Java Interview Questions for Freshers

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JVM (Java Virtual Machine): It is an abstract machine. It is a specification that provides run-time environment in which java bytecode can be executed. It follows three notations:

Specification: It is a document that describes the implementation of the Java virtual machine. It is provided by Sun and other companies. Implementation: It is a program that meets the requirements of JVM specification. Runtime Instance: An instance of JVM is created whenever you write a java command on the command prompt and run the class. JRE (Java Runtime Environment) : JRE refers to a runtime environment in which java bytecode can be executed. It implements the JVM (Java Virtual Machine) and provides all the class libraries and other support files that JVM uses at runtime. So JRE is a software package that contains what is required to run a Java program. Basically, it’s an implementation of the JVM which physically exists.

JDK(Java Development Kit) : It is the tool necessary to compile, document and package Java programs. The JDK completely includes JRE which contains tools for Java programmers. The Java Development Kit is provided free of charge. Along with JRE, it includes an interpreter/loader, a compiler (javac), an archiver (jar), a documentation generator (javadoc) and other tools needed in Java development. In short, it contains JRE + development tools.

public : Public is an access modifier, which is used to specify who can access this method. Public means that this Method will be accessible by any Class.

static : It is a keyword in java which identifies it is class based i.e it can be accessed without creating the instance of a Class.

void : It is the return type of the method. Void defines the method which will not return any value.

main: It is the method where the main execution occurs.

String args[] : It is the parameter passed to the main method.

In Java, constructor refers to a block of code which is used to initialize an object. It must have the same name as that of the class. Also, it has no return type and it is automatically called when an object is created.

There are two types of constructors:

Default constructor

Parameterized constructor

Singleton class is a class whose only one instance can be created at any given time, in one JVM. A class can be made singleton by making its constructor private.

Platform independent practically means “write once run anywhere”. Java is called so because of its byte codes which can run on any system irrespective of its underlying operating system.

Java is not 100% Object-oriented because it makes use of eight primitive datatypes such as boolean, byte, char, int, float, double, long, short which are not objects.

Method Overloading : In Method Overloading, Methods of the same class shares the same name but each method must have different number of parameters or parameters having different types and order.

Method Overloading is to “add” or “extend” more to method’s behavior.

It is a compile time polymorphism.

The methods must have different signature.

It may or may not need inheritance in Method Overloading.

Association is a relationship where all object have their own lifecycle and there is no owner. Let’s take an example of Teacher and Student.

Multiple students can associate with a single teacher and a single student can associate with multiple teachers but there is no ownership between the objects and both have their own lifecycle. These relationship can be one to one, One to many, many to one and many to many.

Aggregation is a specialized form of Association where all object have their own lifecycle but there is ownership and child object can not belongs to another parent object.

Let’s take an example of Department and teacher. A single teacher can not belongs to multiple departments, but if we delete the department teacher object will not destroy.

Composition is again specialized form of Aggregation and we can call this as a “death” relationship. It is a strong type of Aggregation. Child object dose not have their lifecycle and if parent object deletes all child object will also be deleted. Let’s take again an example of relationship between House and rooms. House can contain multiple rooms there is no independent life of room and any room can not belongs to two different house if we delete the house room will automatically delete.

jspDestry() method is invoked from javax.servlet.jsp.JspPage interface whenever a JSP page is about to be destroyed. Servlets destroy methods can be easily overridden to perform cleanup, like when closing a database connection.

JSP is a technology on the server’s side to make content generation simple. They are document centric, whereas servlets are programs. A Java server page can contain fragments of Java program, which execute and instantiate Java classes. However, they occur inside HTML template file. It provides the framework for development of a Web Application.

There are five keywords used to handle exceptions in java:

• try
• catch
• finally
• throw
• throws

Wrapper classes converts the java primitives into the reference types (objects). Every primitive data type has a class dedicated to it. These are known as wrapper classes because they “wrap” the primitive data type into an object of that class.

(((((((((((((((((Note for designer make below things in table format))))))) Array List Vector Array List is not synchronized. Vector is synchronized.

Array List is fast as it’s non-synchronized. Vector is slow as it is thread safe. If an element is inserted into the Array List, it increases its Array size by 50%. Vector defaults to doubling size of its array.

Array List does not define the increment size. Vector defines the increment size. Array List can only use Iterator for traversing an Array List. Except Hashtable, Vector is the only other class which uses both Enumeration and Iterator.

Equals() method is defined in Object class in Java and used for checking equality of two objects defined by business logic.

“==” or equality operator in Java is a binary operator provided by Java programming language and used to compare primitives and objects.

public boolean equals(Object o) is the method provided by the Object class. The default implementation uses == operator to compare two objects.

The major difference between Heap and Stack memory are:

(((((((((((((((((Note for designer make below things in table format))))))) Features Stack Heap

Memory Stack memory is used only by one thread of execution. Heap memory is used by all the parts of the application.

Access Stack memory can’t be accessed by other threads. Objects stored in the heap are globally accessible.> Memory Management Follows LIFO manner to free memory. Memory management is based on generation associated to each object.

Lifetime Exists until the end of execution of the thread. Heap memory lives from the start till the end of application execution.

Usage Stack memory only contains local primitive and reference variables to objects in heap space. Whenever an object is created, it’s always stored in the Heap space.

Java Servlet is server side technologies to extend the capability of web servers by providing support for dynamic response and data persistence.

The javax.servlet and javax.servlet.http packages provide interfaces and classes for writing our own servlets.

All servlets must implement the javax.servlet.Servlet interface, which defines servlet lifecycle methods. When implementing a generic service, we can extend the GenericServlet

class provided with the Java Servlet API. The HttpServlet class provides methods, such as doGet() and doPost(), for handling HTTP-specific services.

Most of the times, web applications are accessed using HTTP protocol and thats why we mostly extend HttpServlet class. Servlet API hierarchy is shown in below image.

(((((((((((((((((Note for designer make below things in table format))))))) Abstract Class Interfaces

An abstract class can provide complete, default code and/or just the details that have to be overridden. An interface cannot provide any code at all,just the signature. In case of abstract class, a class may extend only one abstract class. A Class may implement several interfaces.

An abstract class can have non-abstract methods. All methods of an Interface are abstract.

An abstract class can have instance variables. An Interface cannot have instance variables An abstract class can have any visibility: public, private, protected. An Interface visibility must be public (or) none.

If we add a new method to an abstract class then we have the option of providing default implementation and therefore all the existing code might work properly If we add a new method to an Interface then we have to track down all the implementations of the interface and define implementation for the new method An abstract class can contain constructors An Interface cannot contain constructors Abstract classes are fast Interfaces are slow as it requires extra indirection to find corresponding method in the actual class

RequestDispatcher interface is used to forward the request to another resource that can be HTML, JSP or another servlet in same application. We can also use this to include the content of another resource to the response.

There are two methods defined in this interface:

• void forward()
• void include()