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Basic Java Interview Questions and Answers

What is the most important feature of Java?
Java is a platform independent language.

 What do you mean by platform independence?
Platform independence means that we can write and compile the java code in one platform (eg Windows) and can execute the class in any other supported platform eg (Linux,Solaris,etc).

 What is a JVM?
JVM is Java Virtual Machine which is a run time environment for the compiled java class files.

 Are JVM's platform independent?
JVM's are not platform independent. JVM's are platform specific run time implementation provided by the vendor.

 What is the difference between a JDK and a JVM?
JDK is Java Development Kit which is for development purpose and it includes execution environment also. But JVM is purely a run time environment and hence you will not be able to compile your source files using a JVM.

 What is a pointer and does Java support pointers?
Pointer is a reference handle to a memory location. Improper handling of pointers leads to memory leaks and reliability issues hence Java doesn't support the usage of pointers.

    List two Java IDE’s?
  Netbeans, Eclipse, etc.

 List some Java keywords(unlike C, C++ keywords)?
 Some Java keywords are import, super, finally, etc.

 What do you mean by Object?
 Object is a runtime entity and it’s state is stored in fields and behavior is shown via methods. Methods operate on an object's internal state and serve as the primary mechanism for object-to-object communication.

 Define class?
 A class is a blue print from which individual objects are created. A class can contain fields and methods to describe the behavior of an object.

 What is the base class of all classes?

 Does Java support multiple inheritance?
Java doesn't support multiple inheritance.

 Is Java a pure object oriented language?
Java uses primitive data types and hence is not a pure object oriented language.

 Are arrays primitive data types?
In Java, Arrays are objects.

 What is difference between Path and Classpath?
Path and Classpath are operating system level environment variales. Path is used define where the system can find the executables(.exe) files and classpath is used to specify the location .class files.

 What are local variables?
Local varaiables are those which are declared within a block of code like methods. Local variables should be initialised before accessing them.

 What are instance variables?
Instance variables are those which are defined at the class level. Instance variables need not be initialized before using them as they are automatically initialized to their default values.

 How to define a constant variable in Java?
The variable should be declared as static and final. So only one copy of the variable exists for all instances of the class and the value can't be changed also.

static final int PI = 2.14; is an example for constant.

 Should a main() method be compulsorily declared in all java classes?
No not required. main() method should be defined only if the source class is a java application.

 What is the return type of the main() method?

Main() method doesn't return anything hence declared void.

 Why is the main() method declared static?
main() method is called by the JVM even before the instantiation of the class hence it is declared as static.

 What is the arguement of main() method?
main() method accepts an array of String object as arguement.

 Can a main() method be overloaded?
Yes. You can have any number of main() methods with different method signature and implementation in the class.

 Can a main() method be declared final?
Yes. Any inheriting class will not be able to have it's own default main() method.

 Does the order of public and static declaration matter in main() method?
No. It doesn't matter but void should always come before main().

 Can a source file contain more than one class declaration?
Yes a single source file can contain any number of Class declarations but only one of the class can be declared as public.
 What is a package?
Package is a collection of related classes and interfaces. package declaration should be first statement in a java class.
 Which package is imported by default?
java.lang package is imported by default even without a package declaration.

What are Encapsulation, Inheritance and Polymorphism?
- 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.

What is OOPs?- 
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.

What are Class, Constructor and Primitive data types?
- 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, boolean, char.

What is an Object and how do you allocate memory to it?-
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 usingnew operator, memory is allocated to it.

What are methods and how are they defined?-
 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. 

What is the use of bin and lib in JDK?
 Bin contains all tools such as javac, appletviewer, awt tool, etc., whereas lib contains API and all packages.

What is casting?- 
Casting is used to convert the value of one type to another.
How many ways can an argument be passed to a subroutine and explain them?-
 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.

What are different types of access modifiers?
- 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.

What is final, finalize() and finally?
- 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 contingency.

What is UNICODE?
- Unicode is used for internal representation of characters and strings and it uses 16 bits to represent each other.

What is Garbage Collection and how to call it explicitly?-
 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.

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

What are Transient and Volatile Modifiers?
- 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.

What is method overloading and method overriding?
- 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.

What is meant by Inheritance and what are its advantages?
- 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.

What modifiers may be used with top-level class?-
 public, abstract and final can be used for top-level class.

What are inner class and anonymous class?-
 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.

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

What is a reflection package?-
java. lang. reflect package has the ability to analyze itself in runtime.

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About Sridhar Mahendrakar

Its my passion to explore myself with new ideas and i love to write; a compulsion; something that gives me an avenue to express myself. I write when I am happy; when I am sad or when an issue touches my heart.


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Graduate Engineer @ Indian Space Research Organization