Tag Archives: Core Java

Basic Core Java Concepts

First of all, advance bail on my behalf if my content is insignificant at this point. I thought of writing this blog post after searching through Google Trends and noting that there are good amount of search queries till now looking for these basic core java concepts.

I don’t want to split this blogs into multiple parts as i feel it wouldn’t be good for a reader to go onto various parts and would like to keep it an ever evolving blog post with full support from my readers.


My previous blogs could give some inputs in bulleted points to new Java developers as given below:-

Basic Java FAQ – Part 1

Basic Java FAQ – Part 2

Basic Java FAQ – Part 3

SCJP – Quick Notes

and finally and small mock example to check knowledge

Mock Exam

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SCJP – Quick Notes

Topics Covered


General Tips

Language Fundamantals










Primitive Types

Numeric Literals

Character Literals

String Literals

Class Literals

Operators and Assignments


Numeric Promotion

Unary Operators

Arithmetic Operators

Comparison Operators

Assignmant Operators


Ternary Operator

String Operators

Boolean equals()


Bitwise Vs Logical Operators

Method Invocation

Flow Control and Exception handing



Declarations and Access Control

Access Modifiers

Special Modifiers

this and super



Access Control

Garbage Collection

Overloading and Overriding

Inner Classes

Annonymous Classes



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Basic Java FAQ – Part 1

1)What is OOPs?

Ans: Object oriented programming organizes a program around its data,i.e.,objects and a set of well defined interfaces to that data. An object-oriented program can be characterized as data controlling access to code.

2)What is the difference between Procedural and OOPs?

Ans: a) In procedural program, programming logic follows certain procedures and the instructions are executed one after another. In OOPs Program, unit of program is object, which is nothing but combination of data

and code.

b) In procedural program,data is exposed to the whole program whereas in OOPs program,it is accessible with in the object and which in turn assures the security of the code.

3)What are Encapsulation, Inheritance and Polymorphism?

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

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

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

4)What is the difference between Assignment and Initialization?

Ans: Assignment can be done as many times as desired whereas initialization can be done only once.

5)What are Class, Constructor and Primitive data types?

Ans: Class is a template for multiple objects with similar features and it is a blue print for objects. It defines a type of object according to the data the object can hold and the operations the object can perform.

Constructor is a special kind of method that determines how an object is initialized when created.

Primitive data types are 8 types and they are:

byte, short, int, long

float, double



6)What is an Object and how do you allocate memory to it?

Ans: Object is an instance of a class and it is a software unit that combines a structured set of data with a set of operations for inspecting and manipulating that data. When an object is created using new operator, memory is allocated to it.

7)What is the difference between constructor and method?

Ans: Constructor will be automatically invoked when an object is created whereas method has to be called explicitly.

8)What are methods and how are they defined?

Ans: Methods are functions that operate on instances of classes in which they are defined. Objects can

communicate with each other using methods and can call methods in other classes.

Method definition has four parts. They are name of the method, type of object or primitive type the method

returns, a list of parameters and the body of the method. A method’s signature is a combination of the first

three parts mentioned above.

9)What is the use of bin and lib in JDK?

Ans: Bin contains all tools such as javac, appletviewer, awt tool, etc., whereas lib contains API and all packages.

10)What is casting?

Ans: Casting is used to convert the value of one type to another.

11)How many ways can an argument be passed to a subroutine and explain them?

Ans: An argument can be passed in two ways. They are passing by value and passing by reference.

Passing by value: This method copies the value of an argument into the formal parameter of the subroutine.

Passing by reference: In this method, a reference to an argument (not the value of the argument) is passed to

the parameter.

12)What is the difference between an argument and a parameter?

Ans: While defining method, variables passed in the method are called parameters. While using those methods, values passed to those variables are called arguments.

13)What are different types of access modifiers?

Ans: public: Any thing declared as public can be accessed from anywhere.

private: Any thing declared as private can’t be seen outside of its class.

protected: Any thing declared as protected can be accessed by classes in the same package and subclasses in the other packages.

default modifier : Can be accessed only to classes in the same package.

