Java Interview Questions Answers for Freshers and Experienced

What are the features of Java?

Following are the various features of the Java programming language:

• High Performance– Using a JIT (Just-In-Time) compiler allows high performance in Java. The JIT compiler converts the Java bytecode into machine language code, which then gets executed by the JVM.
• Multi-threading– A thread is a flow of execution. The JVM creates a thread which is called the main thread. Java allows the creation of several threads using either extending the thread class or implementing the Runnable interface.
• OOPS Concepts– Java follows various OOPS concepts, namely abstraction, encapsulation, inheritance, object-oriented, and polymorphism
• Platform Independency– Java makes use of the Java Virtual Machine or JVM, which allows a single Java program to operate on multiple platforms without any modifications.

You may want to check out Java features in detail here. 

What is JIT compiler?

Just-In-Time(JIT) compiler: It is used to improve the performance. JIT compiles parts of the bytecode that have similar functionality at the same time, and hence reduces the amount of time needed for compilation. Here the term “compiler” refers to a translator from the instruction set of a Java virtual machine (JVM) to the instruction set of a specific CPU.

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What is the platform?

A platform is the hardware or software environment in which a piece of software is executed. There are two types of platforms, software-based and hardware-based. Java provides the software-based platform.

What are the main differences between the Java platform and other platforms?

There are the following differences between the Java platform and other platforms.

• Java is the software-based platform whereas other platforms may be the hardware platforms or software-based platforms.
• Java is executed on the top of other hardware platforms whereas other platforms can only have the hardware components.

What is the JIT compiler?

Answer: JIT compiler runs after the program is executed and compiles the code into a faster form, hosting CPU’s native instructing set. JIT can access dynamic runtime information, whereas a standard compiler doesn’t and can make better optimizations like inlining functions that are used frequently. 

If I don’t provide any arguments on the command line, then what will the value stored in the String array passed into the main() method, empty or NULL?

It is empty, but not null.

What if I write static public void instead of public static void?

The program compiles and runs correctly because the order of specifiers doesn’t matter in Java.

What is the default value of the local variables?

The local variables are not initialized to any default value, neither primitives nor object references.

What are the various access specifiers in Java?

In Java, access specifiers are the keywords which are used to define the access scope of the method, class, or a variable. In Java, there are four access specifiers given below.

• Public The classes, methods, or variables which are defined as public, can be accessed by any class or method.
• Protected Protected can be accessed by the class of the same package, or by the sub-class of this class, or within the same class.
• Default Default are accessible within the package only. By default, all the classes, methods, and variables are of default scope.
• Private The private class, methods, or variables defined as private can be accessed within the class only.

What is object-oriented paradigm?

It is a programming paradigm based on objects having data and methods defined in the class to which it belongs. Object-oriented paradigm aims to incorporate the advantages of modularity and reusability. Objects are the instances of classes which interacts with one another to design applications and programs. There are the following features of the object-oriented paradigm.

• Follows the bottom-up approach in program design.
• Focus on data with methods to operate upon the object’s data
• Includes the concept like Encapsulation and abstraction which hides the complexities from the user and show only functionality.
• Implements the real-time approach like inheritance, abstraction, etc.
• The examples of the object-oriented paradigm are C++, Simula, Smalltalk, Python, C#, etc.

What is an object?

The Object is the real-time entity having some state and behavior. In Java, Object is an instance of the class having the instance variables as the state of the object and the methods as the behavior of the object. The object of a class can be created by using the new keyword.

What is a package in Java? List down various advantages of packages.

Packages in Java, are the collection of related classes and interfaces which are bundled together. By using packages, developers can easily modularize the code and optimize its reuse. Also, the code within the packages can be imported by other classes and reused. Below I have listed down a few of its advantages:

• Packages help in avoiding name clashes
• They provide easier access control on the code
• Packages can also contain hidden classes which are not visible to the outer classes and only used within the package

Creates a proper hierarchical structure which makes it easier to locate the related classes

Why pointers are not used in Java?

Java doesn’t use pointers because they are unsafe and increases the complexity of the program. Since, Java is known for its simplicity of code, adding the concept of pointers will be contradicting. Moreover, since JVM is responsible for implicit memory allocation, thus in order to avoid direct access to memory by the user,  pointers are discouraged in Java.

What is JIT compiler in Java?

