Learn Programming in JAVA with Free Lessons & Tips

Class is a template or a blueprint which is used to describe an object.

On other hand Object is a reference of a class which follows all the stuff written inside the class.

How about taking the whole tour in the following video

In the previous lession, we have learnt how to use filters and collectors. In filter we have passed the condition to evaluate whether the object is eligible to be filtered or not. Code given below for reference and highlighted in bold.

public class NowJava8 {

public static void main(String[] args) {

List lines = Arrays.asList("abc", "xyz", "shiva");

List result = lines.stream() 
.filter(line -> !"shiva".equals(line)) 
.collect(Collectors.toList()); 

result.forEach(System.out::println); //output : abc,xyz

}

}

Now, what if I would like to same condition in different places to filter on different collections. We would end up repleating same piece of code.
What does it mean? we break DRY (Do not repeat yourself). 

Here to solve this we can use predicates. Example given below.

Java interview your java should be much strong then J2EE. core java and Advance java is the basic foundation for Interview. Some of the topic about which you should know before going for a java interview are mentioned below:

1-JRE, JDK & JVM

2-OOPS

3-Different scenarios related to oops like scenarios based on compile-time and runtime polymorphism, different type of inheritence

4-exception hierarchy, checked and unchecked exception, Different scenarios related to exceptions

5-Threads , their states and ways of creating threads.

6-Collection framework and difference between different collections.

7-Generics.

8-Hashcode and equals and their contract.

9-Difference between:

         String & StringBuffer

         equals and ==

         checked and unchecked exceptions

         interface and abstract class

          Thread class and runnable interface.

• Not purely Object Oriented: It doesn't support multiple inheritence, it supports primitive data types and static members.
• Doesn’t support multiple inheritance: Reason is diamond problem i.e., if both parent classes have same method & not overridden by child, then at runtime ambiguity occurs.
• Platform Independent: Compiled into platform independent bytecode.
• Robust: Provides compile-time and runtime checks in order to eliminate error prone situations.
• Multithreaded: Supports performing multiple tasks simultaneously.
• Interpreted: Java bytecode is translated on the fly to native machine instructions and is not stored anywhere.
• High Performance: Uses JIT compiler.
• Secure.
• Architectural neutral.
• Portable.
• Distributed.
• Dynamic.

Lesson-1:

Class, Object and Methods of Java can be compared with body, heart and brain. Without these 3 things, Java does not exist. If you Google the above 3 items, you will find many definitions. But be honest and think how many of you can really understand the actual concept. Let me explain it in my way:

Class: I would say a class is a community, like Human or Indian.

Object: Object belongs to a particular community/class and holds behaviours of that class. Like you, me, he, she etc who belong to or represent the human community.

Method: Method defines some specific functionality of the belonging class. As for example eating, speaking is some functionalities of human class. But the functionality many vary from object to object. Like you speaks English and I speaks the Hindi language. We both are objects of the Human class.

Prototype Design Pattern:

Ø Prototype pattern refers to creating duplicate object while keeping performance in mind.

Ø This pattern involves implementing a prototype interface which tells to create a clone of the current object.

Ø This pattern is used when creation of object directly is costly. For example, an object is to be created after a costly database operation.

Ø We can cache the object, returns its clone on next request and update the database as and when needed thus reducing database calls.

 Prototype DP involves cloning of object and thus object needs to:

 Ø Implement Cloneable interface

 Ø Override clone() of Object class

 public abstract class Payment implements Cloneable {

       protected double amount;

        public double getAmount() {

                return amount;

           }

           public void setAmount(double amount) {

                this.amount = amount;

           }

     public abstract void makePayment();

     public Object clone() {

                 Object clone = null;

                 try {

                    clone = super.clone();

                 } catch (CloneNotSupportedException e) {

                    e.printStackTrace();

                 }

                 return clone;

              }   

}

public class CreditCard extends Payment {

     public void makePayment() {

           System.out.println("Payment of "+amount+ " by Credit Card");

     }

}

public class CashPayment extends Payment {

     public void makePayment() {

           System.out.println("Payment of "+amount+" by Cash");

     }

}

public class PaymentCache {

     private Map<String, Payment> map = new HashMap();

     public Payment getPayment(String key) {

           Payment payment = map.get(key);

           return (Payment) payment.clone();

     }

     // Map of Objects which are frequently used...

