There is the list of core java interview questions. If there is any core java interview question that have been asked to you, kindly post it in the ask question section. We assure that you will get here the 90% frequently asked interview questions and answers. The answers of the core java interview questions are short and to the point. The core java interview questions are categorized in Basics of java interview questions, OOPs interview questions, String Handling interview questions, Multithreading interview questions,collection interview questions, JDBC interview questions etc.

BASIC JAVA - INTERVIEW QUESTIONS AND ANSWERS

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Java OOPS Concepts ?

Ans. Abstraction , Encapsulation , Inheritance and Polymorphism.

Difference between Abstract and Concrete Classes ?

Ans. Abstract classes are only meant to be sub classed and not meant to be instantiated whereas concrete classes are meant to be instantiated.

Difference between Overloading and Overriding ?

Ans.

Overloading - Similar Signature but different definition , like function overloading.

Overriding - Overriding the Definition of base class in the derived class.

Types of Inner classes ?

Ans. Simple Inner Class, Local Inner Class, Anonymous Inner Class , Static Nested Inner Class.

Difference between TreeMap and HashMap ?

Ans. They are different the way they are stored in memory. TreeMap stores the Keys in order whereas HashMap stores the key value pairs randomly.

Difference between List , Sets and Maps ?

Ans. Lists - Members are stored in sequence in memory and can be accessed through index. Sets - There is no relevance of sequence and index. Sets doesn't contain duplicates whereas multiset can have duplicates. Maps - Contains Key , Value pairs.

Difference between Private , Public and Protected ?

Ans. Private - Not accessible outside object scope. Public - Accessible outside object scope. Protected - Becomes private when inherited.

What is role of Synchronization in Java ?

Ans. Managing exclusive access of the code block to single thread at a time to maintain its integrity.

What is Default Access ?

Ans. Accessible only by the objects in the same package.

Difference between Vector and ArrayList ?

Ans. Vectors are synchronized whereas Array lists are not.

Difference between class and objects ?

Ans. Class is a template using which objects are created in memory.

Describe Garbage Collection in Java ?

Ans. Mechanism by which Java reclaims the memory inaccessible by the application.

What are the types of Polymorphism in Java ?

Ans. Static ( function overloading ) and Run time ( Virtual functions )

Different ways of using threads ?

Ans. Extending Thread class and implementing runnable interface.

Difference between Composition and Inheritence ?

Ans. Has a relationship vs Is a relationship.

What are Static variables ?

Ans. Correspond to class and not objects. All objects share that variable.

Difference between Interfaces and Abstract classes ?

Ans. Abstract classes have a body ( member elements ) where as Interfaces just have methods and static member declarations.

Constructor ?

Ans. Its an operation that creates and initializes the object.

Dynamic Binding ?

Ans. Association of function call to function definition during run time.

Volatile ?

Ans. Volatile is a declaration that a variable can be accessed by multiple threads and hence shouldn't be cached.

Serialization ?

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

Transient ?

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

Static Methods ?

Ans. Can only operates on static variables.

Final variable ?

Ans. Variable constant. Variable value can't be changed after instantiation.

Final Methods ?

Ans. Can't be overridden

Final Classes ?

Ans. Can't be sub classed.

28 Immutable Object ?

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

checked and unchecked exceptions ?

Ans. For checked exceptions compiler throws a errors if they are not checked whereas unchecked exceptions and caught during runtime only and hence can't be checked.

30 Example of checked and unchecked exception ?

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

Singleton Class ?

Ans. Using which only 1 object can be created.

Name few Java exceptions ?

Ans. IndexOutofBound , NoClassDefFound , OutOfMemory , IllegalArgument.

Name few Design Patterns used for designing Java applications ?

Ans. Singleton , Factory , Abstract Factory , Proxy , Command , Builder.

Give an example of Runtime Polymorphism ?

Ans. Action Mapping in a web application , Generics.

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

Ans. Composition.

