1. Features of JAVA?

1. Object-Oriented – java is based on object-oriented programming where the class and methods describe about the state and behavior of object.
2. Portable – Java program gets converted into Java Byte Codes that can be executed on any platform without any dependency.
3. Platform independent – java works on “write once and run anywhere” as it supports multiple platforms like Windows, Linux, Mac, Sun Solaris, etc.
4. Robust – Java has a strong memory management as there is no pointer allocations. It has automatic garbage collection that prohibits memory leaks.
5. Interpreted – java compiler converts the codes into Java Byte Codes which are then interpreted and executed by Java Interpreter.

2. OOPS Concepts in Java?

Java is based on Object Oriented Programming Concepts, following are some of the OOPS concepts implemented in java programming.

1. Abstraction
2. Encapsulation
3. Polymorphism
4. Inheritance
5. Association
6. Aggregation
7. Composition

3. Difference between JDK and JRE and JVM

1. JDK - Java Development Kit is the core component of Java Environment and provides all the tools, executables and binaries required to compile, debug and execute a Java Program. JDK is a platform specific software and thats why we have separate installers for Windows, Mac and Unix systems. We can say that JDK is superset of JRE since it contains JRE with Java compiler, debugger and core classes. Current version of JDK is 1.7 also known as Java 7.
2. JVM - JVM is the heart of java programming language. When we run a program, JVM is responsible to converting Byte code to the machine specific code. JVM is also platform dependent and provides core java functions like memory management, garbage collection, security etc. JVM is customizable and we can use java options to customize it, for example allocating minimum and maximum memory to JVM. JVM is called virtual because it provides a interface that does not depend on the underlying operating system and machine hardware. This independence from hardware and operating system is what makes java program write-once run-anywhere.
3. JRE - JRE is the implementation of JVM, it provides platform to execute java programs. JRE consists of JVM and java binaries and other classes to execute any program successfully. JRE doesn’t contain any development tools like java compiler, debugger etc. If you want to execute any java program, you should have JRE installed but we don’t need JDK for running any java program.

4. What is Java Virtual Machine?

Java Virtual Machine (JVM) is the heart of java programming language. JVM is responsible for converting byte code into machine readable code. JVM is not platform independent, thats why you have different JVM for different operating systems. We can customize JVM with Java Options, such as allocating minimum and maximum memory to JVM. It’s called virtual because it provides an interface that doesn’t depend on the underlying OS.

5. What is the main method in Java?

main() method is the entry point of any standalone java application. The syntax of main method is public static void main(String args[]). Java main method is public and static so that Java runtime can access it without initializing the class. The input parameter is an array of String through which we can pass runtime arguments to the java program.

6. What is overloading and overriding in java?

When we have more than one method with the same name in a single class but the arguments are different, then it is called as method overloading. Overriding concept comes in picture with inheritance when we have two methods with same signature, one in parent class and another in child class. We can use @Override annotation in the child class overridden method to make sure if parent class method is changed, so as child class.

7. What is Exception in Java?

Exception is an error event that can happen during the execution of a program and disrupts it’s normal flow. Exception can arise from different kind of situations such as wrong data entered by user, hardware failure, network connection failure etc. Java Exception handling framework is used to handle runtime errors only, compile time errors are not handled by exception handling framework.

8. What are the throw, try-catch-finally Exception Handling Keywords in Java?

1. throw: Sometimes we explicitly want to create exception object and then throw it to halt the normal processing of the program. throw keyword is used to throw exception to the runtime to handle it.
2. throws: When we are throwing any checked exception in a method and not handling it, then we need to use throws keyword in method signature to let caller program know the exceptions that might be thrown by the method. The caller method might handle these exceptions or propagate it to it’s caller method using throws keyword. We can provide multiple exceptions in the throws clause and it can be used with main() method also.
3. try-catch: We use try-catch block for exception handling in our code. try is the start of the block and catch is at the end of try block to handle the exceptions. We can have multiple catch blocks with a try and try-catch block can be nested also. catch block requires a parameter that should be of type Exception.
4. finally: finally block is optional and can be used only with try-catch block. Since exception halts the process of execution, we might have some resources open that will not get closed, so we can use finally block. finally block gets executed always, whether exception occurrs or not.

9. What are important methods of Java Exception Class?

1. String getMessage() – This method returns the message String of Throwable and the message can be provided while creating the exception through it’s constructor.
2. String getLocalizedMessage() – This method is provided so that subclasses can override it to provide locale specific message to the calling program. Throwable class implementation of this method simply use getMessage() method to return the exception message.
3. synchronized Throwable getCause() – This method returns the cause of the exception or null id the cause is unknown.
4. String toString() – This method returns the information about Throwable in String format, the returned String contains the name of Throwable class and localized message.
5. void printStackTrace() – This method prints the stack trace information to the standard error stream, this method is overloaded and we can pass PrintStream or PrintWriter as argument to write the stack trace information to the file or stream.

