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Rarely asked Interview Questions and Answers - 72 question(s) found - Order By Newest

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 Q1. What are the common uses of "this" keyword in java ?Core Java
Ans. "this" keyword is a reference to the current object and can be used for following -

1. Passing itself to another method.

2. Referring to the instance variable when local variable has the same name.

3. Calling another constructor in constructor chaining.

  Sample Code for this keyword

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 Q2. Can we use both "this()" and "super()" in a constructor ?Core Java
Ans. No, because both this and super should be the first statement.

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 Q3. How does making string as immutable helps with securing information ? How does String Pool pose a security threat ?Core Java
Ans. String is widely used as parameter for many java classes, e.g. network connection, opening files, etc. Making it mutable might possess threats due to interception by the other code segment or hacker over internet.

Once a String constant is created in Java , it stays in string constant pool until garbage collected and hence stays there much longer than what's needed. Any unauthorized access to string Pool pose a threat of exposing these values.


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   Like         Discuss         Correct / Improve     Security  String pool   string immutable  immutability      expert        rare


 Q4. Can we declare static variables as transient ?Core Java
Ans. It's weird that compiler doesn't complain if we declare transient with static variable because it makes no sense. At least a warning message saying "transient is useless in this situation" would have helped with code cleaning.

Static variables are never serialized and transient is an indication that the specified variable shouldn't be serialized so its kind of double enforcement not to serialize.

It could be that as it makes no different to the variable behavior and hence using both keywords with a variable are permitted.

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 Q5. What are concepts introduced with Java 5 ?Core Java
Ans. Generics , Enums , Autoboxing , Annotations and Static Import.

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 Q6. Does Constructor creates the object ?Core Java
Ans. New operator in Java creates objects. Constructor is the later step in object creation. Constructor's job is to initialize the members after the object has reserved memory for itself.

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 Q7. Can constructors be synchronized in Java ?Core Java
Ans. No. Java doesn't allow multi thread access to object constructors so synchronization is not even needed.

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   Like         Discuss         Correct / Improve     synchronization   synchronize   constructor   java   multithreading   yes-no   advanced     Asked in 1 Companies      expert        rare


 Q8. Will this code give error if i try to add two heterogeneous elements in the arraylist. ? and Why ?

List list1 = new ArrayList<>();
list1.add(5);
list1.add("5");
Ans. If we don't declare the list to be of specific type, it treats it as list of objects.

int 1 is auto boxed to Integer and "1" is String and hence both are objects.

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 Q9. What are the Wrapper classes available for primitive types ?Core Java
Ans. boolean - java.lang.Boolean
byte - java.lang.Byte
char - java.lang.Character
double - java.lang.Double
float - java.lang.Float
int - java.lang.Integer
long - java.lang.Long
short - java.lang.Short
void - java.lang.Void

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 Q10. Which String class does not override the equals() and hashCode() methods, inheriting them directly from class Object?Core Java
Ans. java.lang.StringBuffer.

  Sample Code for StringBuffer

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 Q11. What is a ConcurrentHashMap ?Core Java
Ans. ConcurrentHashMap is a hashMap that allows concurrent modifications from multiple threads as there can be multiple locks on the same hashmap.

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 Q12. Can you write a "Hello World" program without using any ";" within it?Core Java
Ans. Yes, That is possible

class A {
public static void main(String args[]){
if(System.out.printf("Hello World")==null){}
}
}

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 Q13. Can we serialize static variables ?Core Java
Ans. No. Only Object and its members are serialized. Static variables are shared variables and doesn't correspond to a specific object.

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Usually asked only to fresh graduates.
 Q14. What is the difference between time slicing and preemptive scheduling ?Operating System
Ans. In preemptive scheduling, highest priority task continues execution till it enters a not running state or a higher priority task comes into existence. In time slicing, the task continues its execution for a predefined period of time and reenters the pool of ready tasks.

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 Q15. Are constructors inherited? Can a subclass call the parent's class constructor? When?Core Java
Ans. You cannot inherit a constructor. That is, you cannot create a instance of a subclass using a constructor of one of it's superclasses. One of the main reasons is because you probably don't want to override the superclasses constructor, which would be possible if they were inherited. By giving the developer the ability to override a superclasses constructor you would erode the encapsulation abilities of the language.

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 Q16. In which cases , moving methods to utility class could be useful ?Core Java
Ans. It could be worthy to move a method to util class if the method needs to be shared, doesn't require polymorphic behavior and need not be overridden in special cases.

Don't belong to one group through is-a relationship ( You can share through parent class method )

Don't implement a specific interface ( java 8 default methods )

Doesn't involve complex computing as you will be loosing the benefit of object state with just static method.

Doesn't require polymorphic behavior as static methods don't participate in runtime polymorphism.

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   Like         Discuss         Correct / Improve     utility classes   util classes   static methods   application design        rare


 Q17.  What is the role of JSON.stringify ?Json
Ans.  JSON.stringify() turns an object into a JSON text and stores that JSON text in a string. 

So If we stringfy above notation , it will become

{"name":"xyz","gender":"male";"age":30}

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 Q18. What is URL?
Java EE
Ans. URL is Uniform Resource Locator which is representation of HTTP address.

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 Q19. Can I import same package/class twice? Will the JVM load the package twice at runtime?Core Java
Ans. One can import the same package or same class multiple times. Neither compiler nor JVM complains wil complain about it. And the JVM will internally load the class only once no matter how many times you import the same class.

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 Q20. Difference between boolean and Boolean ?Core Java
Ans. boolean is a primitive type whereas Boolean is a class.

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 Q21. Can we declare an abstract method private ?
Ans. No Abstract methods can only be declared protected or public.

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 Q22. What is Externalizable interface?Core Java
Ans. Externalizable is an interface which contains two methods readExternal and writeExternal. These methods give you a control over the serialization mechanism.

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 Q23. Have you heard about the external table feature of Oracle ?Database
Ans. The external tables feature is a complement to existing SQL Loader functionality. It enables to access data in external sources as if it were in a table in the database. We have used it few times for replicating tables across different database systems.

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 Q24. How Marker interfaces works internally ?Core Java
 This question was recently asked at 'GGK Technologies'.This question is still unanswered. Can you please provide an answer.


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 Q25. What is servlet Chaining ?Java EE
Ans. Multiple servlets serving the request in chain.

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 Q26. Which interface does java.util.Hashtable implement?Core Java
Ans. Java.util.Map

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 Q27. Difference between long.Class and Long.TYPE ?
Ans. They both represent the long primitive type. They are exactly the same.

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 Q28. What are transient variables in java?Core Java
Ans. Transient variables are variable that cannot be serialized.

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 Q29. Which memory segment loads the java code ?Core Java
Ans. Code segment.

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 Q30. Can a lock be acquired on a class ?
Ans. Yes, a lock can be acquired on a class. This lock is acquired on the class's Class object.

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