John Justin's blog

In your collection implementation how you will make sure the unique values without implementing set/hashSet interface? otherway how you implement set interface in your java collection data?

For example

public class Test {
    
    public static void main(String[] args)
    {
       
        HashSet<Object> testSet= new HashSet<Object>();
        testSet.add(100);
        testSet.add("hello");
        System.out.println("Result "+testSet);
    }
}

The result will be : Result [100, hello ] 

Now we will add some duplicate elements in the same collection list

public class Test {
    
    public static void main(String[] args)
    {
       
        HashSet<Object> testSet= new HashSet<Object>();
        testSet.add(100);
        testSet.add("hello");

        testSet.add(100);
        testSet.add("hello");
        System.out.println("Result "+testSet);
    }
}

The result will be : Result [100, hello ] 

Eventhough when we add duplicate element in this collection set with add method why the duplicated data not inserted? because add method returs false when we try to add duplicate elements. How the add method returns false?
The HashSet implementation of the add() method in java is internally implemented by HashMap implementation.

Whenever we tries to create an object of HashSet , it internally create an object of HashMap 

public class HashSet
extends AbstractSet
implements Set, Cloneable, java.io.Serializable

{
    private transient HashMap map;
           
    private static final Object PRESENT = new Object();
        
                public HashSet() {
                      map = new HashMap<>();
                    }
    
       
               public boolean add(E e) {
                       return map.put(e, PRESENT)==null;
                   }
    
       

}


The key is unique in hashmap , when we are calling add(Element E) this element E will be added as key in HashMap and the value for key will be added by the java developer api by using the variable PRESENT , it will create ( new Object() ) which is a dummy value referred by Object reference PRESENT

when you are adding a line in HashSet like  hashset.add(100)   what java does internally is that it will put that element E here 100 as a key in the HashMap(created during HashSet object creation) and some dummy value that is Object's object is passed as a value to the key .

The main point to notice in above code is that put (key,value) will return

1.  null , if key is unique and added to the map
2.  Old Value of the key , if key is duplicate

So , in HashSet add() method ,  we check the return value of map.put(key,value) method with null value
i.e.

   public boolean add(E e) {
            return map.put(e, PRESENT)==null;
       }

So , if map.put(key,value) returns null ,then
map.put(e, PRESENT)==null      will return true and element is added to the HashSet.

So , if map.put(key,value) returns old value of the key ,then
map.put(e, PRESENT)==null      will return false and element is  not added to the HashSet .

What is a Collection? 
• An object that groups multiple elements into a single unit.
 • Sometimes called a container






     img courtesy - http://javahungry.blogspot.com/

There are two ways to traverse collections:
1) using Iterators.
2) with the (enhanced) for-each construct

Iterators 

• An Iterator is an object that enables you to traverse through a collection and to remove elements from the collection selectively, if desired.

• You get an Iterator for a collection by calling its iterator method.

• Suppose collection is an instance of a Collection. Then to print out each element on a separate line:

Iterator it = collection.iterator(); 
while (it.hasNext())
 System.out.println(it.next());

 • Note that next() does two things:
                    1. Returns the current element (initially the first element)
                    2. Steps to the next element and makes it the current element. 

    Levels of Abstraction 

• Recall that Java supports three levels of abstraction: –

 - Interface 

             • Java expression of an ADT
             • Includes method declarations with arguments of specified types, but with empty bodies

 – Abstract Class 

             • Implements only a subset of an interface.
             • Cannot be used to instantiate an object. 

– (Concrete) Classes 

            • May extend one or more abstract classes
            • Must fully implement any interface it implements 
            • Can be used to instantiate objects. 

Iterating Over a Map 

• Because Map does not extend Iterable, but Collection (hence Set) does extend Iterable, you can (only) iterate over a Map using one of its three views:
           – Keys: Set keySet() 
           – Values: Collection values() 
           – Pairs: Set


 What is the root interface in collection hierarchy ? 

Root interface in collection hierarchy is Collection interface . Few interviewer may argue that 
Collection interface extends Iterable interface. So iterable should be the root interface. But you should reply iterable interface present in java.lang package not in java.util package .It is clearly mentioned in Oracle Collection  docs , that Collection interface is a member of the Java Collections framework.  For Iterable interface Oracle doc , iterable interface is not mentioned as a part of the Java Collections framework .So if the question includes  collection hierarchy , then you should answer the question as Collection interface (which is found in java.util package).

 What is the difference between Collection and Collections ?

Collection is  an interface while Collections is a java class , both are present in java.util package and  part of java collections framework.

Camel Book in one page

  Camel books Camel in action



 What is the difference between enrich() and pollEnrich()?

  Content enricher here  ( Link)

 Content enrichment using the enrich DSL element Camel comes with two flavors of content enricher in the DSL enrich pollEnrich enrich uses a Producer to obtain the additional data. It is usually used for Request Reply messaging, for instance to invoke an external web service. pollEnrich on the other hand uses a Polling Consumer to obtain the additional data. It is usually used for Event Message messaging, for instance to read a file or download a FTP file.

• enrich assumes you want to use an incoming Exchange as a parameter to an another service request. for example, your incoming Exchange could be a userID, but you really need the entire User object, so you could enrich it by passing the userID to a REST service that returns the User object which becomes the Exchange, etc.
• pollEnrich assumes the incoming Exchange is a simple trigger that tell a PollingConsumer to look for data and create an Exchange (ignoring the contents of the incoming Exchange). For example, you could have a timer or other business process event that requires picking up a file for processing, etc. that said, the incoming Exchange data is NOT used to dynamically configure the PollingConsumer...only the URI is used for this.