Difference between linked list and array data structure in Java Programming

This makes it even more important to learn and understand difference between an array and a linked list.             

                Well there are lot of difference between these two starting from how they store data, to how you retrieve data from them. Main difference comes from the fact that array elements are stored in contiguous memory location, which makes it easy to retrieve them in quick time, while linked list elements are scattered through out memory, where one element knows address of other, it makes it hard to retrieve element from linked list in quick time. Some of the differences which we saw in ArrayList vs LinkedList also applicable at data structure level, because ArrayList is backed by array and LinkedList is internally backed by double linked list in Java. 

Array vs linked list in Java

Here is my list of differences between array and linked list. Though data structure concept are independent of any programming language and more or less applicable in all programming language including C and C++, I have explained differences in Java's context.

1. First and major difference between linked list and array data structure is that former doesn't support random access, while later support random access. linked list is sequential, in order to retrieve an element, you need to traverse till that, while if you know index, you can retrieve an element from array very quickly, because it doesn't involved traversal.

2. Second major difference between array and linked-list data structure is that, array needs contiguous memory allocation, which may result in java.lang.OutOfMemoryError: Java Heap Space if there is not enough contiguous ( a big chunk) of memory in Java Heap. On the other hand, linked list is distributed data structure, it's element are scattered over heap and doesn't need a contiguous memory allocation. This makes linked list ideal, if you have scattered memory.

3. Third major difference is fixed length, array is a fixed length data structure, you provide length or size of array at the time of creation, later you can not modify that size. On the other hand, linked list is dynamic data structure, it can grow and doesn't required size to be specified at the time of creation, because each node keep tracks of other.

4. It's easy to insert and delete elements from linked list than array, especially inserting element at beginning of linked list, and deleting element from end of linked list is O(1) operation. On the other hand array is fixed length data structure, so memory is allocated during initialization, and doesn't really change due to addition and removal of elements.  Though you can set a particular index null, to cut the reference count of that object.

5. Array is ideal for implementing fast caches e.g. HashMap or Hashtable, which requires constant time retrieval e.g. Map data structure provides O(1) performance for get(Key key) operation, while linked list based structure provides liner performance i.e. O(n) for retrieval operation, where n is the number of elements in linked list.

6. Array can be one or multi-dimensional, while linked list can be singly, doubly or circular linked list. Two dimensional array are most common in multi-dimensional and used to represent matrix in Java. You can use two dimensional array to represent a plain of x,y coordinates, frequently used in Game programming. Java programming language provides support for creating array at syntax level, it supports both single and multidimensional array. Java API also provides a class called java.util.LinkedList, which is an implementation of doubly linked list data structure.

That's all on my list of differences between array and linked list data structure. I strongly suggest to get a good hold of these data structure, especially linked list, which is very popular among data structure interview questions. Questions like appending elements into linked list, deleting elements, reversing linked list are quite common in various programming jobs. At very least, knowledge of fundamental data structure is essential to do well in programming jobs.

Don't store sensitive data in String

String pose security threat if used for storing sensitive data like passwords, SSN or any other sensitive information. Since String is immutable in Java there is no way you can erase contents of String and since they are kept in String pool (in case of String literal) they stay longer on Java heap ,which exposes risk of being seen by anyone who has access to Java memory, like reading from memory dump. Instead char[] should be used to store password or sensitive information. See Why char[] is more secure than String for storing passwords in Java for more details.