Core Java - Interview Questions and Answers for 'Core java' | Search Interview Question

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Q961. Different ways of implementing Threads in Java ?

Core Java

Ans. Threads in Java can be implement either by Extending Thread class or implementing runnable interface.

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Frequently asked question for intermediate developers. Frequently asked in HCL Technologies and EPAM.

Q962. What is Volatile keyword used for ?

Core Java

Ans. Volatile is a declaration that a variable can be accessed by multiple threads and hence shouldnt be cached.

Sample Code for volatile

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Frequently asked in Cognizant (CTS)

Q963. What is Serialization ? Why do we need it ?

Core Java

Ans. Storing the state of an object in a file or other medium is called serialization.

Classes can communicate only if they are built together ( as they need Byte code for communication ). What if we need to enable communication between different applications ( i.e they have been built independently or even they reside at different locations ), We need a mechanism that will transfer the Bean state to a Medium than can be transferred to the receiving application.

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Q964. What is the use of Transient Keyword ?

Core Java

Ans. It in Java is used to indicate that a field should not be serialized.

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Q965. What is a Final Method ?

Core Java

Ans. A Method that cannot be overriden in the sub class.

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Very frequently asked. Usually asked with questions related to String. Frequently asked at CTS / Cognizant

Q966. What is an Immutable Object ?

Core Java

Ans. Object that can't be changed after instantiation.

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Very Frequently asked. Usually asked along with String Class related questions.

Q967. What is an immutable class ?

Core Java

Ans. Class using which only immutable (objects that cannot be changed after initialization) objects can be created.

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Q968. How to implement an immutable class ?

Core Java

Ans. We can make a class immutable by

1. Making all methods and variables as private.

2. Setting variables within constructor.

Public Class ImmutableClass{

private int member;
ImmutableClass(int var){
member=var;
}
}

and then we can initialize the object of the class as

ImmutableClass immutableObject = new ImmutableClass(5);

Now all members being private , you cant change the state of the object.

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Q969. Does Declaring an object "final" makes it immutable ?

Core Java

Ans. Only declaring variables as final makes them immutable. Making objects final means that the object handler cannot be used to target some other object but the object is still mutable.

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Q970. Difference between object instantiation and construction ?

Core Java

Ans. Though It's often confused with each other, Object Creation ( Instantiation ) and Initialization ( Construction ) are different things in Java. Construction follows object creation.

Object Creation is the process to create the object in memory and returning its handler. Java provides New keyword for object creation.

Initialization is the process of setting the initial / default values to the members. Constructor is used for this purpose. If we don't provide any constructor, Java provides one default implementation to set the default values according to the member data types.

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Q971. Can we override static methods ? Why ?

Core Java

Ans. No.

Static methods belong to the class and not the objects. They belong to the class and hence doesn't fit properly for the polymorphic behavior.

A static method is not associated with any instance of a class so the concept of overriding for runtime polymorphism using static methods is not applicable.

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Q972. Can we access instance variables within static methods ?

Core Java

Ans. Yes.we cannot access them directly but we can access them using object reference.Static methods belong to a class and not objects whereas non static members are tied to an instance. Accessing instance variables without the instance handler would mean an ambiguity regarding which instance the method is referring to and hence its prohibited.

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Q973. Can we reduce the visibility of the inherited or overridden method ?

Core Java

Ans. No.

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Q974. Give an Example of checked and unchecked exception ?

Core Java

Ans. ClassNotFoundException is checked exception whereas NoClassDefFoundError is a unchecked exception.

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Q975. Name few Java Exceptions ?

Core Java

Ans. IndexOutofBoundException ,
NoClassDefFoundException ,
OutOfMemoryException ,
IllegalArgumentException,
NullPointerException,
IllegalStateException

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Frequently asked to fresh graduates and less experienced.

Q976. Which of the following is tightly bound ? Inheritance or Composition ?

Core Java

Ans. Inheritance.

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Q977. Explain the use of "Native" keyword ?

Core Java

Ans. Used in method declarations to specify that the method is not implemented in the same Java source file, but rather in another language

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Q978. Difference between boolean and Boolean ?

Core Java

Ans. boolean is a primitive type whereas Boolean is a class.

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Q979. What is a finalize method ?

Core Java

Ans. finalize() method is called just before an object is destroyed.

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Very frequently asked in phone and walk in interviews.

Q980. What are Marker Interfaces ? Name few Java marker interfaces ?

Core Java

Ans. An interface without any method declaration is called as marker interface. there are 3 in-built interfaces in JVM i.e. serializable, clonable, remote

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Q981. Is runnable a Marker interface ?

Core Java

Ans. No , it has run method declared.

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Q982. Difference between Serialization and Deserialization ?

Core Java

Ans. Serialization is the process of writing the state of an object to a byte stream. Deserialization is the process of restoring these objects.

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Q983. Explain Autoboxing ?

Core Java

Ans. Autoboxing is the automatic conversion that the Java compiler makes between the primitive types and their corresponding object wrapper classes

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Q984. What is an Enum type ?

Core Java

Ans. An enum type is a special data type that enables for a variable to be a set of predefined constants

Sample Code for enum

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Q985. What are Wrapper Classes ? What are Primitive Wrapper Classes ?

Core Java

Ans. A wrapper class is any class which "wraps" or "encapsulates" the functionality of another class or component. A Wrapper Class that wraps or encapsulates the primitive data type is called Primitive Wrapper Class.

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Q986. What is "Import" used for ?

Core Java

Ans. Enables the programmer to abbreviate the names of classes defined in a package.

Sample Code for import

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Q987. Different types of memory used by JVM ?

Core Java

Ans. Class , Heap , Stack , Register , Native Method Stack.

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Q988. What is a class loader ? What are the different class loaders used by JVM ?

Core Java

Ans. Part of JVM which is used to load classes and interfaces.

Bootstrap , Extension and System are the class loaders used by JVM.

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Q989. What is a Static import ?

Core Java

Ans. By static import , we can access the static members of a class directly without prefixing it with the class name.

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Very frequently asked. Usually difference between String,StringBuffer and StringBuilder is asked in different variations.

Q990. Difference between StringBuffer and StringBuilder ?

Core Java

Ans. StringBuffer is synchronized whereas StringBuilder is not.

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