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1) What is Java?
Java is the high-level, object-oriented, robust, secure programming language, platform- independent, high performance, Multithreaded, and portable programming language. It was developed by James Gosling in June 1991. It can also be known as the platform as it provides its own JRE and API.
2) What are the differences between C++ and Java?
The differences between C++ and Java are given in the following table.
Comparison
Index
C++ Java
Platform- independent C++ is platform-dependent. Java is platform- independent. Mainly used for C++ is mainly used for system programming. Java is mainly used for application programming. It is widely used in window, web-based, enterprise and mobile applications. Design Goal C++ was designed for systems and applications programming. It was an extension of C programming language. Java was designed and created as an interpreter for printing systems but later extended as a support network computing. It was designed with a goal of being easy to use and accessible to a broader audience. Goto C++ supports the goto statement. Java doesn't support the goto statement.
Multiple inheritance C++ supports multiple inheritance. Java doesn't support multiple inheritance through class. It can be achieved by interfaces in java. Operator Overloading C++ supports operator overloading. Java doesn't support operator overloading. Pointers C++ supports pointers. You can write pointer program in C++. Java supports pointer internally. However, you can't write the pointer program in java. It means java has restricted pointer support in Java. Compiler and Interpreter C++ uses compiler only. C++ is compiled and run using the compiler which converts source code into machine code so, C++ is platform dependent. Java uses compiler and interpreter both. Java source code is converted into bytecode at compilation time. The interpreter executes this bytecode at runtime and produces output. Java is interpreted that is why it is platform independent. Call by Value and Call by reference C++ supports both call by value and call by reference. Java supports call by value only. There is no call by reference in java. Structure and Union C++ supports structures and unions. Java doesn't support structures and unions. Thread Support C++ doesn't have built-in support for threads. It relies on third-party libraries for thread support. Java has built- in thread support. Documentation comment C++ doesn't support documentation comment. Java supports documentation comment
gets derived from java.lang.Object.
3) List the features of Java Programming language.
There are the following features in Java Programming Language. o Simple: Java is easy to learn. The syntax of Java is based on C++ which makes easier to write the program in it. o Object-Oriented: Java follows the object-oriented paradigm which allows us to maintain our code as the combination of different type of objects that incorporates both data and behavior. o Portable: Java supports read-once-write-anywhere approach. We can execute the Java program on every machine. Java program (.java) is converted to bytecode (.class) which can be easily run on every machine. o Platform Independent: Java is a platform independent programming language. It is different from other programming languages like C and C++ which needs a platform to be executed. Java comes with its platform on which its code is executed. Java doesn't depend upon the operating system to be executed. o Secured: Java is secured because it doesn't use explicit pointers. Java also provides the concept of ByteCode and Exception handling which makes it more secured. o Robust: Java is a strong programming language as it uses strong memory management. The concepts like Automatic garbage collection, Exception handling, etc. make it more robust. o Architecture Neutral: Java is architectural neutral as it is not dependent on the architecture. In C, the size of data types may vary according to the architecture ( bit or 64 bit) which doesn't exist in Java. o Interpreted: Java uses the Just-in-time (JIT) interpreter along with the compiler for the program execution.
o High Performance: Java is faster than other traditional interpreted programming languages because Java bytecode is "close" to native code. It is still a little bit slower than a compiled language (e.g., C++). o Multithreaded: We can write Java programs that deal with many tasks at once by defining multiple threads. The main advantage of multi-threading is that it doesn't occupy memory for each thread. It shares a common memory area. Threads are important for multi-media, Web applications, etc. o Distributed: Java is distributed because it facilitates users to create distributed applications in Java. RMI and EJB are used for creating distributed applications. This feature of Java makes us able to access files by calling the methods from any machine on the internet. o Dynamic: Java is a dynamic language. It supports dynamic loading of classes. It means classes are loaded on demand. It also supports functions from its native languages, i.e., C and C++.
4) What do you understand by Java virtual machine?
Java Virtual Machine is a virtual machine that enables the computer to run the Java program. JVM acts like a run-time engine which calls the main method present in the Java code. JVM is the specification which must be implemented in the computer system. The Java code is compiled by JVM to be a Bytecode which is machine independent and close to the native code.
5) What is the difference between JDK, JRE, and JVM?
JVM
JVM is an acronym for Java Virtual Machine; it is an abstract machine which provides the runtime environment in which Java bytecode can be executed. It is a specification which specifies the working of Java Virtual Machine. Its implementation has been provided by Oracle and other companies. Its implementation is known as JRE. JVMs are available for many hardware and software platforms (so JVM is platform dependent). It is a runtime instance which is created when we run the Java class. There are three notions of the JVM: specification, implementation, and instance.
8) 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.
9) What are the main differences between the Java
platform and other platforms?
There are the following differences between the Java platform and other platforms. o Java is the software-based platform whereas other platforms may be the hardware platforms or software-based platforms. o Java is executed on the top of other hardware platforms whereas other platforms can only have the hardware components.
10) 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.
11) 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.
- 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.
- 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.
12) Is Empty .java file name a valid source file name?
