You need a two-, three-, or more dimensional array or ArrayList.

No problem. Java supports this.

As mentioned back in Section 7.2, Java arrays can hold any reference type. Since an array is a reference type, it follows that you can have arrays of arrays or, in other terminology, multidimensional arrays. Further, since each array has its own length attribute, the columns of a two-dimensional array, for example, do not all have to be the same length (see Figure 7-2).

Figure 7-2. Multidimensional arrays

Here is code to allocate a couple of two-dimensional arrays, one using a loop and the other using an initializer. Both are selectively printed.

/** Show Two-Dimensional Array of Objects */
public class ArrayTwoDObjects {

    /** Return list of subscript names (unrealistic; just for demo). */
    public static String[][] getArrayInfo(  ) {
        String info[][];
        info = new String[10][10];
        for (int i=0; i < info.length; i++) {
            for (int j = 0; j < info[i].length; j++) {
                info[i][j] = "String[" + i + "," + j + "]";
            }
        }
        return info;
    }

    /** Return list of allowable parameters (Applet method). */
    public static String[][] getParameterInfo(  ) {
        String param_info[][] = {
            {"fontsize",    "9-18",    "Size of font"},
            {"URL",    "-",    "Where to download"},
        };
        return param_info;
    }

    /** Run both initialization methods and print part of the results */
    public static void main(String[] args) {
        print("from getArrayInfo", getArrayInfo(  ));
        print("from getParameterInfo", getParameterInfo(  ));
    }

    /** Print selected elements from the 2D array */
    public static void print(String tag, String[][] array) {
        System.out.println("Array " + tag + " is " + array.length + " x " + 
            array[0].length);
        System.out.println("Array[0][0] = " + array[0][0]);
        System.out.println("Array[0][1] = " + array[0][1]);
        System.out.println("Array[1][0] = " + array[1][0]);
        System.out.println("Array[0][0] = " + array[0][0]);
        System.out.println("Array[1][1] = " + array[1][1]);
    }
}

Running it produces this output:

> java ArrayTwoDObjects
Array from getArrayInfo is 10 x 10
Array[0][0] = String[0,0]
Array[0][1] = String[0,1]
Array[1][0] = String[1,0]
Array[0][0] = String[0,0]
Array[1][1] = String[1,1]
Array from getParameterInfo is 2 x 3
Array[0][0] = fontsize
Array[0][1] = 9-18
Array[1][0] = URL
Array[0][0] = fontsize
Array[1][1] = -       
>

The same kind of logic can be applied to any of the Collections. You could have an ArrayList of ArrayLists, or a Vector of linked lists, or whatever your little heart desires.

As Figure 7-2 shows, it is not necessary for the array to be “regular.” That is, it’s possible for each column of the 2D array to have a different height. That is why in the code example I used array[0].length for the length of the first column.