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Primitive arrays map the key type (int) to a value (the type of the array). In this lesson, we will explore mapping with letters as keys.

array[0] = 1;

Notice the 0 as the accessing index. This is the key. With arrays, the key is always an int.

However, we can take advantage of the fact that a char can be easily cast to an int.


Let’s try a problem:

Find the number of occurrences of each letter in a word.

We need to map each letter to an index in the array. Let’s use a to 0, b to 1, c to 2, … , z to 25. Conveniently, ASCII values of letters are in the same order, just with a different starting number.

String word = "hello";
int[] occ = new int[26]; // Occurrences
for (char c : word.toCharArray()) {
	occ[c - 'a']++; // 'a' for lowercase and 'A' for uppercase

Mapping isn’t limited to only letters though. Just use a bigger array to support more characters.

String word = "";
int[] occ = new int[256]; // Many more ASCII values
for (char c : word.toCharArray()) {

// Print the occurences of only lowercase letters
for (char i = 'a'; i <= 'z'; i++) {
	System.out.println("The letter " + i + " occurs " + occ[i] + " times");

Array mapping can also be used with other keys such as digits: counting the occurrences of digits. Later on, we will learn mapping with other data types, such as double, String, and even custom classes.


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