Implementing List instead of ArrayList while using generics instead of raw types

Question

After going through many posts and suggestions, I have found that instead of using a concrete implementation such as ArrayList, I should use List instead, to allow flexibility between different implementations of the List Interface. So far, I have seen that many programmers suggest the following line of code:

List list = new ArrayList();

However, this would give a warning in the compiler for using the raw types List and ArrayList and that they should actually be parameterized.

Synonymous to these warnings, I have found several posts telling me that raw types should never be used and that I should take advantage in using generics that java offers so conveniently.

Personally, I am trying to implement a class that acts as a table requiring a 2 Dimensional List structure with ArrayLists being used internally. I am trying to implement the following lines of code:

List<List> table;
table = new ArrayList();
table.add(new ArrayList());

Envisioned in my head, the table structure should be able to hold multiple variable types such as the raw data types along with the String variable type. I have tried to implement generics such as using

List<List<Object>> table = new ArrayList<ArrayList<Object>>();

but I have received many errors and hence failed so far.

I am relatively new to programming, pursing a computer science major, so forgive me if I have any horrible misunderstanding of the lines of code that I have exemplified above.

Thank You.

You're still using raw types here List<List>. Read this which addresses your last snippet of code. — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Dec 16, 2014 at 14:48
this should be List<List<Object>> table = new ArrayList<List<Object>>(); and you're probably looking for List<List<?>> table = new ArrayList<List<?>>(); but this still just stores Object and not necessarily String or anything. — EpicPandaForce
– EpicPandaForce, Commented Dec 16, 2014 at 14:51

Konstantin Tarashchanskiy · Accepted Answer · 2014-12-16 14:52:36Z

2

You want to do this:

List<List<Foo>> table = new ArrayList<List<Foo>>();
table.add(new ArrayList<Foo>())

Where Foo is the type stored in your table.

You want the generic parameter of the type and value to be the same.

answered Dec 16, 2014 at 14:52

Konstantin Tarashchanskiy

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1 Comment

Rezwan Azfar Haleem Over a year ago

It looks so simple and yet I couldn't figure it out. I feel relieved that I will not get more of those warnings and still use generics. Thank you very much.

Trying · Accepted Answer · 2014-12-16 15:16:21Z

1

import java.util.ArrayList;
import java.util.List;

public class Sample<T> {

    private final int x;
    private final int y;
    private final List<List<T>> list;

    public Sample(final int x, final int y) {
        this.x = x;
        this.y = y;
        list = new ArrayList<>();
        for(int k=0; k<y; k++) {
           list.add(k, new ArrayList<T>());
        }
    }


    public T get(final int indexX, final int indexY) {
        if(indexX >= x) {
            return null;
        }
        if(indexY >= y) {
            return null;
        }
        return list.get(indexX).get(indexY);
    }

Now you can call Sample<String> s = new Sample<>(); and done. Hope it answer your query.

edited Dec 16, 2014 at 15:16

answered Dec 16, 2014 at 15:04

Trying

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1 Comment

Rezwan Azfar Haleem Over a year ago

Thank you very much for the effort. I understand it now.

John Smith · Accepted Answer · 2014-12-16 15:10:48Z

You should make your class generic to avoid warnings/errors. Maybe this small class that I wrote will help you out:

import java.util.ArrayList;
import java.util.List;

/**
* Creates a List of Lists of the given type
* @param <T> - The type of the table elements
*/
public class Table <T> {
    private final List<List<T>> data;

    public Table(int rows, int cells) {
        data = new ArrayList<List<T>>(rows);
        for(int i=0; i<rows; i++) {
            data.add(new ArrayList<T>(cells));
        }
    }

    public static void main(String[] args) {
        //create a table of strings
        Table<String> table = new Table<String>(10, 10);
        //do something with table
    }
}

If you want the table to contain various elements, create it like that:

Table<Object> table = new Table<Object>(10, 10);

This should only serve to demonstrate generics, I'm not claiming this is the best way to create a table. Also, I'm skipping the implementation of other methods you will surely need (such as accessors for the table elements, etc).

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