Daily Archives: March 26, 2017

How arrayList works Internally

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  1. Internally an ArrayList uses an Object[] Array. 
           private transient Object[] elementData;
  2. As you add items to an ArrayList, the list checks to see if the backing array has room left. If there is room, the new item is just added at the next empty space. If there is not room, a new, larger, array is created, and the old array is copied into the new one.
  3. When we actually create an arrayList following piece of code is executed – 
      this.elementData=new Object[initial capacity];
  4. ArrayList can be created in two ways-

    List<String> myList=new ArrayList<String>();
  5. When we create an ArrayList in this way, default constructor is invoked and will internally create an array of Object with default size, which is 10.

    List<String> myList=new ArrayList<String>(5);

    When we create an ArrayList in this way, constructor with an integer argument is invoked and will internally create an array of Object with the size, specified in the constructor argument, which happens to be 5 in this case.

  6. Inside add() method Check is made, before adding element into the array it will check what is the current size of filled elements and what is the maximum size of the array. If size of filled elements is greater than maximum size of the array then size of the array must be increased. But we know that the size of the array cannot be increased dynamically.

    So what happens internally is a new Array is created with size 1.5*currentSize and the data from old Array is copied into this new Array.

Internally an ArrayList uses an Object[].

Capacity vs Size

The capacity is how many elements the list can potentially accommodate without reallocating its internal structures.

The size is the number of elements in the list

If you allocate a new array with arr = new Employee[100], the size of that array (arr.length) is going to be 100. It has 100 elements. All the elements are initially null (as this is an array of object references), but still, there are 100 elements.

If you do something like list = new ArrayList (100), and try to check list.size(), you’ll get 0. There are no elements in the list.

Internally, it’s true that the ArrayList allocates enough place to put 100 items before it needs to extend its capacity, but that’s an internal implementation detail, and the list presents its content to you as “no items stored”. Only if you actually do list.add(something), you’ll have items in the list.

So although the list allocates storage in advance, the API with which it communicates with the program tells you there are no items in it. The null items in its internal array are not available to you – you cannot retrieve them or change them.

An ArrayList is just one way to represent an abstract list, and the capacity of an ArrayList is an implementation detail of how the system implements the logical list.

An ArrayList stores the elements of a list by using an actual array “under the covers.” The actual realization of the array in computer memory has a certain size when it is allocated; this size is the ArrayList’s capacity. The ArrayList emulates a variable-sized list by storing the logical length of the list in addition to the fixed-length array. Thus if you have an ArrayList with a capacity 10 which contains 4 logical elements, the ArrayList can be represented as a length and an array

(4) | e1 | e2 | e3 | e4 | __ | __ | __| __ | __ | __ |

where the (4) is the logical length of the list and ‘__’ represent data that is ignored because it is not part of the logical list. If you attempt to access the 5th element of this ArrayList, it will throw an exception because it knows that the fifth element has not been initialized. If we then append an extra element e5 to the list, the ArrayList becomes

(5) | e1 | e2 | e3 | e4 | e5 | __ | __ | __ | __ | __ |

Note that the capacity has not changed, while the logical length has, because the underlying array is still able to handle all the data in the logical list.

If you manage to add more than ten elements to this list, the ArrayList will not break. The ArrayList is an abstraction meant to be compatible with all array operations. Rather, the ArrayList changes its capacity when its logical length exceeds its original capacity. If we were to add the elements (a1, a2, …, a7) to the above list, the resulting ArrayList might look like

(12) | e1 | e2 | e3 | e4 | e5 | a1 | a2 | a3 | a4 | a5 | a6 | a7 | __ | __ | __ | __ | __ | __ | __ | __ |

with a capacity of 20.

Once you have created an ArrayList, you can ignore the capacity in all programming that follows; the logic is unaffected. However, the performance of the system under certain kinds of operations can be affected. Increasing the capacity, for instance, might well involved allocating a larger array, copying the first array into the second and then performing the operations. This can be quite slow compared to, e.g. the same operation on a linked list. Thus it is sensible to choose the capacity of an ArrayList to be bigger than, or at least comparable to, the actual number of elements expected in the real runtime environment.

Is there a Way to Create ArrayList of Fixed size in Java

 List<String> fixedSizeList = Arrays.asList(new String[100]);

You cannot insert new Strings to the fixedSizeList (it already has 100 elements). You can only set its values like this:

fixedSizeList.set(7, "new value");

What would be the Output of the Following Code

List<Employee> employees = new ArrayList<>(100);
int size = employes.size();

size will be 0 while the initial capacity is 100.

Interface extend Multiple Interfaces in Java

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Yes. An interface can extend multiple interfaces, as shown here:

interface Maininterface extends inter1, inter2, inter3{  
  // methods
}

A single class can also implement multiple interfaces

interface A
{
    void test();
}

interface B 
{
    void test();
}

class C implements A, B
{
    @Override
    public void test() {

    }     
}

Single implementation works for both.

Can Abstract class can have Private Methods and Constructors

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Yes.You can use any kind of method in an abstract class

public abstract class AbstractDAO{

public void save(){
  validate();
  //save
}
  private void validate(){ // we are hiding this method
  }
}

Private Constructors
If Private constructor is the only constructor of the class, then the reason is clear: to prevent subclassing. Some classes serve only as holders for static fields/methods and do not want to be either instantiated or subclassed. Note that the abstract modifier is in this case redundant—with or without it there would be no instantiation possible. The abstract modifier is also bad practice because it sends wrong signals to the class’s clients. The class should in fact have been final.