     // Original objects are acting as template objects...

     public void loadCache() {

           CashPayment cash = new CashPayment();

           cash.setAmount(2000);

           map.put("cash", cash);

           CreditCard credit = new CreditCard();

           credit.setAmount(4000);

           map.put("credit", credit);

     }

}

public class Client {

     public static void main(String[] args) {

           PaymentCache cache= new PaymentCache();

           cache.loadCache();

           Payment payment=cache.getPayment("cash");

           payment.makePayment();

           Payment payment1=cache.getPayment("credit");

           payment1.makePayment();

     }

}

Output:

Payment of 2000.0 by Cash

Payment of 4000.0 by Credit Card

Application:

Session replication from one server to another server.

Generating the GUI having many numbers of similar controls.

Advantage of Prototype Pattern:

It reduces the need of subclassing.

It hides complexities of creating objects.

The clients can get new objects without knowing which type of object it will be.

It lets you add or remove objects at runtime.

Usage of Prototype Pattern:

When the classes are instantiated at runtime.

When the cost of creating an object is expensive or complicated.

When you want to keep the number of classes in an application minimum.

When the client application needs to be unaware of object creation and representation.

How to create a Singleton class:

Q) What is a singleton class?

A) In simple words, a singleton class is a class which can have only one instance at any point of time throughout the application and provides a global point of access to its instance.

Q) How can you create a singleton class?

A)

class Singleton {

       public static final Singleton INSTANCE = new Singleton();

       private Singleton() {

       }

}

                 This is a simple example for a singleton class where I used a public static final INSTANCE variable and initialized it with a Singleton object. Since INSTANCE is public static final, it can’t be reassigned and it is thread safe as well.

                 Singleton’s constructor is made private to prevent the instantiation of Singleton class from other classes. That means no other class can create an object for Singleton class with the new() operator and also no other class can extend this Singleton class with extends keyword. If any class wants to have an instance of Singleton it can have by accessing INSTANCE field of Singleton class like Singelton. INSTANCE as its static.

1. Q) So here, Singleton instance will be created at the time of class loading that means when the Singleton class is loaded by JVM into memory. So it’s an early loading/instantiation of Singleton class and it occupies memory even when its not required yet, so what If we want this singleton instance to be created when it’s really required?

1. A) I understand that you are interested in lazy loading/instantiation of Singleton. Here it is

class Singleton {

       private static Singleton INSTANCE;

       private Singleton() {

       }

       public static synchronized Singleton getInstance() {

              if (INSTANCE == null) {

                     INSTANCE = new Singleton();

              }

              return INSTANCE;

       }

}

1. Q) You synchronized the whole method

       public static synchronized Singleton getInstance()

Don’t you see any performance overhead with this approach?

Only one thread can access the method getInstance at a time to get the Singleton instance and rest of all other threads need to wait even though INSTANCE is not null as its already created.

A) Let me create it using synchronized block instead of synchronized method, here how it goes

class Singleton {

       private static Singleton INSTANCE;

       private Singleton() {

       }

       public static Singleton getInstance() {

              if (INSTANCE == null) {             //----- 1st

                     synchronized (Singleton.class) {

                           if (INSTANCE == null) {   //----- 2nd

                                  INSTANCE = new Singleton();

                           }

                     }

              }

              return INSTANCE;

       }

}

1. Q) Why do you use null check twice here?
2. A) I think I don’t need to explain you why did I use first null check, as you know I am going to create Singleton instance in case INSTANCE is null otherwise I ll return the already created instance.

2nd null check is being used to ensure thread safety.