14)What is final, finalize() and finally?

Ans: final : final keyword can be used for class, method and variables.

A final class cannot be subclassed and it prevents other programmers from subclassing a secure class to invoke insecure methods.

A final method can’ t be overridden

A final variable can’t change from its initialized value.

finalize( ) : finalize( ) method is used just before an object is destroyed and can be called just prior to

garbage collection.

finally : finally, a key word used in exception handling, creates a block of code that will be executed after a

try/catch block has completed and before the code following the try/catch block. The finally block will execute whether or not an exception is thrown.

For example, if a method opens a file upon exit, then you will not want the code that closes the file

to be bypassed by the exception-handling mechanism. This finally keyword is designed to address this


15)What is UNICODE?

Ans: Unicode is used for internal representation of characters and strings and it uses 16 bits to represent each other.

16)What is Garbage Collection and how to call it explicitly?

Ans: When an object is no longer referred to by any variable, java automatically reclaims memory used by that

object. This is known as garbage collection.

System.gc() method may be used to call it explicitly.

17)What is finalize() method ?

Ans: finalize () method is used just before an object is destroyed and can be called just prior to garbage collection.

18)What are Transient and Volatile Modifiers?

Ans: Transient: The transient modifier applies to variables only and it is not stored as part of its object’s

Persistent state. Transient variables are not serialized.

Volatile: Volatile modifier applies to variables only and it tells the compiler that the variable modified by

volatile can be changed unexpectedly by other parts of the program.

19)What is method overloading and method overriding?

Ans: Method overloading: When a method in a class having the same method name with different arguments is said to be method overloading.

Method overriding : When a method in a class having the same method name with same arguments is said to be method overriding.

20)What is difference between overloading and overriding?

Ans: a) In overloading, there is a relationship between methods available in the same class whereas in overriding, there is relationship between a superclass method and subclass method.

b) Overloading does not block inheritance from the superclass whereas overriding blocks inheritance from the superclass.

c) In overloading, separate methods share the same name whereas in overriding,subclass method replaces the superclass.

d) Overloading must have different method signatures whereas overriding must have same signature.

21) What is meant by Inheritance and what are its advantages?

Ans: Inheritance is the process of inheriting all the features from a class. The advantages of inheritance are reusability of code and accessibility of variables and methods of the super class by subclasses.

22)What is the difference between this() and super()?

Ans: this() can be used to invoke a constructor of the same class whereas super() can be used to invoke a super class constructor.

23)What is the difference between superclass and subclass?

Ans: A super class is a class that is inherited whereas sub class is a class that does the inheriting.

24) What modifiers may be used with top-level class?

Ans: public, abstract and final can be used for top-level class.

25)What are inner class and anonymous class?

Ans: Inner class : classes defined in other classes, including those defined in methods are called inner classes.

An inner class can have any accessibility including private.

Anonymous class : Anonymous class is a class defined inside a method without a name and is instantiated and declared in the same place and cannot have explicit constructors.

26)What is a package?

Ans: A package is a collection of classes and interfaces that provides a high-level layer of access protection and name space management.

27) What is a reflection package?

Ans: java.lang.reflect package has the ability to analyze itself in runtime.

28) What is interface and its use?

Ans: Interface is similar to a class which may contain method’s signature only but not bodies and it is a formal set of method and constant declarations that must be defined by the class that implements it.

Interfaces are useful for:

a)Declaring methods that one or more classes are expected to implement

b)Capturing similarities between unrelated classes without forcing a class relationship.

c)Determining an object’s programming interface without revealing the actual body of the class.

29) What is an abstract class?

Ans: An abstract class is a class designed with implementation gaps for subclasses to fill in and is deliberately incomplete.

30) What is the difference between Integer and int?

Ans: a) Integer is a class defined in the java.lang package, whereas int is a primitive data type defined in the Java language itself. Java does not automatically convert from one to the other.

b) Integer can be used as an argument for a method that requires an object, whereas int can be used for calculations.