Abstract Class with Private Constructor is same as Final Class. Both serves the same purpose.

Singleton vs Factory pattern

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Q1.What is the difference between Singleton vs Factory pattern?
A singleton pattern ensures that you always get back the same instance of whatever type you are retrieving, whereas the factory pattern generally gives you a different instance of each type.

The purpose of the singleton is where you want all calls to go through the same instance. An example of this might be a class that manages a disk cache, or gets data from a static dictionary; wherever it is important only one known instance interacts with the resource. This does make it less scalable.The purpose of the factory is to create and return new instances. Often, these won’t actually be the same type at all, but they will be implementations of the same base class. However, there may be many instances of each type

Q2.How to stop create instance via Reflection in Singleton

private Singleton() {
    if (singleton != null) {
        throw new IllegalStateException("Singleton already constructed");
    }
}

Q3.What are different ways of preventing object creation in singleton?
There are 5 ways of creating Objects. The below code explains how to prevent object creation by all 5 methods. Instead of
doing all these we can create singleton by using ENUM
Singleton.java

public class Singleton implements Serializable 
{
 private static final long serialVersionUID = 3119105548371608200 L;
 private static final Singleton singleton = new Singleton();
 
 //Prevents Class creation by Constructor
 private Singleton() 
 {
     //Prevents Object creation by reflection
     if( Singleton.singleton != null ) 
     {
        throw new InstantiationError( "Creating of this object is not allowed." );
     }
 }
 
 public static Singleton getInstance() 
 {
  return singleton;
 }
 
 //Prevents Class creation during cloning
 @Override
 protected Object clone() throws CloneNotSupportedException 
 {
  throw new CloneNotSupportedException("Cloning of this class is not allowed");
 }
 
 //Prevents New Object Creation by returning same singleton object
 protected Object readResolve() 
 {
  return singleton;
 }
}

Main.java

try {
    Class<Singleton> singletonClass = (Class<Singleton>) Class.forName("test.singleton.Singleton");
    Singleton singletonReflection = singletonClass.newInstance();
} catch (ClassNotFoundException e) {
    e.printStackTrace();
} catch (CloneNotSupportedException e) {
    e.printStackTrace();
} catch (IllegalAccessException e) {
    e.printStackTrace();
}

JAXWS vs JAXRS

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JAX-WS – is Java API for the XML-Based Web Services – a standard way to develop a Web- Services in SOAP notation (Simple Object Access Protocol).

Calling of the Web Services is performed via remote procedure calls. For the exchange of information between the client and the Web Service is used SOAP protocol. Message exchange between the client and the server performed through XML- based SOAP messages.

Clients of the JAX-WS Web- Service need a WSDL file to generate executable code that the clients can use to call Web- Service.

JAX-RS – Java API for RESTful Web Services. RESTful Web Services are represented as resources and can be identified by Uniform Resource Identifiers (URI). Remote procedure call in this case is represented a HTTP- request and the necessary data is passed as parameters of the query. Web Services RESTful – more flexible, can use several different MIME- types. Typically used for XML data exchange or JSON (JavaScript Object Notation) data exchange

Plain Old Java Object vs Java Beans

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POJO Plain Old Java Object. Basically a class with attributes and its getters and setters.

public class User
{
 private String name;
 private int age;

 public void setName(String name){
    this.name = name;
 }

 public String getName(){
    return this.name;
 }

 //same for age

}

POJO vs Java Beans
A JavaBean is a Java object that satisfies certain programming conventions:

  1. all JavaBean properties must have public setter and getter methods (as appropriate);
  2. all JavaBean instance variables should be private.
  3. the JavaBean class must have a no-arg constructor
  4. the JavaBean class must implement either Serializable or Externalizable;

Advantages of Bean

  1. A Bean obtains all the benefits of Java’s “write-once, run-anywhere” paradigm.
  2. The properties, events, and methods of a Bean that are exposed to an application
    builder tool can be controlled.
  3. A Bean may be designed to operate correctly in different locales, which makes it
    useful in global markets.
  4. The configuration settings of a Bean can be saved in persistent storage and restored
    at a later time.
  5. A Bean may register to receive events from other objects and can generate events that
    are sent to other objects.

Advantages of POJO

  1. Getter & setter methods allow you to change the underlying data type without breaking the public interface of your class which makes it (and your application) more robust and resilient to changes
  2. You might want to call some other code such as raising a notification when the value is obtained or changed like in java bean. This is not possible with your current class.
  3. You can expose values that are not backed by a field I.E. calculated values such as getFullName() which is a concatenation of getFirstName() and getLastName() which are backed by fields.
  4. You can add validation to your setter methods to ensure that the values being passed are correct. This ensures that your class is always in a valid state.
  5. If the field is an object (I.E. not a primitive type) then the internal state of your class can be modified by other objects which can lead to bugs or security risks. You can protect against this scenario in your POJO’s getter by returning a copy of the object so that clients can work with the data without affecting the state of your object. Note that having a final field does not always protect you against this sort of attack as clients can still make changes to the object being referenced (providing that object is itself mutable) you just cannot point the field at a different reference once it has been set.