Do you know - Series - 4: Boxing and Unboxing - Integer assignment and comparison

We know about boxing and unboxing in Java. It is about automatic conversion of primitive type value into its Wrapper Object equivalent type value and viz.

Below is the simple example to understand this.

      int a = getCustomerNumber(); //getCustomerNumber() returns an int
      Integer aObj = a; //boxing

      Integer bObj = getAccountNumber(); //getAccountNumber() returns an Integer
      int b = bObj; //unBoxing

      if(a == bObj) {
            System.out.println("a and bObj are equal");
      } else {
           System.out.println("a and bObj are not equal");
      }

The above code will work fine if the method getAccountNumber() returns an Integer value. But, the problem starts when this getAccountNumber() method returns a null value. When it returns null you could see a NullPointerException in line where we have "int b = bObj;". The reason is that we are trying to set 'null' into a primitive type.

The simplest solution to fix this is change the type of b into Integer or you may need to check for the 'null' value from the getAccountNumber() and then assign proper 'int' value to 'b'.

Now, the corrected statement will be

Integer b = bObj;


Great! We fixed the issue.

Now, when you try to run again and you would notice the same exception, but in line where we have the if() condition. Again, the reason is comparing 'null' with primitive type variable 'a'. Now, to fix this you may suggest to change the type of 'a' into 'Integer' or simply typecast 'a' into Integer within the if() condition as below.

if((Integer)a == bObj)


Excellent! We fixed the second issue!

But, things are not settled down still. If you notice the 'if()' condition we have  '==' to compare reference types. Definitely, this is not the right way to compare references, right?

Lets change the condition with if(((Integer)a).equals(bObj)) .

Finally, we are able to run the code. The final version of code section is given below.

      int a = getCustomerNumber(); //getCustomerNumber() returns an int
      Integer aObj = a; //boxing

      Integer bObj = getAccountNumber(); //getAccountNumber() returns an Integer
      Integer b = bObj;

      if(((Integer)a).equals(bObj)) {
            System.out.println("a and bObj are equal");
      } else {
           System.out.println("a and bObj are not equal");
      }


We now understand that things look simple externally, but they come with some complexities internally...

Do you know - Series - 3: Arrays as List: Java

Mose of the time we use the method Arrays.asList(T… a) to covert an array into a List. But, what we forget is that the returned list is not modifiable. It will be a fixed list backed up by the given array of elements.

This fixed list won't stop you using the method add(), to add an element to the list and remove(), to remove an element at a particular index, but will throw UnSupportedOperationException at runtime.

If you look at the Arrays class it has its own implementation of ArrayList and that is what returned by the asList() method. Though the ArrayList class extends the AbstractList, but does not override the add() and remove() methods which is actually the reason for the UnSupportedOperationException.

So, the use of Arrays.asList(T… a) is to just get an readonly List from an array of elements. It just acts a bridge between array and collection based APIs.

If you need a modifiable List then you may wrap this unmodifiable list into an ArrayList as given in the below sample code.

public List getList(String[] array) {
    List unModifiableList = Arrays.asList(array);
    unModifiableList.add("abc"); //not supported
    unModifiableList.remove(1); //not supported
 
    List modifiableList = new ArrayList(unModifiableList); //wrapper to read-only list
    modifiableList.add("abc"); //supported
    modifiableList.remove(1);  //supported
}

Do you know - Series 2: Stream Operation: Java

This post talks about different options available for iterating over a List.

• using iterator  - Jdk 1.2

          List listItems = new LinkedList();
          listItems.add("a");
          . . .
          listItems.add(n);
          Iterator itr = listItems.iterator();
          while(itr.hasNext()) {
                String item = itr.next();
                doSomething(item);
         }

• using for loop - Jdk 1.5

          for(String item: listItems) {

                 doSomething(item);

          }

• using for...each loop - Jdk 1.8

          listItems.forEach(item -> doSomething(item));

• using Streams  - Jdk 1.8

          listItems.stream().filter(something).map(item -> doSomething(item)).count();

          or

          Stream.of("a", "b", "c").filter(something).map(item -> doSomething(item)).count();

Please note that the Streams can not be reused once it is done with any terminal    operations.  When you try to reuse you will get IlleagalStateException as the stream is already operated and closed. 

If you want to reuse and avoid this exception then you may keep the above statement local scope or create a new stream every time using stream Supplier as below.

     
          Supplier < Stream < String >> streamSupplier = () -> Stream.of("a", "b", "c");

          

                  

Do you know - Series 1: Null Check: Java

The idea of this post is to just highlight some of the features in Java which we may need in our day to day development work.

• The different ways to check null value of an Object

• null != obj

• Objects.nonNull(obj) - from java.util

• filter(Objects::nonNull) - when used with Lambda

• Optional.ofNullable(obj).ifPresent(o —> o.getValue());

• StringUtils.isNotEmpty(obj) - from org.apache.commons.lang3
• this takes care of both empty and null checks

• filter(StringUtils::isNotEmpty) - when used with Lambda

• Do something if nonNull and something else if Null

• Optional.ofNullable(obj).map(obj -> {
      doSomething(obj)
      return obj;
  }).orElseGet(() -> {
      logForNull();
      return null;
  });
  
  
  Here, If the object is nonNull, then only the value will be applied to the mapping function

When can I use Interface and Abstract class?

