Why do I need to override the equals and hashCode methods in Java?

The equals() and hashCode() methods are two methods that are defined in the Object class, which is the superclass of all classes in Java. The equals() method is used to determine if two objects are equal, and the hashCode() method is used to generate a unique integer value (the hash code) for an object.

By default, the equals() method compares the references of two objects to determine if they are equal, rather than their contents. This means that if you have two objects that are logically equal, but have different references, the equals() method will return false when you compare them.

For example:

String str1 = new String("hello");
String str2 = new String("hello");

if (str1.equals(str2)) {  // this will return false
    // do something
}

In this example, the str1 and str2 objects are logically equal (they have the same contents), but they are different objects with different references, so the equals() method returns false when they are compared.

To compare the contents of two objects rather than their references, you need to override the equals() method in your class and implement your own logic for determining if two objects are equal.

The hashCode() method is used to generate a unique integer value for an object, based on its contents. This value is called the hash code of the object. The hashCode() method is used by certain Java collections (such as HashMap and HashSet) to efficiently store and retrieve objects based on their hash code.

If you override the equals() method in your class, you should also override the hashCode() method and implement your own logic for generating the hash code of an object. This is because the default implementation of the hashCode() method is based on the object's reference, rather than its contents. If you override the equals() method to compare the contents of two objects, you should also override the hashCode() method to generate hash codes based on the contents of the objects.

For example:

class MyClass {
    private String str;

    public MyClass(String str) {
        this.str = str;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        MyClass myClass = (MyClass) o;
        return Objects.equals(str, myClass.str);
    }

    @Override
    public int hashCode() {
        return Objects.hash(str);
    }