This is the second set of questions that are generally asked for Collection Framework questions. These are a bit more advanced then the earlier set of questions.
Question 1: What is difference between ArrayList and LinkedList?
There are quiet a few difference between ArrayList and LinkedList:
Question 3: What is the difference between HashMap and HashTable?
The main difference is that HashTable is synchronised whereas HashMap is not. You need to provide external synchronisation if you want to synchronise the HashMap. Another difference is that HashTable does not allows null as keys or values whereas HashMap allow one null key and any number of null values.
Question 4: What is the difference between HashMap and TreeMap?
The basic difference is that in TreeMap objects are stored in an order decided either by the natural ordering of key or by the comparator that is defined at the the instantiation time of the TreeMap whereas HashMap does not guarantee any ordering. Since the elements are stored in an order, I think it is safe to say that the insertion of element in TreeMap will be slower as compared to HashMap whereas retrieval will be faster than HashMap.
Question 5: Explain how HashMap works or how the hashcode() and equals() method is used by the HashMap or how the get() method works in HashMap?
This question generally is the starting point for more complicated questions on HashMap. Basically what happens is when we call get(), put() method of HashMap. The HashMap uses the hashcode() method of the key to find the hashcode, then it uses its internal hashing mechanism to find the index of the correct "bucket" where the value might be stored. This bucket contains a list of Map.Entry objects in the form of a linked list. Once the bucket is identified, the map will traverse through the Map.Entry to find the exact key by using equals method, once found it will overwrite or return the value.
Question 6: How is HashSet implemented? or How will you implement the HashSet using HashMap?
Actually if you look closely on the HashMap methods there is already a set in the HashMap, they keySet. It has all the properties like no duplicates (HashMap does not allow duplicate keys). So, all you need to do is following, when you insert an object in HashSet, you insert the object as key in the HashMap and put the value as an EMPTY object. Same is the case when retrieving a value from HashSet, rather returning a value just return the key.
Question 7: Is Collection.synchronisedMap() is really thread-safe?
Question 8: What is the difference between ArrayList and Vector?
As per the Java API, the main difference between the ArrayList and Vector is that Vector is synchronised whereas ArrayList is not. The other difference is the way there size is incremented, Vector always doubles the size whereas ArrayList increases the size by half the initial capacity. So if there is a need for thread-safety is advisable to use Vector but since synchronisation takes a hit on the performance we may consider using the ArrayList and synchronise it using the Collections utility class.
Question 9: What are the mandatory methods you should override while using TreeMap and why?
Question 1: What is difference between ArrayList and LinkedList?
There are quiet a few difference between ArrayList and LinkedList:
- ArrayList uses a primitive array to store the values internally. However, LinkedList is made up of nodes where each nodes has three elements: value, pointer to next element and pointer to previous element.
- ArrayList implements the RandomAccess interface whereas LinkedList does not. RandomAccess is a marker interface which states that the implementation implements the fast algorithm to access the Nth element. So accessing Nth element is fast and takes constant-time for ArrayList whereas LinkedList has to scan the whole list to find the Nth element, hence making it slower for random access.
- Inserting and deleting an element at the start and end is faster in LinkedList as compared to ArrayList. While inserting an element in the LinkedList at the start and the beginning , we just need to create a node and assign the pointers whereas in ArrayList if you are inserting an element at the start, then first it will copy all the elements in a different list and then adds them after it, similarly while adding an element at the end means ArrayList will have to scan the whole list for inserting an element.
- LinkedList usually takes more memory than ArrayList as each node in the LinkedList will also store the next and previous pointers.
- ArrayList may also have issue if your list fills up quiet frequently as the ArrayList will create a new list with increased capacity and then copies all the elements whereas no such thing is required for LinkedList.
Question 2: Where will you use ArrayList and where will you use LinkedList?
If you have add elements at the start and end frequently or iterate over it for deleting elements then you should consider using LinkedList as it requires constant time for these operations and linear-time in ArrayList. If you have to access the positional elements then you should consider using ArrayList as it takes constant time for that and LinkedList take linear time for positional access.
The main difference is that HashTable is synchronised whereas HashMap is not. You need to provide external synchronisation if you want to synchronise the HashMap. Another difference is that HashTable does not allows null as keys or values whereas HashMap allow one null key and any number of null values.
Question 4: What is the difference between HashMap and TreeMap?
The basic difference is that in TreeMap objects are stored in an order decided either by the natural ordering of key or by the comparator that is defined at the the instantiation time of the TreeMap whereas HashMap does not guarantee any ordering. Since the elements are stored in an order, I think it is safe to say that the insertion of element in TreeMap will be slower as compared to HashMap whereas retrieval will be faster than HashMap.
Question 5: Explain how HashMap works or how the hashcode() and equals() method is used by the HashMap or how the get() method works in HashMap?
This question generally is the starting point for more complicated questions on HashMap. Basically what happens is when we call get(), put() method of HashMap. The HashMap uses the hashcode() method of the key to find the hashcode, then it uses its internal hashing mechanism to find the index of the correct "bucket" where the value might be stored. This bucket contains a list of Map.Entry objects in the form of a linked list. Once the bucket is identified, the map will traverse through the Map.Entry to find the exact key by using equals method, once found it will overwrite or return the value.
Question 6: How is HashSet implemented? or How will you implement the HashSet using HashMap?
Actually if you look closely on the HashMap methods there is already a set in the HashMap, they keySet. It has all the properties like no duplicates (HashMap does not allow duplicate keys). So, all you need to do is following, when you insert an object in HashSet, you insert the object as key in the HashMap and put the value as an EMPTY object. Same is the case when retrieving a value from HashSet, rather returning a value just return the key.
Question 7: Is Collection.synchronisedMap() is really thread-safe?
Question 8: What is the difference between ArrayList and Vector?
As per the Java API, the main difference between the ArrayList and Vector is that Vector is synchronised whereas ArrayList is not. The other difference is the way there size is incremented, Vector always doubles the size whereas ArrayList increases the size by half the initial capacity. So if there is a need for thread-safety is advisable to use Vector but since synchronisation takes a hit on the performance we may consider using the ArrayList and synchronise it using the Collections utility class.
Question 9: What are the mandatory methods you should override while using TreeMap and why?