There is always a question of when should you go for using an interface and/or an abstract class. You may consider the below scenarios to decide.

This post won't explain what is an Interface and an Abstract class or their differences rather it would explain the scenarios for the usage of these.

Scenario 1: I want to have some predefined common functionality to my child classes and also want to enforce them some contract. These contracts might be suitable for all the classes in the same hierarchical system. But, I don't think the contract(s) might be useful to other systems (classes other than this hierarchical structure) or it would make sense to them.

Solution: Use Abstract class - Define concrete methods (for predefined common functionality) and declare abstract methods (contract with child classes)

Example:

public abstract class Vehicle  {
          public abstract void startVehicle();
          public abstract void stopVehicle();
          public final void printVehicleDetails() { }
}

public class Plane extends Vehicle {
         public void startVehicle() { }
         public void stopVehicle() { }
}
public class Bicycle extends Vehicle  {
         public void startVehicle() { }
         public void stopVehicle() { }
}

Explanation: In the above example the abstract methods, startVehicle() and stopVehicle() are needed for this hierarchical system. All the Vehicles must have these details. But, it may not be useful to other systems and hence don't want to make it a generic one (interface type. Please note that interface is something which can be used by any one; its public)


Scenario 2: I want to have some common functionality to my child classes and also want to enforce them some contract. I also feel that the contract might be useful to other systems(classes other than this hierarchical structure). But, at the same time it may not be relevant to some other classes in the same hierarchical systems.

Solution: Use Interface - declare abstract methods (contract with child classes). Define concrete methods (for predefined common functionality) in the non-abstract parent class.

Example:

public interface Flight {
       public void fly();
} 

public abstract class Vehicle {
         public abstract void fly(); //wrong to have here; this will make bicycle to fly
         public final void printVehicleDetails() { }
}

public class Plane extends Vehicle implements Flight {    
         public void fly() { }
}
public class Helicopter extends Vehicle implements Flight {
         public void fly() { }
}

          public class Bicycle extends Vehicle {
          }

Explanation: In this example you can not have abstract method fly() in your abstract parent class, as it would lead to different behavior to other child classes like Bicycle; these can't fly. Hence, move this fly() behavior to an Interface and use it in the child classes like Plane, Helicopter where it is relevant.


Scenario 3: Combination of Scenario 1 and 2. I want to have

• Common functionality to the child classes
• Some contracts which are suitable for all the classes in the same hierarchical systems, but may not be useful outside the same hierarchical systems (candidate for Abstract class)
• Some contracts which may not be relevant to all the classes in the same hierarchical systems, but, might be useful outside this hierarchical system (candidate for Interface)

Solution: Abstract class and Interface

Example:

public interface Flight {
       public void fly();
} 

public abstract class Vehicle {
         public abstract void startVehicle();
         public abstract void stopVehicle();
         public final void printVehicleDetails() { }
}

public class Plane extends Vehicle implements Flight { //start(), stop(), and fly()  
         public abstract void startVehicle() { }
         public abstract void stopVehicle() { }
         public void fly() { }
}
public class Helicopter extends Vehicle implements Flight { //start(), stop(), and fly()
         public abstract void startVehicle() { }
         public abstract void stopVehicle() { }
         public void fly() { }
}

          public class Bicycle extends Vehicle { //will have only start() and stop()
                   public abstract void startVehicle() { }
                   public abstract void stopVehicle() { }
          }

Explanation:
In the above example the concrete methods startVehicle(), and stopVehicle() are suitable for all the Vehicle types. Hence, you can accommodate into an Abstract class. But, the fly() abstract method is not relevant to all the Vehicle types. Hence, move this behavior to an Interface.

How do I understand Node.js, as a Java Developer?

Whenever we want to learn something new we relate the new thing to the one which we have already known. This helps learning things quickly. I had once learnt DotNet using my Java experience and knowledge.

In the similar passion I wanted to learn Node.js using my existing knowledge on Java. It helped really to understand the basics of Node.js quickly and easily.

I have shared my learnings as an article in DZone.  I hope you would find it interesting and useful!

Happy learning!!!

Simplifying Validations Using the Strategy(Enum) Pattern

In one of my previous projects we had to perform series of complex validations to prepare some input data. Below will explain the scenario.

• Call a service and perform some complex validations based on this service response to prepare one of the input values for using it in another service call. 

• You may need to check for multiple inputs with separate complex logic, but at the end you would select only a particular input for another service call.

• You decide the number of inputs. The validation will be performed for all these inputs. You may add or remove number of inputs at any point of time so, we need a flexible way to specify that.

Generally, we use multiple if...else blocks, but, it would lead to readability, maintainability issues. Hence, we came up with an approach of using strategy pattern via Enum. 

The approach is explained with working sample code and posted in DZone site. Hope this will be useful.