Java Hash table

Time to go back to basics and just try to solve problems here and there using Java. Java and PHP and my favorite languages so lets stay current by keeping our skills up to date.

A hash table is a data structure that is used to store keys and values in a way that allows for fast insertion, deletion, and lookup of values. In Java, a hash table is implemented using the HashMap class, which is part of the Java Collections Framework.

To learn about hash tables in Java, you can start by reading about the HashMap class in the Java documentation. This will give you an understanding of how to use the class to create a hash table and perform various operations on it, such as inserting, deleting, and looking up values.

You can also try working through some Java exercises or tutorials that involve hash tables. This will give you hands-on experience using the HashMap class and help you understand how it works in practice.

Here is a simple example of how to create and use a HashMap in Java:

import java.util.HashMap;

public class HashTableExample {
    public static void main(String[] args) {
        // Create a hash table
        HashMap<String, Integer> table = new HashMap<>();

        // Add some key-value pairs to the table
        table.put("apple", 1);
        table.put("banana", 2);
        table.put("orange", 3);

        // Look up a value by key
        int value = table.get("banana");
        System.out.println(value);  // Outputs 2

        // Check if the table contains a key
        boolean containsKey = table.containsKey("apple");
        System.out.println(containsKey);  // Outputs true

        // Check if the table contains a value
        boolean containsValue = table.containsValue(3);
        System.out.println(containsValue);  // Outputs true

        // Remove a key-value pair from the table
        table.remove("orange");

        // Print the size of the table
        System.out.println(table.size());  // Outputs 2
    }
}

The example creates a HashMap that maps strings (the keys) to integers (the values). It adds three key-value pairs to the table, looks up a value by key, checks if the table contains a specific key or value, removes a key-value pair from the table, and prints the size of the table.

Here is an analysis of the time complexity of the operations in the example code provided:

table.put(key, value): This operation has an average time complexity of O(1) in the best case and an average time complexity of O(n) in the worst case, where n is the number of key-value pairs in the HashMap. This is because the HashMap uses a hash function to map keys to indices in an underlying array, and the time it takes to insert a new key-value pair into the array depends on the length of the array and the distribution of the keys. In the best case, the array is large enough and the keys are distributed evenly, so the operation takes constant time. In the worst case, the array is small and the keys are not distributed evenly, so the operation takes longer.

table.get(key): This operation has an average time complexity of O(1) in the best case and an average time complexity of O(n) in the worst case, for the same reasons as the put operation.

table.containsKey(key): This operation has an average time complexity of O(1) in the best case and an average time complexity of O(n) in the worst case, for the same reasons as the put operation.

table.containsValue(value): This operation has a time complexity of O(n), because it requires iterating through all of the values in the HashMap to check if the given value is present.

table.remove(key): This operation has an average time complexity of O(1) in the best case and an average time complexity of O(n) in the worst case, for the same reasons as the put operation.

table.size(): This operation has a time complexity of O(1), because the HashMap stores the number of key-value pairs that it contains, so it can return the size in constant time.

It’s worth noting that the time complexity of these operations is an average case, because the actual time it takes to perform them may vary depending on the specific input and the implementation of the HashMap.

Here are some additional resources that you might find helpful:

The Java tutorial on the HashMap class: https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/HashMap.html

A tutorial on hash tables in Java: https://www.geeksforgeeks.org/hashmap-in-java/

A tutorial on hash tables and hash functions: https://www.tutorialspoint.com/data_structures_algorithms/hash_data_structure.htm

Java HashMap W3 Schools: https://www.w3schools.com/java/java_hashmap.asp