Java Hashtable class

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The Hashtable class in Java is a legacy data structure that provides a way to store and retrieve key-value pairs in a table-like structure. It is similar to the HashMap class in Java, but is synchronized, which means that it is thread-safe and can be accessed by multiple threads concurrently.

Here are some key features of the Hashtable class:

  • It stores key-value pairs as entries in a hash table.
  • Keys are unique and are used to retrieve values.
  • Keys must be immutable objects, such as strings or integers.
  • Values can be any object.
  • The Hashtable class is synchronized, which means that it is thread-safe.
  • It does not allow null keys or values.
  • It is part of the Java Collections Framework and implements the Map interface.

Here is an example of how to create a Hashtable and add entries to it:

Hashtable<String, Integer> scores = new Hashtable<String, Integer>();
scores.put("Alice", 95);
scores.put("Bob", 87);
scores.put("Charlie", 92);
Hashtable<String, Integer> scores = new Hashtable<String, Integer>();
scores.put("Alice", 95);
scores.put("Bob", 87);
scores.put("Charlie", 92);

In this example, the Hashtable is created with keys of type String and values of type Integer. Three entries are added to the Hashtable, with “Alice” mapping to 95, “Bob” mapping to 87, and “Charlie” mapping to 92.

To retrieve a value from the Hashtable, you can use the get() method:

int aliceScore = scores.get("Alice");

This retrieves the value associated with the key “Alice” (in this case, 95) and stores it in the aliceScore variable.

Overall, the Hashtable class can be a useful tool for storing and retrieving key-value pairs in a thread-safe manner. However, it is important to note that it is a legacy class and has been largely replaced by the HashMap class, which provides similar functionality but is not synchronized.

Hashtable class declaration:

The Hashtable class in Java is part of the java.util package, which means that you need to import it before you can use it in your code. Here is the declaration of the Hashtable class:

public class Hashtable<K,V>
    extends Dictionary<K,V>
    implements Map<K,V>, Cloneable, Serializable

Let’s break down this declaration:

  • public – This indicates that the class is accessible from anywhere in your code.
  • class – This indicates that Hashtable is a class.
  • Hashtable<K,V> – This is the name of the class, with K and V being the type parameters for the keys and values, respectively.
  • extends Dictionary<K,V> – This indicates that Hashtable extends the Dictionary class, which is an abstract class that provides the basic functionality for a dictionary.
  • implements Map<K,V>, Cloneable, Serializable – This indicates that Hashtable implements the Map interface, which means that it provides methods for storing and retrieving key-value pairs, as well as the Cloneable and Serializable interfaces, which allow Hashtable objects to be cloned and serialized.

In summary, the Hashtable class is a concrete implementation of the Map interface that provides a thread-safe way to store and retrieve key-value pairs. It is similar to the HashMap class, but is synchronized and has been largely replaced by the latter.

Hashtable class Parameters:

The Hashtable class in Java is a generic class, which means that it can take two type parameters, K and V, to represent the types of the keys and values, respectively. Here’s a breakdown of these parameters:

  • K: This represents the type of the keys that are stored in the Hashtable. Keys must be unique and immutable objects. Examples of valid key types include String, Integer, and Enum.
  • V: This represents the type of the values that are associated with the keys in the Hashtable. Values can be any object, including null. Examples of valid value types include String, Integer, and custom classes.

When creating a Hashtable object, you specify the types of the keys and values as follows:

Hashtable<String, Integer> scores = new Hashtable<String, Integer>();

In this example, the Hashtable object scores has keys of type String and values of type Integer.

You can also omit the type parameters and let the compiler infer them, like this:

Hashtable<String, Integer> scores = new Hashtable<>();

This is equivalent to the previous example, but the compiler infers the types of the keys and values based on the type of the variable (scores).

In summary, the Hashtable class in Java is a generic class that takes two type parameters, K and V, which represent the types of the keys and values, respectively.

Constructors of Java Hashtable class:

The Hashtable class in Java provides several constructors that can be used to create Hashtable objects. Here are the available constructors:

  1. Hashtable(): Creates an empty Hashtable with default initial capacity (11) and load factor (0.75).
  2. Hashtable(int initialCapacity): Creates an empty Hashtable with the specified initial capacity and default load factor (0.75).
  3. Hashtable(int initialCapacity, float loadFactor): Creates an empty Hashtable with the specified initial capacity and load factor.
  4. Hashtable(Map<? extends K, ? extends V> t): Creates a new Hashtable with the same mappings as the specified Map.

