, but it's a little bit of work. Let me know if you'd like that.

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