JIT stands for Just-In-Time compiler in Java. It is a program that helps in converting the Java bytecode into instructions that are sent directly to the processor. By default, the JIT compiler is enabled in Java and is activated whenever a Java method is invoked. The JIT compiler then compiles the bytecode of the invoked method into native machine code, compiling it “just in time” to execute. Once the method has been compiled, the JVM summons the compiled code of that method directly rather than interpreting it. This is why it is often responsible for the performance optimization of Java applications at the run time.

What are the features of JAVA?

Answer: Features of Java are as follows:

• OOP concepts
  • Object-oriented
  • Inheritance
  • Encapsulation
  • Polymorphism
  • Abstraction

• Platform independent: A single program works on different platforms without any modification.
• High Performance: JIT (Just In Time compiler) enables high performance in Java. JIT converts the bytecode into machine language and then JVM starts the execution.

Multi-threaded: A flow of execution is known as a Thread. JVM creates a thread which is called the main thread. The user can create multiple threads by extending the thread class or by implementing the Runnable interface.

How does Java enable high performance?

Answer: Java uses Just In Time compiler to enable high performance. It is used to convert the instructions into bytecodes.

Name the Java IDE’s?

Answer: Eclipse and NetBeans are the IDE’s of JAVA.

What do you mean by Constructor?

Answer: Constructor can be explained in detail with enlisted points:

• When a new object is created in a program a constructor gets invoked corresponding to the class.
• The constructor is a method which has the same name as the class name.
• If a user doesn’t create a constructor implicitly a default constructor will be created.
• The constructor can be overloaded.

If the user created a constructor with a parameter then he should create another constructor explicitly without a parameter.

What is meant by the Local variable and the Instance variable?

Answer:

Local variables are defined in the method and scope of the variables that exist inside the method itself.

Instance variable is defined inside the class and outside the method and the scope of the variables exists throughout the class.

What is a Class?

Answer: All Java codes are defined in a Class. It has variables and methods.

Variables are attributes which define the state of a class.

Methods are the place where the exact business logic has to be done. It contains a set of statements (or) instructions to satisfy the particular requirement.

Example:

public class Addition{ //Class name declaration
int a = 5; //Variable declaration
int b= 5;
public void add(){ //Method declaration
int c = a+b;
}
}

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 What is an Object?

Answer: An instance of a class is called an object. The object has state and behavior.

Whenever the JVM reads the “new()” keyword then it will create an instance of that class.

Example:

public class Addition{
public static void main(String[] args){
Addion add = new Addition();//Object creation
}
}

The above code creates the object for the Addition class.

What is the difference between an object-oriented programming language and object-based programming language?

There are the following basic differences between the object-oriented language and object-based language.

• Object-oriented languages follow all the concepts of OOPs whereas, the object-based language doesn’t follow all the concepts of OOPs like inheritance and polymorphism.
• Object-oriented languages do not have the inbuilt objects whereas Object-based languages have the inbuilt objects, for example, JavaScript has window object.
• Examples of object-oriented programming are Java, C#, Smalltalk, etc. whereas the examples of object-based languages are JavaScript, VBScript, etc.

What is the purpose of static methods and variables?

The methods or variables defined as static are shared among all the objects of the class. The static is the part of the class and not of the object. The static variables are stored in the class area, and we do not need to create the object to access such variables. Therefore, static is used in the case, where we need to define variables or methods which are common to all the objects of the class.

For example, In the class simulating the collection of the students in a college, the name of the college is the common attribute to all the students. Therefore, the college name will be defined as static.

What are the advantages of Packages in Java?

There are various advantages of defining packages in Java.

• Packages avoid the name clashes.
• The Package provides easier access control.
• We can also have the hidden classes that are not visible outside and used by the package.

It is easier to locate the related classes.

Which Java IDE to use, and why?

Answer: A Java IDE is a software that allows Java developers to easily write as well as debug Java programs. It is basically a collection of various programming tools, accessible via a single interface, and several helpful features, such as code completion and syntax highlighting. Codenvy, Eclipse, and NetBeans are some of the most popular Java IDEs.

Java is a platform-independent language. Why?

Answer: Java programming language does not depend on any particular hardware or software because it is compiled by the compiler and then converted into byte code. Byte code is platform-independent and can run on multiple systems. The only requirement is Java needs a runtime environment, i.e., JRE, which is a set of tools used for developing Java applications.

Explain Typecasting

Answer: The concept of assigning a variable of one data type to a variable of another data type. It is not possible for the boolean data type.