Here’s an example of how to use each constructor:

// Create an empty Hashtable with default initial capacity and load factor
Hashtable<String, Integer> scores1 = new Hashtable<>();

// Create an empty Hashtable with initial capacity of 20
Hashtable<String, Integer> scores2 = new Hashtable<>(20);

// Create an empty Hashtable with initial capacity of 50 and load factor of 0.5
Hashtable<String, Integer> scores3 = new Hashtable<>(50, 0.5f);

// Create a Hashtable with the same mappings as a HashMap
Map<String, Integer> hashMap = new HashMap<>();
hashMap.put("Alice", 95);
hashMap.put("Bob", 87);
Hashtable<String, Integer> scores4 = new Hashtable<>(hashMap);

Note that the Hashtable class does not allow null keys or values, so if you attempt to insert a null key or value, a NullPointerException will be thrown.

In summary, the Hashtable class in Java provides several constructors that can be used to create Hashtable objects, including empty Hashtable objects with default or specified initial capacity and load factor, and Hashtable objects with the same mappings as other maps.

Methods of Java Hashtable class:

The Hashtable class in Java provides a variety of methods for adding, removing, and querying key-value pairs. Here are some of the most commonly used methods:

  1. put(K key, V value): Associates the specified value with the specified key in the Hashtable. If the key already exists in the Hashtable, the previous value is replaced with the new value.
  2. get(Object key): Returns the value to which the specified key is mapped in the Hashtable, or null if the key is not present in the Hashtable.
  3. remove(Object key): Removes the key-value pair with the specified key from the Hashtable, if it is present.
  4. containsKey(Object key): Returns true if the Hashtable contains a mapping for the specified key.
  5. containsValue(Object value): Returns true if the Hashtable contains one or more keys mapped to the specified value.
  6. isEmpty(): Returns true if the Hashtable contains no key-value mappings.
  7. size(): Returns the number of key-value mappings in the Hashtable.
  8. clear(): Removes all key-value mappings from the Hashtable.
  9. keySet(): Returns a Set of all keys in the Hashtable.
  10. values(): Returns a Collection of all values in the Hashtable.
  11. entrySet(): Returns a Set of all key-value mappings in the Hashtable.

Here’s an example that demonstrates the use of some of these methods:

Hashtable<String, Integer> scores = new Hashtable<>();
scores.put("Alice", 95);
scores.put("Bob", 87);
scores.put("Charlie", 92);

System.out.println(scores.get("Bob")); // prints "87"
System.out.println(scores.containsKey("Charlie")); // prints "true"

scores.remove("Charlie");
System.out.println(scores.containsKey("Charlie")); // prints "false"

System.out.println(scores.size()); // prints "2"

System.out.println(scores.keySet()); // prints "[Alice, Bob]"

System.out.println(scores.values()); // prints "[95, 87]"

System.out.println(scores.entrySet()); // prints "[Alice=95, Bob=87]"

In summary, the Hashtable class in Java provides a variety of methods for adding, removing, and querying key-value pairs, as well as for accessing the keys, values, and key-value mappings as collections.

Java Hashtable Example:

Sure! Here’s an example of how to use the Hashtable class in Java:

import java.util.Hashtable;

public class HashtableExample {
    public static void main(String[] args) {
        Hashtable<String, Integer> scores = new Hashtable<>();

        // add some key-value pairs to the Hashtable
        scores.put("Alice", 95);
        scores.put("Bob", 87);
        scores.put("Charlie", 92);

        // print the value associated with a key
        System.out.println("Bob's score: " + scores.get("Bob"));

        // remove a key-value pair
        scores.remove("Charlie");

        // print the number of key-value pairs in the Hashtable
        System.out.println("Number of scores: " + scores.size());

        // print all the keys in the Hashtable
        System.out.println("Keys: " + scores.keySet());

        // print all the values in the Hashtable
        System.out.println("Values: " + scores.values());

        // print all the key-value pairs in the Hashtable
        System.out.println("Entries: " + scores.entrySet());
    }
}

This program creates a Hashtable object called scores, adds some key-value pairs to it, and then demonstrates how to retrieve, remove, and print information about the key-value pairs in the Hashtable. When you run this program, you should see output similar to the following:

Bob's score: 87
Number of scores: 2
Keys: [Alice, Bob]
Values: [95, 87]
Entries: [Alice=95, Bob=87]

Java Hashtable Example: remove()

Sure! Here’s an example of how to use the remove() method of the Hashtable class in Java:

import java.util.Hashtable;

public class HashtableRemoveExample {
    public static void main(String[] args) {
        Hashtable<String, String> dictionary = new Hashtable<>();

        // add some key-value pairs to the Hashtable
        dictionary.put("apple", "a round fruit with red or green skin");
        dictionary.put("banana", "a long curved fruit with a yellow skin");
        dictionary.put("cherry", "a small round fruit with a red skin");

        // remove a key-value pair
        dictionary.remove("banana");

        // print the updated Hashtable
        System.out.println(dictionary);
    }
}

In this example, we create a Hashtable object called dictionary, and add some key-value pairs to it representing definitions of various fruits. Then, we use the remove() method to remove the key-value pair with the key "banana" from the Hashtable. Finally, we print the updated Hashtable using the toString() method (which is called automatically when we pass the Hashtable object to System.out.println()).

