Can a vector reference a 2-D array in Java?

Question

Basically, I want to create a 2-D array that grows in size at runtime. Could I do this using a vector in Java?

Would these be right then?

int [] [] x = new x [100] [100];

Vector x= new Vector();

To add elements into the array-

for i=0 to 99
   for j=0 to 99
   x.addElement(x[i] [j]);

essentially, it would be just the same like referencing any other object, only here, I've to specify the index numbers as well, right?

I'm a novice at Java. So I'd appreciate any help!

Answer 1

It's hard to know what to suggest to you without knowing the cases you need to optimize for.

For example, this simple wrapper around a map is a sparse 2D matrix optimized for setting and getting specific indices (doesn't need to "grow" at all), but bad for iterating over all indices.

public class SparseMatrix<T> {
    private final Map<Coordinates, T> map = new HashMap<Coordinates, T>();
    private final T defaultValue;

    public SparseMatrix(T defaultValue) {
       this.defaultValue = defaultValue;
    }

    private static class Coordinates {
        private final int[] coordinates;

        Coordinates(int... coordinates) {
            this.coordinates = coordinates;
        }

        @Override
        public boolean equals(Object o) {
            return Arrays.equals(coordinates, ((Coordinates)o).coordinates);
        }

        @Override
        public int hashCode() {
            return Arrays.hashCode(coordinates);
        }
    }

    public T get(int x, int y) {
        T value = map.get(new Coordinates(x, y));
        if ( value == null ) {
            return defaultValue;
        }
    }

    public T set(int x, int y, T val) {
        return map.put(new Coordinates(x, y), val);
    }
} 

Usage:

SparseMatrix<Integer> matrix = new SparseMatrix<Integer>(0);
matrix.set(3, 5, 7);
int seven = matrix.get(3, 5);
int zero = matrix.get(3, 6); //not set yet, uses default    

It can also be very easily adapted to N-dimensions. Of course in production code you wouldn't roll your own, you'd use a library that does a better job.

Answer 2

You can use ArrayList which is equivalent to dynamic array.

ArrayList<ArrayList<Integer>> aDynamicArray = new ArrayList<ArrayList<Integer>>(2);
aDynamicArray.add(new ArrayList<Integer>());
aDynamicArray.add(new ArrayList<Integer>());

Now you can use you for loop as following:

int [] [] x = new x [2] [100];
for i=0 to 1 
   for j=0 to 99
       aDynamicArray.get(i).add(x[i] [j]);

EDIT:

if i wanted to access x[2] [3] , how would i do that in the arraylist?

x[2][3] means value at 3rd row and 4th column. So, Fisrt line of code will change as follows: (to accomodate 3 rows)

ArrayList<ArrayList<Integer>> aDynamicArray = new ArrayList<ArrayList<Integer>>(3);
for(int i=0; i<3, i++)
{
    aDynamicArray.add(new ArrayList<Integer>());
}

then following line will give you access to x[2][3]:

aDynamicArray.get(2).get(3);

Answer 3

Unfortunately, Java has neither 2D arrays nor 2D growing vectors.

It has array of arrays or vector of vectors.

In your example, you created 1D vector of Objects:

Vector x= new Vector();

is similar to growing array of type Object[] x

So, when you do

y.addElement(x[i][j]);

Java does "boxing", i.e. it does

y.addElement(new Integer(x[i][j]));

Since you added 100x100 elements into 1D array, you will need to calculate location yourself

y.get(i*numcols+j)

So, to avoid all this, use vector of vectors like example below. The example copies newly fixed size array into vector of vectors:

   // creates fixed size 2D array with zeros
    int [] [] x = new int [50][50]; 

    // creates empty vector of vectors of integers
    // y is of type Vector<Vector<Integer>>
    // y.get(row) is of type Vector<Integer>>
    // y.get(row).get(col) is of type Integer
    Vector<Vector<Integer>> y = new Vector<Vector<Integer>>(); 

    // set the size of vector of vectors (number of rows)
    // each row will be null for now
    y.setSize(x.length);

    // enumerating rows
    for(int row=0; row<x.length; ++row) {

        log.info("row {}", row);

        // assign empty vector for row
        y.set(row, new Vector<Integer>());

        // set row size (number of columns)
        y.get(row).setSize(x[row].length);

        // enumerating columns of current row
        for(int col=0; col<x[row].length; ++col) {

            // setting the value for a cell
            y.get(row).set(col, x[row][col]);

        }
    }

Answer 4

In java would be something like:

int [][] x = new int [100] [100]

ArrayList <Integer> y = new ArrayList();

y.add(some number);

More information about Arraylist

Java let you have a collection compose by different data types. For Example

ArrayList<Object> list = new ArrayList<Object>();
list.add(1);
list.add("Java");
list.add(3.14);

Thus you can also use ArrayList of ArrayList and so on.

Answer 5

If you know the size of array (you have mentioned as 99), you can use int[][] intA = new int[100][100] and If you are familiar in java, you can implement your own Growing int[][] by monitoring array size when adding element.

Assume you are adding last element intA[99][99] = someInt at this time you can create new array int[][] newIntA = new int[intA.length+25][100]; then you need to copy content of existing intA array to newIntA

or you can use ArrayList<ArrayList<Integer>> to add your values or use ArrayList[] its your choice.