#linkedhashmap

Video Tutorial: Java Hash and Maps

In this video, we get a closer glimpse at Map, HashMap, LinkedHashMap, TreeMap, and HashTable in Java with sample courses in Java Collections. Let’s get began! by Ram N · Sep. 29, 21 · Java Zone · Tutorial Supply link

Java: Map Interface Methods and Examples

In the collection framework, a map contains values on the basis of key and value pairs. This pair is known as an entry.

Points to Remember

Map contains unique keys.

Map allows duplicate values.

Map is useful to search, update or delete elements on the basis of a key.

Map is the root interface in the Map hierarchy for Collection Framework.

Map interface is extended by SortedMap and implemented by

LinkedHashMap Example: LRU Cache Implementation

LRU cache can be implemented fairly easily using LinkedHashMap. In this article we examine what is LRU cache and how to implement it in Java.

What is LRU Cache?

LRU cache stands for least-recently-used cache. Before we examine the LRU policy, let’s take a moment to see what a cache is.

What is a cache?

In computer science a cache is a limited storage facility where items are stored for faster…

The simplest way to create a cache using

LinkedHashMap

is to extend it. Here is an example:

public class Cache extends    LinkedHashMap {  private final int capacity;  public Cache(int capacity)  {    super(capacity + 1, 1.1f, true);    this.capacity = capacity;  }  protected boolean removeEldestEntry(Entry eldest)  {    return size() > capacity;  } }

Note that the constructor takes as an argument the maximum number of entries we want a Cache object to hold. The superclass constructor has three arguments: the initial capacity of the map, the load factor, and a boolean argument that tells the

LinkedHashMap

constructor to keep entries in access order instead of the default insertion order. (See the

java.util.HashMap

API documentation for a description of the initial capacity and load factor.) In this case we set the initial capacity to be one more than our required cache size - this is because new entries are added before any are removed, so for example if we want to hold 100 entries the cache will actually contain 101 for an instant when new data is added. Setting the load factor to 1.1 ensures that the rehashing mechanism of the underlying

HashMap

class isn't triggered - this isn't a vital point but helps a little with efficiency at run-time.

The

removeEldestEntry()

method overrides a default implementation in

LinkedHashMap

and is where we determine the policy for removing the oldest entry. In this case, we return

true

LinkedHashMap

  LinkedHashMap class extends HashMap.LinkedHashMap is a hashmap with predictable iteration order.   If you add entries to a LinkedHashMap and then retrieve them through an iterator, they will be returned in the same order in which they were inserted.  

We can also create a LinkedHashMap that returns its elements in the order they were last accessed.   LinkedHashMap maintains a…