Dicionários e Tabelas Hash em Java 17: Guia Detalhado

Dictionaries and Hash Tables in Java

What is a dictionary and why is it needed?

Imagine you have a notebook. In it, you look up a friend's phone number by their name. The name is the key, and the phone number is the value. In Java, for such "key-value" pairs, there is a special tool — HashMap (dictionary or hash table).

Dictionaries are convenient when you need to quickly find something by a unique name (key). For example:

• Country → Greeting in its language
• Person's name → Phone number
• Day of the week → To-do list
• Product name → Price

Unlike lists (ArrayList), where we search by number (index), in a dictionary we search by key. This is very fast!

Basic HashMap Syntax

To use a dictionary, you need to import the java.util.HashMap library.

import java.util.HashMap;

public class Main {
    public static void main(String[] args) {
        // Create a dictionary where the key is String, the value is String
        HashMap<String, String> greetings = new HashMap<>();

        // Add pairs (key, value)
        greetings.put("Russia", "Hello");
        greetings.put("France", "Bonjour");
        greetings.put("Italy", "Ciao");

        // Get the value by key
        String russianGreeting = greetings.get("Russia");
        System.out.println("In Russia they say: " + russianGreeting);

        // Check if a key exists
        if (greetings.containsKey("Italy")) {
            System.out.println("Italy is in the dictionary!");
        }

        // How many entries total
        System.out.println("Total countries: " + greetings.size());

        // Delete an entry
        greetings.remove("France");
        System.out.println("After deletion: " + greetings.size() + " countries");
    }
}

Program output:

In Russia they say: Hello
Italy is in the dictionary!
Total countries: 3
After deletion: 2 countries

Key and Value Types

Keys and values can be of different types. The main rule: keys must be unique (non-repeating). If you add an existing key, the old value will be replaced with the new one.

import java.util.HashMap;

public class Main {
    public static void main(String[] args) {
        // Key — String (product name), value — Double (price)
        HashMap<String, Double> prices = new HashMap<>();
        prices.put("Apples", 89.50);
        prices.put("Milk", 75.00);
        prices.put("Bread", 45.00);

        // Print the price of apples
        System.out.println("Price of apples: " + prices.get("Apples") + " rub.");

        // Replace the price (key already exists)
        prices.put("Apples", 95.00);
        System.out.println("New price of apples: " + prices.get("Apples") + " rub.");
    }
}

Program output:

Price of apples: 89.5 rub.
New price of apples: 95.0 rub.

How to iterate over all entries in a dictionary?

Often you need to go through all "key-value" pairs. For this, use a for-each loop.

import java.util.HashMap;

public class Main {
    public static void main(String[] args) {
        HashMap<String, String> phoneBook = new HashMap<>();
        phoneBook.put("Masha", "8-912-345-67-89");
        phoneBook.put("Petya", "8-923-456-78-90");
        phoneBook.put("Anya", "8-934-567-89-01");

        // Iterate over all entries
        for (String name : phoneBook.keySet()) {
            String phone = phoneBook.get(name);
            System.out.println(name + ": " + phone);
        }
    }
}

Program output:

Masha: 8-912-345-67-89
Petya: 8-923-456-78-90
Anya: 8-934-567-89-01

Analysis of Typical Problems

Problem 1: Greeting for a traveler

We have a dictionary of countries and greetings. We need to find the greeting for a given country.

import java.util.HashMap;
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        HashMap<String, String> greetings = new HashMap<>();
        greetings.put("England", "Hello");
        greetings.put("Spain", "Hola");
        greetings.put("Germany", "Guten Tag");
        greetings.put("Japan", "Konnichiwa");

        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter country: ");
        String country = scanner.nextLine();

        if (greetings.containsKey(country)) {
            System.out.println("Greeting: " + greetings.get(country));
        } else {
            System.out.println("Sorry, this country is not in the dictionary.");
        }
        scanner.close();
    }
}

Problem 2: Phone directory

We look up a phone number by name. If the name is not found, we report it.

import java.util.HashMap;
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        HashMap<String, String> contacts = new HashMap<>();
        contacts.put("Ivan", "8-111-222-33-44");
        contacts.put("Olga", "8-555-666-77-88");
        contacts.put("Sergey", "8-999-000-11-22");

        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter name: ");
        String name = scanner.nextLine();

        String phone = contacts.get(name);
        if (phone != null) {
            System.out.println("Phone number: " + phone);
        } else {
            System.out.println("Contact not found.");
        }
        scanner.close();
    }
}

Problem 3: Deleting a contact

Delete an entry from the digital notebook by name.

import java.util.HashMap;
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        HashMap<String, String> notebook = new