How does LinkedHashMap work under the hood?

Question

I read about LinkedHashMap and from the description (although very interesting) I couldn't understand how it actually does its job under the hood. As a side note, I know how a HashMap works underneath in Java.
So I review the source and still can not figure out how it works. Perhaps I am not grasping something fundamental in OOP in this case so bear with me.
To summarize the part that is confusing to me is the following:
The LinkedHashMap delegates all the calls to its parent HashMap.
Internally it overrides the HashMap.Entry to implement the various recordAccess and recordRemoval methods which seem to implement the logic of the LinkedHashMap
But the actual Entries are inside the table of the base class i.e. the HashMap which instantiates a table of HashMap.Entry and not of LinkedHashMap.Entry.
So I can't figure out how the various recordAccess and recordRemove etc are actually being called.
So can anyone help me understand what's going on here?
Am I correct to think that somehow the LinkedHashedMap.Entry is the type of table created by the HashMap? But how?

UPDATE:
My question is how do the recordAccess are being called. My experiment on this using a derived version of HashMap failed for the reason of Shengyuan Lu (+1) - My bad there

UPDATE:
The following which I tried is the same (I think) as what the LinkedHashMap is doing:

package delete;  

public class Base<T> {  

    Entry<T>[] table;  
    int idx = 0;  
    @SuppressWarnings("unchecked")  
    public Base(){  
        System.out.println("In base");  
        table = new Entry[10];  
    }

    public void add(T x){  
        table[idx] = new Entry(x);  
        table[idx].doSomething();  
    }  

    static class Entry<T>{  
        T value;  

        Entry(T x){  
            this.value = x;  
            System.out.println("Entry::Base");  
        }

        void doSomething(){  
            System.out.println("In Entry base, doing something");  
        }  
    }  

}  




public class Derived<T> extends Base<T> {  

    static class Entry<T> extends Base.Entry<T>{  

        Entry(T x) {  
            super(x);  
            System.out.println("In Entry derived");  
        }  

        int val;  

        @Override  
        void doSomething() {  
            System.out.println("In Entry derived doing something really smart!");  
        }       
    }  

    /**
     * @param args
     */
    public static void main(String[] args) {  

        Base<String> b = new Derived<String>();  
        b.add("Test string");  

    }  

}

But it prints:

In base  
Entry::Base     
In Entry base, doing something

So the derived Entry is never called.
Is my example different somehow? I can't understand how this works for LinkedHashMap

for _Why does LinkedHashMap is defined as extends HashMap implements Map_ read [this](http://stackoverflow.com/q/2165204/1393766) — Pshemo, Jun 03 '12 at 12:49
@Pshemo:Thank you. I updated OP after the explanation of your link. Now all I need is help in my core question — Cratylus, Jun 03 '12 at 12:51

score 4 · Accepted Answer · edited Jun 03 '12 at 14:37

4

If you define MyLinkedHashMap under package java.util, it will compile;)

Because HashMap.HashEntry is package visibility.

PLUS:

I think the major thing puzzled you is LinkedHashMap.Entry vs HashMap.Entry. The point is LinkedHashMap.Entry is-a HashMap.Entry. Actually HashMap.table stores LinkedHashMap.Entry in LinkedHashMap.

Regarding to recordAccess and recordRemoval, they both override HashMap.Entry versions. You could find out the references both in LinkedHashMap and HashMap.

Merge comments here: You sample code is not same as LinkedHashMap implementation. See LinkedHashMap.addEntry() instead.

edited Jun 03 '12 at 14:37

Cratylus

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answered Jun 03 '12 at 12:41

卢声远 Shengyuan Lu

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But how?`HashMap` does `table = new Entry[DEFAULT_INITIAL_CAPACITY];`. – Cratylus Jun 03 '12 at 14:01
@user384706 Think about `Object[] table=new Object[1]; table[0] = "I am subclass of Object"; ` – 卢声远 Shengyuan Lu Jun 03 '12 at 14:06

score 2 · Answer 2 · answered Jun 03 '12 at 13:08

Ctrl+F is your friend here, especially when it allows you to search multiple files at once. recordAccess is called on the accessed/created entry by the put and get methods of the map. (The calls are in HashMap.put, HashMap.putForNullKey and LinkedHashMap.get.) It is only relevant if you use the constructor of the LinkedHashMap that takes a boolean parameter and passed true to that. The effect of that is, that whenever you touch the map, the touched entry will be moved to the front of the internal linked list.

Quoting the documentation:

A special constructor is provided to create a linked hash map whose order of iteration is the order in which its entries were last accessed, from least-recently accessed to most-recently (access-order). This kind of map is well-suited to building LRU caches. Invoking the put or get method results in an access to the corresponding entry (assuming it exists after the invocation completes). The putAll method generates one entry access for each mapping in the specified map, in the order that key-value mappings are provided by the specified map's entry set iterator. No other methods generate entry accesses. In particular, operations on collection-views do not affect the order of iteration of the backing map.

The removeEldestEntry(Map.Entry) method may be overridden to impose a policy for removing stale mappings automatically when new mappings are added to the map.

Similarly recordRemoval is called from HashMap.removeEntryForKey and HashMap.removeMapping.

How does LinkedHashMap work under the hood?

2 Answers2