added PhoneNumberMnemonics

Author: davecom

CCSPiJ/src/chapter9/PhoneNumberMnemonics.java

@@ -0,0 +1,57 @@
+package chapter9;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Map;
+import java.util.Scanner;
+
+public class PhoneNumberMnemonics {
+	Map<Character, String[]> phoneMapping = Map.of(
+			'1', new String[] { "1" },
+			'2', new String[] { "a", "b", "c" },
+			'3', new String[] { "d", "e", "f" },
+			'4', new String[] { "g", "h", "i" },
+			'5', new String[] { "j", "k", "l" },
+			'6', new String[] { "m", "n", "o" },
+			'7', new String[] { "p", "q", "r", "s" },
+			'8', new String[] { "t", "u", "v" },
+			'9', new String[] { "w", "x", "y", "z" },
+			'0', new String[] { "0", });
+	private final String phoneNumber;
+
+	public PhoneNumberMnemonics(String phoneNumber) {
+		this.phoneNumber = phoneNumber;
+	}
+
+	public static String[] cartesianProduct(String[] first, String[] second) {
+		ArrayList<String> product = new ArrayList<>(first.length * second.length);
+		for (String item1 : first) {
+			for (String item2 : second) {
+				product.add(item1 + item2);
+			}
+		}
+		return product.toArray(String[]::new);
+	}
+
+	public String[] getMnemonics() {
+		String[] mnemonics = { "" };
+		for (Character digit : phoneNumber.toCharArray()) {
+			String[] combo = phoneMapping.get(digit);
+			if (combo != null) {
+				mnemonics = cartesianProduct(mnemonics, combo);
+			}
+		}
+		return mnemonics;
+	}
+
+	public static void main(String[] args) {
+		System.out.println("Enter a phone number:");
+		Scanner scanner = new Scanner(System.in);
+		String phoneNumber = scanner.nextLine();
+		scanner.close();
+		System.out.println("The possible mnemonics are:");
+		PhoneNumberMnemonics pnm = new PhoneNumberMnemonics(phoneNumber);
+		System.out.println(Arrays.toString(pnm.getMnemonics()));
+	}
+
+}