What is equals () and hashCode () contract in Java?

What is equals () and hashCode () contract in Java?

If two objects are equal(according to equals() method) then the hashCode() method should return the same integer value for both the objects. But, it is not necessary that the hashCode() method will return the distinct result for the objects that are not equal (according to equals() method).

What is hashCode and equals method?

Hashcode value is mostly used in hashing based collections like HashMap, HashSet, HashTable…. etc. This method must be overridden in every class which overrides equals() method. Syntax : public int hashCode() // This method returns the hash code value // for the object on which this method is invoked.

What is difference between and equals in Java with example?

equals() method. The major difference between the == operator and . equals() method is that one is an operator, and the other is the method. Both these == operators and equals() are used to compare objects to mark equality.

How hashCode () and equals () method are used in HashMap?

In HashMap, hashCode() is used to calculate the bucket and therefore calculate the index. equals method is used to check that 2 objects are equal or not. This method is provided by Object class. You can override this in your class to provide your own implementation.

Can two objects have same hashCode?

1) If two objects are equal (i.e. the equals() method returns true), they must have the same hashcode. 2) If the hashCode() method is called multiple times on the same object, it must return the same result every time. 3) Two different objects can have the same hash code.

Why is 31 used in hashCode?

The value 31 was chosen because it is an odd prime. If it were even and the multiplication overflowed, information would be lost, as multiplication by 2 is equivalent to shifting. The advantage of using a prime is less clear, but it is traditional.

Can two string have same hashCode?

Two same strings/value must have the same hashcode, but the converse is not true. There might be another string which can match the same hash-code, so we can’t derive the key using hash-code. The reason for two different string to have the same hash-code is due to the collision.

What is equals method in Java?

Java String equals() Method The equals() method compares two strings, and returns true if the strings are equal, and false if not. Tip: Use the compareTo() method to compare two strings lexicographically.

Where can we use == and equals () Write suitable example?

We can use == operators for reference comparison (address comparison) and . equals() method for content comparison….equals method to check whether two objects contain the same data or not.

• In the above example, we create 3 Thread objects and 2 String objects.
• In the first comparison, we check whether t1 == t3 or not.

What is the difference between HashMap and Hashtable?

Hashmap vs Hashtable It is thread-safe and can be shared with many threads. HashMap allows one null key and multiple null values whereas Hashtable doesn’t allow any null key or value. HashMap is generally preferred over HashTable if thread synchronization is not needed.

Can HashMap have null key?

HashMap allows one null key and multiple null values whereas Hashtable doesn’t allow any null key or value.