PhoneNumberMnemonics.java

// PhoneNumberMnemonics.java
// From Classic Computer Science Problems in Java Chapter 9
// Copyright 2020 David Kopec
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

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()));
	}

}