HashMap Machanism

When you store the values in HashMap, the values are actually stored in a set of buckets. Each of those buckets has been assigned a number which is use to identify it. HashMap implementation calls hashCode method on Key object and assigns the key object hashcode number to bucket to identify it. When you put a value in the HashMap, it stores the data in one of those buckets. Which bucket is used depends on the hashCode that will return by your object. 

Important point to mention is that HashMap in Java stores both key and value object as Map.Entry in bucket which is essential to understand the retrieving logic.

What will happen if two different objects have same hashcode?

equals() and hashCode() contract  that two unequal object in Java can have same hashcode

Since hashcode is same, bucket location would be same and collision will occur in HashMap, Since HashMap use LinkedList to store object, this entry (object of Map.Entry comprise key and value )  will be stored in LinkedList.