When you run this program, you should see output like the following:

{apple=a round fruit with red or green skin, cherry=a small round fruit with a red skin}

As you can see, the key-value pair with the key "banana" has been removed from the Hashtable.

Java Hashtable Example: getOrDefault()

Sure! Here’s an example of how to use the getOrDefault() method of the Hashtable class in Java:

import java.util.Hashtable;

public class HashtableGetOrDefaultExample {
    public static void main(String[] args) {
        Hashtable<String, Integer> scores = new Hashtable<>();

        // add some key-value pairs to the Hashtable
        scores.put("Alice", 95);
        scores.put("Bob", 87);

        // try to get the value associated with a key that doesn't exist
        int charlieScore = scores.getOrDefault("Charlie", 0);
        System.out.println("Charlie's score (default value): " + charlieScore);

        // try to get the value associated with a key that does exist
        int aliceScore = scores.getOrDefault("Alice", 0);
        System.out.println("Alice's score: " + aliceScore);
    }
}

In this example, we create a Hashtable object called scores, and add some key-value pairs to it representing test scores for various students. Then, we use the getOrDefault() method to try to retrieve the value associated with two different keys: "Charlie", which does not exist in the Hashtable, and "Alice", which does exist. When we try to get the value associated with "Charlie", the getOrDefault() method returns the default value of 0, because that key is not in the Hashtable. When we try to get the value associated with "Alice", the getOrDefault() method returns the actual score associated with that key, which is 95.

When you run this program, you should see output like the following:

Charlie's score (default value): 0
Alice's score: 95

Java Hashtable Example: putIfAbsent()

Sure! Here’s an example of how to use the putIfAbsent() method of the Hashtable class in Java:

import java.util.Hashtable;

public class HashtablePutIfAbsentExample {
    public static void main(String[] args) {
        Hashtable<String, Integer> scores = new Hashtable<>();

        // add some key-value pairs to the Hashtable
        scores.put("Alice", 95);
        scores.put("Bob", 87);

        // try to add a key-value pair that doesn't exist yet
        scores.putIfAbsent("Charlie", 92);

        // try to add a key-value pair that already exists
        scores.putIfAbsent("Alice", 90);

        // print the updated Hashtable
        System.out.println(scores);
    }
}

In this example, we create a Hashtable object called scores, and add some key-value pairs to it representing test scores for various students. Then, we use the putIfAbsent() method to try to add two more key-value pairs to the Hashtable: "Charlie", 92, which does not exist yet, and "Alice", 90, which already exists. When we try to add the pair "Charlie", 92", the putIfAbsent() method successfully adds it to the Hashtable, because that key did not exist before. When we try to add the pair "Alice", 90", the putIfAbsent() method does not add it to the Hashtable, because that key already exists.

When you run this program, you should see output like the following:

{Alice=95, Bob=87, Charlie=92}

As you can see, the Hashtable now contains the key-value pairs for "Alice", "Bob", and "Charlie", but the value for "Alice" has not been changed to 90, because the putIfAbsent() method did not add a new key-value pair for that key.

Java Hashtable Example: Book

Sure! Here’s an example of how to use the Hashtable class in Java to store and retrieve information about books:

import java.util.Hashtable;

public class BookExample {
    public static void main(String[] args) {
        Hashtable<Integer, Book> library = new Hashtable<>();

        // create some Book objects
        Book book1 = new Book(1, "To Kill a Mockingbird", "Harper Lee");
        Book book2 = new Book(2, "1984", "George Orwell");

        // add the Book objects to the Hashtable
        library.put(book1.getId(), book1);
        library.put(book2.getId(), book2);

        // retrieve a Book object from the Hashtable
        Book retrievedBook = library.get(1);
        System.out.println("Retrieved book: " + retrievedBook);

        // remove a Book object from the Hashtable
        library.remove(2);
        System.out.println("Updated library: " + library);
    }
}

class Book {
    private int id;
    private String title;
    private String author;

    public Book(int id, String title, String author) {
        this.id = id;
        this.title = title;
        this.author = author;
    }

    public int getId() {
        return id;
    }

    public String getTitle() {
        return title;
    }

    public String getAuthor() {
        return author;
    }

    @Override
    public String toString() {
        return "Book{" +
                "id=" + id +
                ", title='" + title + '\'' +
                ", author='" + author + '\'' +
                '}';
    }
}

In this example, we create a Hashtable object called library to store information about books. We also define a Book class with an id, title, and author, and create two Book objects to add to the Hashtable. We then use the get() method to retrieve a Book object with a specific id, and the remove() method to remove a Book object from the Hashtable.

When you run this program, you should see output like the following:

Retrieved book: Book{id=1, title='To Kill a Mockingbird', author='Harper Lee'}
Updated library: {1=Book{id=1, title='To Kill a Mockingbird', author='Harper Lee'}}

As you can see, we were able to store information about two books in the Hashtable, retrieve information about one of them using its id, and remove information about the other one.