In Java, HashMap is a widely used data structure that stores elements in key-value
pairs. It provides constant-time performance for basic operations like adding, removing,
and accessing elements. The implementation of HashMap relies heavily on the equals
and hashCode methods, which are used to ensure the uniqueness of keys and the
efficiency of lookup operations. In this article, we’ll dive deeper into how these methods
are used in HashMap and what effect overriding them can have.

The equals() Method in HashMap:

The equals method is used to compare the equality of two objects. In the context of
HashMap is used to compare the keys of the HashMap. When a key is added to the
HashMap, the key’s equals method is called to check if there is an existing key with the
same value. If there is, the new value replaces the old one. If not, the key-value pair is
added to the HashMap.
The default implementation of the equals method in Java compares the memory
addresses of the two objects. However, in most cases, we want to compare the content
of the objects instead. Therefore, it is often necessary to override the equals method in
the critical class to ensure that it compares the contents of the objects.
For example, we have a Student class with name and id attributes. We want to
use the id as the key in a HashMap, so we need to override the equals method to
compare the ids instead of the memory addresses.

public class Student {
private int id;
private String name;
public Student(int id, String name) {
this.id = id;
this.name = name;
}
public int getId() {
return id;
}
public String getName() {

return name;
}

@Override

public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
Student student = (Student) o;
return id == student.id;
}
}

The hashCode() Method in HashMap:

The hashCode method returns a hash value that represents the object. In the context of
HashMap is used to determine the index of the bucket where the key-value pair is
stored.

The default implementation of the hashCode method in Java returns the
memory address of the object as an integer. However, this implementation is not
suitable for most use cases, as it can result in poor performance and collisions.
To improve the performance of HashMap, we need to override the hashCode method in
the key class to produce a more evenly distributed hash value.

The general contract of
the hashCode method is that if two objects are equal according to the equals method,
they must have the same hash code. Therefore, it is important to ensure that the
implementation of the hashCode method is consistent with the implementation of the
equals method.

For example, in our Student class, we can override the hashCode method to return the
hash code of the id attribute:

@Override

public int hashCode() {
return Objects.hash(id);
}

Effect of Overriding equals and hashCode Methods in HashMap:

When we override the equals and hashCode methods in the key class, we need to
ensure that the implementation is consistent with the semantics of the class. If two
objects are equal according to the equals method, they must have the same hash code.
If two things have the same hash code, they may or may not be equal according to the
equals method.

If the implementation of the equals and hashCode methods is incorrect, it can result in
unexpected behavior, such as incorrect lookup results or the loss of key-value pairs.
Therefore, it is important to test the implementation of these methods thoroughly and
ensure that they adhere to the contracts.
In conclusion, the equals and hashCode methods are essential components of the
HashMap implementation