Explain some technics to handle collision in Hash Tables

There are multiple techniques available to handle collision:

• Chaining: In chaining we use array indexes to store the values. If hash code of second value also points to the same index then we replace that index value with an linked list and all values pointing to that index are stored in the linked list and actual array index points to the head of the the linked list. But if there is only one hash code pointing to an index of array then the value is directly stored in that index. Same logic is applied while retrieving the values. This is used in Java HashMap/Hashtable to avoid collisions.
• Linear probing: This technique is used when we have more index in the table than the values to be stored. Linear probing technique works on the concept of keep incrementing until you find an empty slot. The pseudo code looks like this:

index = h(k) 
while( val(index) is occupied) 
   index = (index+1) mod n

• Double hashing technique: In this technique we use two hashing functions h1(k) and h2(k). If the slot at h1(k) is occupied then the second hashing function h2(k) used to increment the index. The pseudo-code looks like this:

index = h1(k)
while( val(index) is occupied)
   index = (index + h2(k)) mod n

Linear probing and double hashing techniques are part of open addressing technique and it can only be used if available slots are more than the number of items to be added. It takes less memory than chaining because there is no extra structure used here but its slow because of lot of movement happen until we find an empty slot. Also in open addressing technique when an item is removed from a slot we put an tombstone to indicate that the item is removed from here that is why its empty.