What gives Java its ‘write once and run anywhere’ nature?

The bytecode. Java compiler converts the Java programs into the class file (Byte Code) which is the intermediate language between source code and machine code. This bytecode is not platform specific and can be executed on any computer.

 What is classloader?

Classloader is a subsystem of JVM which is used to load class files. Whenever we run the java program, it is loaded first by the classloader. There are three built-in classloaders in Java.

1. Bootstrap ClassLoader: This is the first classloader which is the superclass of Extension classloader. It loads the rt.jar file which contains all class files of Java Standard Edition like java.lang package classes, java.net package classes, java.util package classes, java.io package classes, java.sql package classes, etc.
2. Extension ClassLoader: This is the child classloader of Bootstrap and parent classloader of System classloader. It loads the jar files located inside $JAVA_HOME/jre/lib/ext directory.

System/Application ClassLoader: This is the child classloader of Extension classloader. It loads the class files from the classpath. By default, the classpath is set to the current directory. You can change the classpath using “-cp” or “-classpath” switch. It is also known as Application classloader.

 How does Java enable high performance?

Answer: In the Just-in-Time compilation, the required code is executed at run time. Typically, it involves translating bytecode into machine code and then executing it directly. For enabling high performance, Java can make use of the Just-In-Time compilation. The JIT compiler is enabled by default in Java and gets activated as soon as a method is called. It then compiles the bytecode of the Java method into native machine code. After that, the JVM calls the compiled code directly instead of interpreting it. This grants a performance boost.

What are the various access specifiers for Java classes?

Ans: In Java, access specifiers are the keywords used before a class name which defines the access scope. The types of access specifiers for classes are:

1. Public : Class,Method,Field is accessible from anywhere.

2. Protected:Method,Field can be accessed from the same class to which they belong or from the sub-classes,and from the class of same package,but not from outside.

3. Default: Method,Field,class can be accessed only from the same package and not from outside of it’s native package.

4. Private: Method,Field can be accessed from the same class to which they belong.

Pointers are used in C/ C++. Why does Java not make use of pointers?

Pointers are quite complicated and unsafe to use by beginner programmers. Java focuses on code simplicity, and usage of pointers can make it challenging. Pointer utilization can also cause potential errors. Moreover, security is also compromised if pointers are used because the users can directly access memory with the help of pointers.

Thus, a certain level of abstraction is furnished by not including pointers in Java. Moreover, the usage of pointers can make the procedure of garbage collection quite slow and erroneous. Java makes use of references as these cannot be manipulated, unlike pointers.

What’s the purpose of Static methods and static variables?

Ans: When there is a requirement to share a method or a variable between multiple objects of a class instead of creating separate copies for each object, we use static keyword to make a method or variable shared for all objects.

Define a Java Class.

A class in Java is a blueprint which includes all your data.  A class contains fields (variables) and methods to describe the behavior of an object. Let’s have a look at the syntax of a class.

class Abc {
member variables // class body
methods}

public class switchExample {
    int score = 4;
    public static void main(String args[]) {
        switch (score) {
            case 1:
                system.out.println("Score is 1");
                break;
            case 2:
                system.out.println("Score is 2");
                break;
            default:
                system.out.println("Default Case");
        }
    }
}

for (;;)
{
    // Statements to execute

    // Add any loop breaking logic
}

What are the main concepts of OOPs in Java?

Object-Oriented Programming or OOPs is a programming style that is associated with concepts like:

1. Inheritance: Inheritance is a process where one class acquires the properties of another.
2. Encapsulation: Encapsulation in Java is a mechanism of wrapping up the data and code together as a single unit.
3. Abstraction: Abstraction is the methodology of hiding the implementation details from the user and only providing the functionality to the users. 

Polymorphism: Polymorphism is the ability of a variable, function or object to take multiple forms.

What is the difference between a local variable and an instance variable?

In Java, a local variable is typically used inside a method, constructor, or a block and has only local scope. Thus, this variable can be used only within the scope of a block. The best benefit of having a local variable is that other methods in the class won’t be even aware of that variable.

Example

if(x > 100)
{
String test = “Edureka”;
}

Whereas, an instance variable in Java, is a variable which is bounded to its object itself. These variables are declared within a class, but outside a method. Every object of that class will create it’s own copy of the variable while using it. Thus, any changes made to the variable won’t reflect in any other instances of that class and will be bound to that particular instance only.

class Test{
public String EmpName;
public int empAge;
}