10. What is difference between throw and throws in Java?

throws keyword is used with method signature to declare the exceptions that the method might throw whereas throw keyword is used to disrupt the flow of program and handing over the exception object to runtime to handle it.

11. Can we overload main method?

Yes, we can have multiple methods with name “main” in a single class. However if we run the class, java runtime environment will look for main method with syntax as public static void main(String args[]).

12. What is Java Package and which package is imported by default?

Java package is the mechanism to organize the java classes by grouping them. The grouping logic can be based on functionality or modules based. A java class fully classified name contains package and class name. For example, java.lang.Object is the fully classified name of Object class that is part of java.lang package. java.lang package is imported by default and we don’t need to import any class from this package explicitly.

13. What are access modifiers?

Java provides access control through public, private and protected access modifier keywords. When none of these are used, it’s called default access modifier. A java class can only have public or default access modifier. Read Java Access Modifiers to learn more about these in detail.

14. What is static keyword?

tatic keyword can be used with class level variables to make it global i.e all the objects will share the same variable. static keyword can be used with methods also. A static method can access only static variables of class and invoke only static methods of the class.

15. What is finally and finalize in java?

inally block is used with try-catch to put the code that you want to get executed always, even if any exception is thrown by the try-catch block. finally block is mostly used to release resources created in the try block. finalize() is a special method in Object class that we can override in our classes. This method gets called by the garbage collector when the object is getting garbage collected. This method is usually overridden to release system resources when the object is garbage collected.

16. What is an interface in JAVA?

Interfaces are core part of java programming language and used a lot not only in JDK but also java design patterns, most of the frameworks and tools. Interfaces provide a way to achieve abstraction in java and used to define the contract for the subclasses to implement.

17. What is an abstract class in JAVA?

Abstract classes are used in java to create a class with some default method implementation for subclasses. An abstract class can have abstract method without body and it can have methods with implementation also.

18. What is the difference between abstract class and interface?

1. abstract keyword is used to create abstract class whereas interface is the keyword for interfaces.
2. Abstract classes can have method implementations whereas interfaces can’t.
3. A class can extend only one abstract class but it can implement multiple interfaces.
4. We can run abstract class if it has main() method whereas we can’t run an interface.

19. What is Classloader in Java?

Java Classloader is the program that loads byte code program into memory when we want to access any class. We can create our own classloader by extending ClassLoader class and overriding loadClass(String name) method.

20. What is ternary operator in java?

Java ternary operator is the only conditional operator that takes three operands. It’s a one liner replacement for if-then-else statement and used a lot in java programming. We can use ternary operator if-else conditions or even switch conditions using nested ternary operators.

21. What is break and continue statement?

We can use break statement to terminate for, while, or do-while loop. We can use break statement in switch statement to exit the switch case. You can see the example of break statement at java break. We can use break with label to terminate the nested loops.

22. What is default constructor?

No argument constructor of a class is known as default constructor. When we don’t define any constructor for the class, java compiler automatically creates the default no-args constructor for the class. If there are other constructors defined, then compiler won’t create default constructor for us.

23. What is Garbage Collection in JAVA?

Garbage Collection is the process of looking at heap memory, identifying which objects are in use and which are not, and deleting the unused objects. In Java, process of deallocating memory is handled automatically by the garbage collector. We can run the garbage collector with code Runtime.getRuntime().gc() or use utility method System.gc(). For a detailed analysis of Heap Memory and Garbage Collection,

24. What is difference between Heap and Stack Memory?

1. Heap memory is used by all the parts of the application whereas stack memory is used only by one thread of execution.
2. Whenever an object is created, it’s always stored in the Heap space and stack memory contains the reference to it. Stack memory only contains local primitive variables and reference variables to objects in heap space.
3. Memory management in the stack is done in LIFO manner whereas it’s more complex in Heap memory because it’s used globally.

25. What is JDBC API?

Java DataBase Connectivity API allows us to work with relational databases. JDBC API interfaces and classes are part of java.sql and javax.sql package. We can use JDBC API to get the database connection, run SQL queries and stored procedures in the database server and process the results. JDBC API is written in a way to allow loose coupling between our Java program and actual JDBC drivers that makes our life easier in switching from one database to another database servers easily.

26. What is JDBC Connection?

JDBC Connection is like a Session created with the database server. You can also think Connection is like a Socket connection from the database server.

27. What is the use of JDBC DriverManager class?

JDBC DriverManager is the factory class through which we get the Database Connection object. When we load the JDBC Driver class, it registers itself to the DriverManager,

28. What is JDBC PreparedStatement?

JDBC PreparedStatement object represents a precompiled SQL statement. We can use it’s setter method to set the variables for the query.

29. What is JDBC ResultSet?

JDBC ResultSet is like a table of data representing a database result set, which is usually generated by executing a statement that queries the database.

30. What is the difference between execute, executeQuery, executeUpdate?

1. 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.
2. 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”.
3. 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.