31) What is a cloneable interface and how many methods does it contain?

Ans- It is not having any method because it is a TAGGED or MARKER interface.

32) What is the difference between abstract class and interface?

Ans: a) All the methods declared inside an interface are abstract whereas abstract class must have at least one abstract method and others may be concrete or abstract.

b) In abstract class, key word abstract must be used for the methods whereas interface we need not use that keyword for the methods.

c) Abstract class must have subclasses whereas interface can’t have subclasses.

33) Can you have an inner class inside a method and what variables can you access?

Ans: Yes, we can have an inner class inside a method and final variables can be accessed.

34) What is the difference between String and String Buffer?

Ans: a) String objects are constants and immutable whereas

StringBuffer objects are not.

b) String class supports constant strings whereas

StringBuffer class supports growable and modifiable strings.

35) What is the difference between Array and vector?

Ans: Array is a set of related data type and static whereas vector is a growable array of objects and dynamic.

36) What is the difference between exception and error?

Ans: The exception class defines mild error conditions that your program encounters.

Ex: Arithmetic Exception, FilenotFound exception

Exceptions can occur when

— try to open the file, which does not exist

— the network connection is disrupted

— operands being manipulated are out of prescribed ranges

— the class file you are interested in loading is missing

The error class defines serious error conditions that you should not attempt to recover from. In most cases it is advisable to let the program terminate when such an error is encountered.

Ex: Running out of memory error, Stack overflow error.

37) What is the difference between process and thread?

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

38) What is multithreading and what are the methods for inter-thread communication and what is the class in which these methods are defined?

Ans: Multithreading is the mechanism in which more than one thread run independent of each other within the process.

wait (), notify () and notifyAll() methods can be used for inter-thread communication and these methods are in Object class.

wait( ) : When a thread executes a call to wait( ) method, it surrenders the object lock and enters into a waiting state.

notify( ) or notifyAll( ) : To remove a thread from the waiting state, some other thread must make a call to notify( ) or notifyAll( ) method on the same object.

39) What is the class and interface in java to create thread and which is the most advantageous method?

Ans: Thread class and Runnable interface can be used to create threads and using Runnable interface is the most advantageous method to create threads because we need not extend thread class here.

40) What are the states associated in the thread?

Ans: Thread contains ready, running, waiting and dead states.

41) What is synchronization?

Ans: Synchronization is the mechanism that ensures that only one thread is accessed the resources at a time.

42) When you will synchronize a piece of your code?

Ans: When you expect your code will be accessed by different threads and these threads may change a particular data causing data corruption.

43) What is deadlock?

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

44) What is daemon thread and which method is used to create the daemon thread?

Ans: Daemon thread is a low priority thread which runs intermittently in the back ground doing the garbage collection operation for the java runtime system. setDaemon method is used to create a daemon thread.

45) Are there any global variables in Java, which can be accessed by other part of your program?

Ans: No, it is not the main method in which you define variables. Global variables is not possible because concept of encapsulation is eliminated here.

46)What is an applet?

Ans: Applet is a dynamic and interactive program that runs inside a web page displayed by a java capable browser.

47)What is the difference between applications and applets?

Ans: a)Application must be run on local machine whereas applet needs no explicit installation on local machine.

b)Application must be run explicitly within a java-compatible virtual machine whereas applet loads and runs itself automatically in a java-enabled browser.

d)Application starts execution with its main method whereas applet starts execution with its init method.

e)Application can run with or without graphical user interface whereas applet must run within a graphical user interface.

48)How does applet recognize the height and width?

Ans:Using getParameters() method.

49)When do you use codebase in applet?

Ans:When the applet class file is not in the same directory, codebase is used.

50)What is the lifecycle of an applet?

Ans:init( ) method – Can be called when an applet is first loaded

start( ) method – Can be called each time an applet is started

paint( ) method – Can be called when the applet is minimized or maximized

stop( ) method – Can be used when the browser moves off the applet’s page

destroy( ) method – Can be called when the browser is finished with the applet

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