Top 20 Java Interview Questions

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 name of the method which is searched by JVM as a starting point for an application with a particular signature only. It is the method where the main execution occurs.
String args[] : It is the parameter passed to the main method.

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.

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.

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. For example: method can be overridden like String class. equals() method is used to compare the values of two objects.

Polymorphism is briefly described as “one interface, many implementations”. Polymorphism is a characteristic of being able to assign a different meaning or usage to something in different contexts – specifically, to allow an entity such as a variable, a function, or an object to have more than one form. There are two types of polymorphism:

Compile time polymorphism
Run time polymorphism

Compile time polymorphism is method overloading whereas Runtime time polymorphism is done using inheritance and interface.

If a child class inherits the property from multiple classes is known as multiple inheritance. Java does not allow to extend multiple classes.

The problem with multiple inheritance is that if multiple parent classes have a same method name, then at runtime it becomes difficult for the compiler to decide which method to execute from the child class.

Therefore, Java doesn’t support multiple inheritance. The problem is commonly referred as Diamond Problem.

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.

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.

There are 5 stages in the lifecycle of a servlet:

Servlet is loaded
Servlet is instantiated
Servlet is initialized
Service the request
Servlet is destroyed

Cookies are text data sent by server to the client and it gets saved at the client local machine.

Servlet API provides cookies support through javax.servlet.http.Cookie class that implements Serializable and Cloneable interfaces.

HttpServletRequest getCookies() method is provided to get the array of Cookies from request, since there is no point of adding Cookie to request, there are no methods to set or add cookie to request.

Similarly HttpServletResponse addCookie(Cookie c) method is provided to attach cookie in response header, there are no getter methods for cookie.

JDBC Driver is a software component that enables java application to interact with the database. There are 4 types of JDBC drivers:

JDBC-ODBC bridge driver
Native-API driver (partially java driver)
Network Protocol driver (fully java driver)
Thin driver (fully java driver)

Registering the driver class
Creating connection
Creating statement
Executing queries
Closing connection

The java.sql package contains interfaces and classes for JDBC API.

Interfaces:
Connection
Statement
PreparedStatement
ResultSet
ResultSetMetaData
DatabaseMetaData
CallableStatement etc.

Classes:
DriverManager
Blob
Clob
Types
SQLException etc.

The DriverManager class manages the registered drivers. It can be used to register and unregister drivers. It provides factory method that returns the instance of Connection.

The ResultSet object represents a row of a table. It can be used to change the cursor pointer and get the information from the database.

Statement execute(String query) is used to execute any SQL query and it returns TRUE if the result is an ResultSet such as running Select queries. The output is FALSE when there is no ResultSet object such as running Insert or Update queries. We can use getResultSet() to get the ResultSet and getUpdateCount() method to retrieve the update count.

Statement executeQuery(String query) is used to execute Select queries and returns the ResultSet. ResultSet returned is never null even if there are no records matching the query. When executing select queries we should use executeQuery method so that if someone tries to execute insert/update statement it will throw java.sql.SQLException with message “executeQuery method can not be used for update”.

Statement executeUpdate(String query) is used to execute Insert/Update/Delete (DML) statements or DDL statements that returns nothing. The output is int and equals to the row count for SQL Data Manipulation Language (DML) statements. For DDL statements, the output is 0.

You should use execute() method only when you are not sure about the type of statement else use executeQuery or executeUpdate method.

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