For suppose, 2 threads are concurrently trying to get the access for getInstance() and two threads are gone through the 1st null check and trying to enter into synchronized block, but only one thread(assume thread-1) got the lock on Singleton.class’s  object and enters the synchronized block while the other thread (assume thread-2)waiting for the lock. Once thread-1 went through 2nd null check then it creates a Singleton instance as INSTANCE is null and leaves the synchronized block and thus releases lock. Now, the waiting thread ( thread-2) holds the lock and enters the synchronized block.

If there is no 2nd null check there then there is a chance for the thread-2 to create another instance of Singleton class and thus it breaks the singleton here. For that reason we have 2nd null check here. So the thread-2 sees that INSTANCE is not null and comes out from the synchronized block without creating Singleton instance.

1. Q) Ok. So double checking is for thread safety of a singleton.

1. A) yes

Q) Do you think that, this Singleton class is still singleton?

I can break your singleton with the help of reflection like below

public class Test {

       public static void main(String args[]) {

              Singleton singleton1 = Singleton.getInstance();

              Singleton singleton2 = null;

              try {

                     // Test is package name contains Singleton class

                     Class singletonClass = Class.forName("Test.Singleton");

                     // as we know there is only one private constructor

                     Constructor cons = singletonClass.getDeclaredConstructor();

                     cons.setAccessible(true);

                     singleton2 = (Singleton) cons.newInstance();

                     System.out.println(singleton1 == singleton2 ? "Objects are equal" : "Objects are different");

              } catch (ClassNotFoundException | InstantiationException

                           | IllegalAccessException | IllegalArgumentException

                           | InvocationTargetException | NoSuchMethodException

                            | SecurityException e) {

                     e.printStackTrace();

              }

       }

}

1. A) In that case I can implement Singleton like below for not to be broken with reflection

class Singleton {

       private static Singleton INSTANCE;

       private Singleton() {

              if (INSTANCE != null)

                     throw new RuntimeException(

                                  "Singleton can not be created more than once.");

       }

       public static Singleton getInstance() {

              if (INSTANCE == null) {

                     synchronized (Singleton.class) {

                           if (INSTANCE == null) {

                                  INSTANCE = new Singleton();

                           }

                     }

              }

              return INSTANCE;

       }

}

Private constructor will be called to instantiate the Singleton object through reflection. So we can throw some exception If INSTANCE field is not null (that means its already created).

1. Q) So you think this implementation saves your Singleton from being broken from reflection. In my previous program, I am trying to create a Singleton object (singleton1) with the help of getInstance() and after that trying to create another object (singleton2) with the help of reflection. What happens If I do this in reverse like below

public class Test {

       public static void main(String args[]) {

//            Singleton singleton1 = Singleton.getInstance();

              Singleton singleton2 = null;

              try {

                     // Test is package name contains Singleton class

                     Class singletonClass = Class.forName("Test.Singleton");

                     // as we know there is only one private constructor

                     Constructor cons = singletonClass.getDeclaredConstructor();

                     cons.setAccessible(true);

                     singleton2 = (Singleton) cons.newInstance();

                     Singleton singleton1 = Singleton.getInstance();

                     System.out.println(singleton1 == singleton2 ? "Objects are equal" : "Objects are different");

              } catch (ClassNotFoundException | InstantiationException

                            | IllegalAccessException | IllegalArgumentException

                           | InvocationTargetException | NoSuchMethodException

                           | SecurityException e) {

                     e.printStackTrace();

              }

       }

}

Output:

 Objects are different

Here, If I try to create a Singleton object (singleton2) with the help of reflection first then it can proceed to create an Singleton object as the static field INSTANCE is still null at that time as getInstance() is not yet invoked. So we are allowed to create an many as objects we want for the Singleton using reflection before getInstance() is invoked and thus breaks the singleton. What do you say?

1. A) Yeah, you are right. Let me come to you with slightly modified implementation of Singleton which protects itself from being broken with reflection.

class Singleton {

       private static Singleton INSTANCE;

       private static boolean instanceCreated = false;

       private Singleton() {

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