In this article, we will discuss on Java collection framework (JCF) overview and quick pointers about each important collection classes
Java Collection framework hierarchy
Source: Team BenchResources.Net
ArrayList (java.util.ArrayList)
- extends java.util.AbstractList and implements java.util.List
- uses dynamic array to store elements inside ArrayList
- size increases by 50% of current array size when ArrayList exceeds its capacity
- allows to insert/add duplicate elements
- maintains insertion order
- accessing elements is faster (as we can use index to access any elements)
- manipulation i.e.; delete is very slow due to shifting of elements
- ArrayList is non-synchronized
- Read more about ArrayList class
LinkedList (java.util.LinkedList)
- extends java.util.AbstractList and implements java.util.List & java.util.Deque
- uses dynamic array to store elements inside LinkedList
- allows to insert/add duplicate elements
- maintains insertion order
- manipulation i.e.; delete is very faster as no shifting is required (the links break and new link is formed while deleting any element from LinkedList)
- LinkedList is non-synchronized
- Read more about LinkedList class and
- difference between ArrayList v/s LinkedList
Vector (java.util.Vector)
- extends java.util.AbstractList and implements java.util.List & java.util.Deque
- uses grow-able array to store elements
- size increases by 100% of current array size when Vector exceeds its capacity
- allows to insert/add duplicate elements
- maintains insertion order
- Vector is legacy and it is synchronized
- Read more about Vector class
Vector v/s ArrayList:
| Vector | ArrayList |
| Vector is legacy (including other 4 viz., HashTable, Stack, Dictionary, Properties) | ArrayList is introduced in Java 1.2 |
| All legacy collection classes are synchronized, thus Vector is synchronized | ArrayList is not synchronized (need to make sure while working in multi-threaded environment) |
| Performance-wise vector is slower comparing with ArrayList due to synchronization | This is comparatively faster as it is non-synchronized |
| Vector increases its size by 100% of current array when its capacity exceeds | ArrayList increases its size by 50% of current array when its capacity exceeds |
| Both Iterator & Enumeration can be used to iterate items/elements inside Vector | Only Iterator is allowed to iterate items/elements inside ArrayList |
Read more about ArrayList v/s Vector
HashSet (java.util.HashSet)
- extends java.util.AbstractSet and implements java.util.Set
- uses hashtable to store elements inside HashSet
- allows only unique elements (doesn’t allow duplicates, if any duplicate is encountered then it is overridden)
- insertion order is not maintained i.e.; items/elements order is random while iterating
- permits null element
- HashSet is non-synchronized
- But it can be easily synchronized using Collections class i.e.; Collections.synchronizedSet(hashSet)
- HashSet is fail-fast –> throws ConcurrentModificationException is modified by any other means than HashSet’s own remove() method
- Read more about HashSet class
LinkedHashSet (java.util.LinkedHashSet)
- extends java.util.HashSet and implements java.util.Set
- allows only unique elements (doesn’t allow duplicates, if any duplicate is encountered then it is overridden)
- allows null element (if more than one is inserted, still it will contain only one null element)
- insertion order is maintained (in the order they are inserted)
- Read more about LinkedHashSet class and
- difference between HashSet v/s LinkedHashSet
TreeSet (java.util.TreeSet)
- extends java.util.SortedSet and implements java.util.NavigableSet
- allows only unique elements (doesn’t allow duplicates, if any duplicate is encountered then it is overridden)
- allows null element (if more than one is inserted, still it will contain only one null element)
- It sort the elements and maintains ascending order
- Read more about TreeSet class
List v/s Set:
- List allows duplicate items/elements
- Set allows only unique items/elements (if same element inserted again, still it will be contain just one element with that value)
- Read more about List v/s Set
PriorityQueue (java.util.PriorityQueue)
- Maintains FIFO order i.e.; orders in the PriorityQueue are in FIFO fashion
- FIFO –> First-In First-Out
- Important methods of PriorityQueue is poll(), peek(), remove(), head()
- Read more about PriorityQueue class
