Frequently Asked JAVA Interview Question and Answer Part 1

Hi Guys, In this post I'll share some interview questions and answers which i have faced or asked in interview panel.

1). What are the important features of Java 8 release?
Ans).Java 8 has been released in March 2014, so it’s one of the hot topic Java 8 has been one of the biggest release after Java 5 annotations and generics. Some of the important features of Java 8 are:

1.  Interface changes with default and static methods

2.   Functional interfaces and Lambda Expressions

3.   Java Stream API for collection classes

4.   Java Date Time API

2). What do you mean by platform independence of Java?
Ans).Platform independence means that you can run the same Java Program in any Operating System. For example, you can write java program in Windows and run it in Mac OS.

3). What is JVM and is it platform independent?
Ans).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, that’s 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.

4). What is the difference between JDK and JVM?
Ans).Java Development Kit (JDK) is for development purpose and JVM is a part of it to execute the java programs.JDK provides all the tools, executables and binaries required to compile, debug and execute a Java Program. The execution part is handled by JVM to provide machine independence.

5). What is the difference between JVM and JRE?
Ans).Java Runtime Environment (JRE) is the implementation of JVM. 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.

6). Why Java doesn’t support multiple inheritance?
Ans).Java doesn’t support multiple inheritance in classes because of “Diamond Problem”.

However multiple inheritance is supported in interfaces. An interface can extend multiple interfaces because they just declare the methods and implementation will be present in the implementing class. So there is no issue of diamond problem with interfaces.

7). Why Java is not pure Object Oriented language?
Ans).Java is not said to be pure object oriented because it support primitive types such as int, byte, short, long etc. I believe it brings simplicity to the language while writing our code. Obviously java could have wrapper objects for the primitive types but just for the representation, they would not have provided any benefit.

As we know, for all the primitive types we have wrapper classes such as Integer, Long etc that provides some additional methods.

8). What is difference between path and classpath variables?
Ans).PATH is an environment variable used by operating system to locate the executables. That’s why when we install Java or want any executable to be found by OS, we need to add the directory location in the PATH variable.

Classpath is specific to java and used by java executables to locate class files. We can provide the classpath location while running java application and it can be a directory, ZIP files, JAR files etc.

9). What is the importance of main method in Java?
Ans).main() method is the entry point of any standalone java application. The syntax of main method is public static void main(String args[]).

main method is public and static so that java 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. 

10). What is overloading and overriding in java?
Ans).When we have more than one method with 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.

11). Can we overload main method?
Ans).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). Can we have multiple public classes in a java source file?
Ans).We can’t have more than one public class in a single java source file. A single source file can have multiple classes that are not public.

13). What is Java Package and which package is imported by default?
Ans).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.

14). What are access modifiers?
Ans).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. 

15). What is final keyword?
Ans).final keyword is used with Class to make sure no other class can extend it, for example String class is final and we can’t extend it.

We can use final keyword with methods to make sure child classes can’t override it.final keyword can be used with variables to make sure that it can be assigned only once. However the state of the variable can be changed, for example we can assign a final variable to an object only once but the object variables can change later on.Java interface variables are by default final and static.

16). What is static keyword?
Ans).static 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.

17). What is finally and finalize in java?
Ans).finally 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 get’s called by garbage collector when the object is getting garbage collected. This method is usually overridden to release system resources when object is garbage collected.

18). Can we declare a class as static?
Ans).We can’t declare a top-level class as static however an inner class can be declared as static. If inner class is declared as static, it’s called static nested class.

Static nested class is same as any other top-level class and is nested for only packaging convenience.

19). What is static import?
Ans).If we have to use any static variable or method from other class, usually we import the class and then use the method/variable with class name.

import java.lang.Math;

//inside classdouble test = Math.PI * 5;

We can do the same thing by importing the static method or variable only and then use it in the class as if it belongs to it.

import static java.lang.Math.PI;

//no need to refer class nowdouble test = PI * 5;

Use of static import can cause confusion, so it’s better to avoid it. Overuse of static import can make your program unreadable and unmaintainable.

20). What is try-with-resources in java?
Ans).One of the Java 7 features is try-with-resources statement for automatic resource management. Before Java 7, there was no auto resource management and we should explicitly close the resource. Usually, it was done in the finally block of a try-catch statement. This approach used to cause memory leaks when we forgot to close the resource.

From Java 7, we can create resources inside try block and use it. Java takes care of closing it as soon as try-catch block gets finished.

21). What is multi-catch block in java?
Ans).Java 7 one of the improvement was multi-catch block where we can catch multiple exceptions in a single catch block. This makes are code shorter and cleaner when every catch block has similar code.

If a catch block handles multiple exception, you can separate them using a pipe (|) and in this case exception parameter (ex) is final, so you can’t change it.

22). What is static block?
Ans).Java static block is the group of statements that gets executed when the class is loaded into memory by Java ClassLoader. It is used to initialize static variables of the class. Mostly it’s used to create static resources when class is loaded.

23). What is an interface?
Ans).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 sub classes to implement.

Interfaces are good for starting point to define Type and create top level hierarchy in our code. Since a java class can implements multiple interfaces, it’s better to use interfaces as super class in most of the cases.

24). What is an abstract class?
Ans).Abstract classes are used in java to create a class with some default method implementation for sub classes. An abstract class can have abstract method without body and it can have methods with implementation also.

abstract keyword is used to create a abstract class. Abstract classes can’t be instantiated and mostly used to provide base for sub-classes to extend and implement the abstract methods and override or use the implemented methods in abstract class..

25). What is the difference between abstract class and interface?
Ans).abstract keyword is used to create abstract class whereas interface is the keyword for interfaces.

Abstract classes can have method implementations whereas interfaces can’t.A class can extend only one abstract class but it can implement multiple interfaces.We can run abstract class if it has main() method whereas we can’t run an interface.

Cheers Guys in next port i'll share more QA.