Give an Example of Generics in Java 5 ?

Ans. List<number> aList = new ArrayList < number>

How can we make sure that a code segment gets executed even in case of uncatched exceptions ?

Ans. By putting it within finally.

Can a class extend an interface ?

Ans. No Class implement the interface. An interface can extend another interface.

Does Java support multiple inheritence ?

Ans. No. We can't extend multiple classes but can implement multiple interfaces.

Can we use an interface reference for referring to an object ?

Ans Yes we can do it for object implementing that interface.

What do you mean by return type Void ?

Ans. Returns Nothing.

Is Java Compiled or an Interpreted language ?

Ans Both

What is a class file ?

Ans. Compiled Java files.

Why Java is a called a platform independent language ?

Ans. Java creates the pre compiled files ( class files ) which can then be interpreted on multiple platforms.

Native ?

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

super ?

Ans. Used to access members of the base class.

What is "this" keyword used for ?

Ans. Used to represent an instance of the class in which it appears

Transient ?

Ans. Declares that an instance field is not part of the default serialized form of an object

What will happen if we declare a class abstract as well as final ?

Ans. Compile time error.

50 Encapsulation ?

Ans. Encapsulation is the mechanism that binds together code and data it manipulates and keeps both safe from outside interference and misuse

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Inheritence ?

Ans. Inheritance is the process by which one object acquires the properties of another object.

Polymorphism ?

Ans. Polymorphism is the feature that allows one interface to be used for general class actions

Difference between assignment and initialization ?

Ans. Assignment can be done anytime but initialization can be done only once during object instantiation.

difference between boolean and Boolean ?

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

Immutable ?

Ans. Objects that can't be changed after initialization ?

Give an example of immutable class ?

Ans. String

Casting ?

Ans. Process to convert one data type to another.

Finalize () ?

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

Marker Interfaces ?

Ans. These are the interfaces which have no declared methods.

Name few Java marker interfaces ?

Ans. Serializable and cloneable.

Is runnable a Marker interface ?

Ans. No , it has run method declared.

62 String and StringBuffer ?

Ans. String is an immutable class where StringBuffer is not.

Process and Thread ?

Ans. Process is a program in execution whereas thread is a separate path of execution in a program.

Thread states ?

Ans. ready, running, waiting and dead.

Deadlock ?

Ans. When two threads are waiting each other and can’t precede the program is said to be deadlock.

Serialization and Deserialization ?

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

Difference between Java 1.4 and Java 5 ?

Ans. Generics , Autoboxing , Enum and Static Imports.

Autoboxing ?

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

Enum ?

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

Wrapper Classes ?

Ans. A wrapper class is any class which "wraps" or "encapsulates" the functionality of another class or component.

Primitive Wrapper Classes ?

Ans. A Wrapper Class that wraps or encapsulates the primitive data type.

What is automatic conversion of primitive data type to its Wrapper class called ?

Ans. Autoboxing.

What Design pattern Wrapper Classes implement ?

Ans. Adapter.

Import ?

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

Servlet Chaining ?

Ans. Servlet chaining is a technique in which two or more servlets can cooperate in servicing a single request.

What is the difference between set and list?

Ans. Set stores elements in an unordered way but does not contain duplicate elements, whereas list stores elements in an ordered way but may contain duplicate elements. List can also holds null elements while Set can't.

Difference between HashMap and HashTable ?

Ans. The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls.

Iterator ?

Ans. Some of the collection classes provide traversal of their contents via a java.util.Iterator interface. This interface allows you to walk through a collection of objects, operating on each object in turn.

JVM ?

Ans. Java Virtual Machine or JVM is an abstract which provides the runtime environment for the Java byte code to run.

JDK ?

Ans. Java Development Kit or JDK is physical entity that comprises of JRE and Development Tools.

JRE ?

Ans. Java Run Time Environment is the implementation of JVM.

different types of memory used by JVM ?