HashMap (java.util.HashMap)
- extends java.util.AbstractMap and implements java.util.Map
- HashMap stores key\value pairs
- allows only unique elements
- maximum of only-one null key is allowed (but it can have any number of null values corresponding to key)
- maintains no order
- HashMap is not synchronized
- But it can be easily synchronized using Collections class i.e.; Collections.synchronizedMap(hashMap)
- Read more about HashMap class
LinkedHashMap (java.util.LinkedHashMap)
- extends java.util.HashMap and implements java.util.Map
- LinkedHashMap stores key\value pairs
- allows only unique elements
- maximum of only-one null key is allowed (but it can have any number of null values corresponding to key)
- maintains insertion order (based on Key)
- LinkedHashMap is not synchronized
- Note: very similar to HashMap except maintaining insertion order
- Read more about LinkedHashMap class and
- difference between HashMap v/s LinkedHashMap
TreeMap (java.util.TreeMap)
- extends java.util.AbstractMap and implements java.util.NavigableMap
- TreeMap stores key\value pair
- allows only unique elements
- Null key is not allowed (but it can have any number of null values corresponding to key)
- maintains ascending order (based on Key)
- TreeMap is not synchronized
- Note: very similar to LinkedHashMap except maintaining ascending order
- Read more about TreeMap class and
- difference between HashMap v/s LinkedHashMap v/s TreeMap
Hashtable (java.util.Hashtable)
- extends java.util.Dictionary and implements java.util.Map
- HashTable stores key\value pairs
- allows only unique elements
- Null key is not allowed and also null value is not allowed
- HashTable is synchronized i.e.; thread-safe
- Doesn’t maintain order (while iterating)
- Read more about Hashtable class
HashTable v/s HaspMap:
| HashTable | HashMap |
| HashTable is legacy (including other 4 viz., Vector, Stack, Dictionary, Properties) | HashMap is introduced in Java 1.2 |
| All legacy collection classes are synchronized, thus HashTable is synchronized | HashMap is not synchronized (need to make sure while working in multi-threaded environment) |
| Performance-wise HashTable is slower comparing with HashMap due to synchronization | This is comparatively faster, as it is non-synchronized |
| HashTable neither allows null key or null values | HashMap allows only one null key (although it allows multiple null values corresponding to key) |
| Both Iterator & Enumeration can be used to iterate over HashTable items/elements | Only Iterator is allowed to iterate over HashMap items/elements |
Read more about difference between HashMap v/s Hashtable
Collection Sorting
We should always check 2 important interfaces while dealing collection framework in Java. Those are
- Comparable
- Comparator
Comparable v/s Comparator:
| Comparable | Comparator |
| Comparable belong to java.lang package | Comparator belong to java.util package |
| Provides compareTo() method to sort items/elements
Signature: public int compareTo(T o); |
Provides compare() method to sort items/elements
Signature: public int compare(T o1, T o2); |
| This is used for single-sorting over the collection items/elements | This is used for multiple-sorting over the collection items/elements |
| Original class needs to be implement java.lang.Comparable interface
Thus affecting original class |
A separate new class implements java.lang.Comparator interface
Thus, doesn’t affects original class |
| Static method “sort(List)” of Collections class is used to sort collection items/elements
Signature: Collections.sort(List lst); |
Static method “sort(List, Comparator)” of Collections class is used to sort collection items/elements
Signature: Collections.sort(List lst, Comparator cmp); |
Read more about
Read also:
- Java Collection Interview question and answers
- Interview Question and Answers on Java Method Overriding
- Interview Question and Answers on Method Overloading
- Difference between Method Overriding and Method Overloading in Java
- Interview Question and Answers on Java Constructor
- Interview Question and Answers on Java Interface
- Java 8: Interface interview question and answers
- Interview Question and Answers on Java Abstract Classes & Methods
- Interview Question and Answers on final keyword in Java
- Interview Questions and Answers on Java JDBC
- Interview program on String using toString() method
- ArrayList v/s LinkedList
Happy Coding !!
Happy Learning !!