arrays.md

Arrays

Declaring and Initializing Arrays

An array holds several elements of the same type. It is declared by adding [] after the element type:

int[] arr;

To initialize an array, you have to use the keyword new and specify how many elements the array contains:

arr = new int[4];

While the distinction is irrelevant in most situations, it's best to think of the array variable as holding a reference to the array:

Note that all of the elements of the array are initialized to default values: 0 for numeric types, false for booleans, and null for reference types (objects and arrays).

If you have a small array and know the specific values you would like it to contain, you can initialize them directly:

String[] names = new String[] {"Akiko", "Bob", "Carlos", "Danielle"};

If the declaration and initialization happen on the same line, as in the example above, you are allowed to omit new String[].

Indexing

Array elements are numbered starting at 0. Thus, arr[0] is the first element of arr, arr[1] is the second, and so on.

You can use this to access or change any element of an array. If we take our array above and set

arr[2] = 8;

then the array looks like this:

Java arrays (unlike C arrays) know how long they are. The length of arr is arr.length (which in this case is 4).

Multidimensional Arrays

A structure like a matrix or a checkerboard can be represented by a multidimensional array. This is declared using multiple pairs of square brackets:

int[][] m = new int[3][4];

A multidimensional array is represented as an array of arrays:

In Java, as in most modern programming languages, the two numbers specify the number of rows and the number of columns, respectively. The lower right box in the diagram above is m[2][3].

Resources

• Sedgewick and Wayne, Introduction to Programming in Java, Section 1.4
• Horstmann, Core Java, Volume I: Fundamentals, 11th Edition, Section 3.10

Questions

1. ⭐ Can an array have length 0?
2. ⭐ What is the index of the last element of an array arr?
3. ⭐ Given

   int[][] m = new int[3][4];

   what is m.length?
4. ⭐ How would you declare a three-dimensional array of doubles?
5. ⭐ How many elements are in the array that is the value of the expression new boolean[2][3][4][5]?
6. ⭐ Is there a good way to remember that it's rows, columns and not the other way around?
7. ⭐ Why does the code below throw a NullPointerException?

   String[] words = new String[8];
   for (String w : words) {
       System.out.println(w.length());
   }

8. ⭐⭐ How can you determine the number of columns in a two-dimensional array m?

Answers

1. Yes, any non-negative integer is a valid length.
2. arr.length - 1. This is because indices start at 0; if there are 10 elements, they are numbered 0 through 9.
3. 3, which is the number of rows. length only follows one reference and counts the number of boxes in the array at the other end.
4. double[][][] arr;.
5. 120, which is the product of the dimensions.
6. Think of a can of RC Cola.
7. While words itself was initialized, its elements were not. They therefore have the default value null.
8. m[0].length.