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

Class Loader ?

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

Does Static Public Void instead of Public static Void gives compilation error ?

Ans. No

Does constructor returns any value ?

Ans. Yes , the current instance or object of the class.

Can we make constructor final ?

Ans. No

Why main method is static ?

Ans. Object needs to be created for calling non static methods.

Static Block ?

Ans. It is executed before main method at the time of class loading.

Can we execute program without mail method ?

Ans. Yes , one way is through static block.

Cloning ?

Ans. Keyword to create the exact copy of the object.

Can we overload main method ?

Ans. Yes.

Can we declare main method as final ?

Ans. Yes.

static binding ?

Ans. Associating function call and its definition during compile time.

Dynamic Binding ?

Ans. Associating function call and its definition during runtime.

Can we declare an interface method static ?

Ans. No

Can an interface be final ?

Ans. No

Can we define private , protected modifiers with variables in interface ?

Ans. No, they are implicitely public.

Static import ?

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

Base class for Error and Exceptions ?

Ans. Throwable

StringBuffer and StringBuilder ?

Ans. StringBuffer is synchronized whereas String Builder is not.

Default Package for Collection classes ?

Ans. Java.Util

Difference between List and Queue ?

Ans. In queue access can only happen at the ends whereas we can access any element of the list.

Difference between Vector and ArrayList ?

Ans. Vectors are synchronized whereas ArrayList are not.

Which is the base interface for all collection classes ?

Ans. Collection interface.

How can we make HashMap synchronized ?

Ans. Map m = Collections.synchronizedMap(hashMap);

Difference between Map and HashMap ?

Ans. Map is an interface where HashMap is the concrete class.

Property Class ?

Ans. The properties class is a subclass of Hashtable that can be read from or written to a stream.

108 Why Java is a preferred technology for Web Applications ?

Ans. Web application involves dealing with multiple requests and thread and hence deals with lot of memory allocation and de allocation. Hence Java with its intrinsic capability of Garbage collection offer a perfect platform.

What are concepts introduced with Java 5 ?

Ans. Generics , Enums , Autoboxing , Annotations and Static Import.

Difference between get and post ?

Ans. Request parameters in get are passed within url whereas with post these are passed within post body as object.

Difference between C++ and Java ?

Ans. We have pointers in C++ , We have garbage collection in Java.

Name few Design Patterns ?

Ans. Singleton , Adapter , Factory , Abstract Factory , Builder , Facade , Observer , Proxy , Prototype.

Give an Example of Factory Design Pattern ?

Ans. Action Mapping within Web Application.

Give an example of Adapter design pattern ?

Ans. Wrapper Classes.

Can we override static methods ?

Ans. Yes , static methods can be overridden by static methods in sub class.

What precaution should be used while dealing with static variables ?

Ans. Static variables are shared by all objects of the class and hence each instance may update it with its local value.

Can we override main method ?

Ans. No

Difference between server side and client side validation ?

Ans. Client side validation is done using scripting technologies like javascript before the form is submitted to server whereas server side validation is done after the form is submitted to server.

Difference between sleep and wait ?

Ans. Sleep puts thread aside for exactly the time specified. Wait causes a wait of up to time specified. A thread could stop waiting earlier if it receives the notify() or notifyAll() call.

Name Wrapper classes available for primitive types ?

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

Explain System.out.println ?

Ans. System is the class having static variable out that defaults to system console and println is the overloaded method.

Explain public static void main() ?

Ans. public means it has public access, static means that the method can be invoked without instantiating the object, void means it returns nothing.

What is Java Byte Code ?

Ans. Java bytecode is the form of instructions that the Java virtual machine executes.

What is the relation between class file and byte code ?

Ans. Class files holds the instructions in byte code.

How can we make the object immutable ?

Ans. By making it a final class and making all members as private with those being initialized through constructor.

What is the purpose of making an object immutable ?

Ans. Since they cannot change state, they cannot be corrupted by thread interference or observed in an inconsistent state.

Explain the scenerios to choose between String , StringBuilder and StringBuffer ?

Ans.

· If the Object value will not change in a scenario use String Class because a String object is immutable.

· If the Object value can change and will only be modified from a single thread, use a StringBuilder because StringBuilder is unsynchronized(means faster).

· If the Object value may change, and can be modified by multiple threads, use a StringBuffer because StringBuffer is thread safe(synchronized).

Explain java.lang.OutOfMemoryError ?

Ans. This Error is thrown when the Java Virtual Machine cannot allocate an object because it is out of memory, and no more memory could be made available by the garbage collector.

Difference between arraylist and linkedlist ?

Ans. Array list maintains indices like array whereas Linked List maintain pointers to the next element.

Difference between hashtable and hashmap ?

Ans. Hashtable are synchronized whereas hashmaps are not. Hashtable doesn't allow null values whereas HashMaps allows null values.

Can we have multiple servlets in a web application and How can we do that ?

Ans. Yes by making entries in web.xml

Are static members serialized ?

Ans. Static members don't belong to a object and hence are not serialized.

How can we manage Error Messages in the web application ?

Ans. Within message.properties file.

What is the use of Marker interface if they don't hold anything ?

Ans. As the name suggest , They just mark a class and just indicate something to compiler and JVM.

Is JVM, a compiler or interpretor ?

Ans. Its an interpretor.

Is Synchronization an overhead ?

Ans. Synchronization has relevance only if a code segment can be accessed by multiple threads and hence may result in ambiguity. Yes Its an overhead if simultaneous access doesn't pose any problem.

Difference between implicit and explicit type casting ?

Ans. An explicit conversion is where you use some syntax to tell the program to do a conversion whereas in case of implicit type casting you need not provide the data type.

Can we have an abstract class without abstract methods ?

Ans. Yes

Can we have abstract methods in a class which is not Abstract ?

Ans. No

Are list , Map and Set Classes or Interfaces ?

Ans. These are interfaces.

Difference between .equals and == ?

Ans. .equals is the method of object class and is used to match the content of object whereas == matches the objects itself. So == looks to see if they are same object.

What are the methods of Object class ?

Ans. toString , clone , finalize , hashcode , equals.

What are the class loaders ?

Ans. JVM loads the classes through class loaders.

What are the different class loaders used by JVM ?

Ans. Bootstrap , Extension and System.

difference between loadClass and Class.forName ?

Ans. loadClass only loads the class but doesn't initialize the object whereas Class.forName initialize the object after loading it.

How do we load the DB drivers ?

Ans. Using Class.forName

Have you ever destroyed objects yourself ?

Ans. Java by its mechanism of Garbage collection does this itself.

Should we override finalize method ?

Ans. Finalize is used by Java for Garbage collection. It should not be done as we should leave the Garbage Collection to Java itself.

Will finally be executed for handled or unhandled exceptions ?

Ans. Both.

Difference between protected and default Modifier ?

Ans. With protected member is accessible in subclass whereas with Default Member is not accessible in subclass.

Can we overload methods on a single parameter with data types as int and long. For example - int calculate(int x) and int calculate(long x) ?

Ans. Yes

What will happen if we call such method using a number lets say calculate(5) ?

Ans. It will call first method with int argument.

Then whats the use of second method. Will that ever be called ?

Ans. we can call by having argument as 5L.

What is servlet Chaining ?

Ans. Multiple servlets serving the request in chain.

In the following code

String str1 = "abc";
String str2 = "abc";

Which of the following will be true.

str == str2
str.equal(str2)

Ans. str.equal(str2) will be true but str==str2 will be false

In the following code

String str1 = "abc";
String str2 = "xyz";
str1=str2;

Which of the following will be true.

str == str2
str.equal(str2)

Ans. both str.equal(str2) and str==str2 will be true
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