Yes, Java allows to save our java file by .java only, we need to compile it by javac .java and run by java classname Let's take a simple example:
- //save by .java only
- class A{
- public static void main(String args[]){
- System.out.println("Hello java");
- }
- }
- //compile by javac .java
- //run by java A compile it by javac .java run it by java A
13) Is delete, next, main, exit or null keyword in java?
No.
14) 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.
15) 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.
20) What is the output of the following Java program?
- class Test
- {
- public static void main (String args[])
- {
- System.out.println( 10 + 20 + "Javatpoint");
- System.out.println("Javatpoint" + 10 + 20 );
- }
- } The output of the above code will be 30Javatpoint Javatpoint Explanation In the first case, 10 and 20 are treated as numbers and added to be 30. Now, their sum 30 is treated as the string and concatenated with the string Javatpoint. Therefore, the output will be 30Javatpoint. In the second case, the string Javatpoint is concatenated with 10 to be the string Javatpoint10 which will then be concatenated with 20 to be Javatpoint1020.
21) What is the output of the following Java program?
- class Test
- {
- public static void main (String args[])
- {
- System.out.println( 10 * 20 + "Javatpoint");
- System.out.println("Javatpoint" + 10 * 20 );
- }
- } The output of the above code will be 200Javatpoint Javatpoint Explanation
In the first case, The numbers 10 and 20 will be multiplied first and then the result 200 is treated as the string and concatenated with the string Javatpoint to produce the output 200Javatpoint. In the second case, The numbers 10 and 20 will be multiplied first to be 200 because the precedence of the multiplication is higher than addition. The result 200 will be treated as the string and concatenated with the string Javatpointto produce the output as Javatpoint200.
22) What is the output of the following Java program?
- class Test
- {
- public static void main (String args[])
- {
- for(int i= 0 ; 0 ; i++)
- {
- System.out.println("Hello Javatpoint");
- }
- }
- } The above code will give the compile-time error because the for loop demands a boolean value in the second part and we are providing an integer value, i.e., 0.
Core Java - OOPs Concepts: Initial OOPs Interview Questions
There is given more than 50 OOPs (Object-Oriented Programming and System) interview questions. However, they have been categorized in many sections such as constructor interview questions, static interview questions, Inheritance Interview questions, Abstraction interview question, Polymorphism interview questions, etc. for better understanding.
23) 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. o Follows the bottom-up approach in program design. o Focus on data with methods to operate upon the object's data
The constructor can be defined as the special type of method that is used to initialize the state of an object. It is invoked when the class is instantiated, and the memory is allocated for the object. Every time, an object is created using the new keyword, the default constructor of the class is called. The name of the constructor must be similar to the class name. The constructor must not have an explicit return type. More Details.
28) How many types of constructors are used in Java?
Based on the parameters passed in the constructors, there are two types of constructors in Java. o Default Constructor: default constructor is the one which does not accept any value. The default constructor is mainly used to initialize the instance variable with the default values. It can also be used for performing some useful task on object creation. A default constructor is invoked implicitly by the compiler if there is no constructor defined in the class. o Parameterized Constructor: The parameterized constructor is the one which can initialize the instance variables with the given values. In other words, we can say that the constructors which can accept the arguments are called parameterized constructors.
29) What is the purpose of a default constructor?
The purpose of the default constructor is to assign the default value to the objects. The java compiler creates a default constructor implicitly if there is no constructor in the class.
- class Student3{
- int id;
- String name;
- void display(){System.out.println(id+" "+name);}
- public static void main(String args[]){
- Student3 s1=new Student3();
- Student3 s2=new Student3();
- s1.display();
- s2.display();
- }
- } Test it Now Output: 0 null 0 null Explanation: In the above class, you are not creating any constructor, so compiler provides you a default constructor. Here 0 and null values are provided by default constructor. More Details.
30) Does constructor return any value?
Ans: yes, The constructor implicitly returns the current instance of the class (You can't use an explicit return type with the constructor). More Details.
- System.out.println(test2.i);
- }
- }
- In the above program, The constructor Test is overloaded with another constructor. In the first call to the constructor, The constructor with one argument is called, and i will be initialized with the value 10. However, In the second call to the constructor, The constructor with the 2 arguments is called, and i will be initialized with the value 15.
34) What do you understand by copy constructor in
Java?
There is no copy constructor in java. However, we can copy the values from one object to another like copy constructor in C++. There are many ways to copy the values of one object into another in java. They are: o By constructor o By assigning the values of one object into another o By clone() method of Object class In this example, we are going to copy the values of one object into another using java constructor.
- //Java program to initialize the values from one object to another
- class Student6{
- int id;
- String name;
- //constructor to initialize integer and string
- Student6(int i,String n){
- id = i;
- name = n;
- }
- //constructor to initialize another object
- Student6(Student6 s){
- id = s.id;
- name =s.name;
- }
- void display(){System.out.println(id+" "+name);}
- public static void main(String args[]){
- Student6 s1 = new Student6( 111 ,"Karan");
- Student6 s2 = new Student6(s1);
- s1.display();
- s2.display();
- }
- } Test it Now Output: 111 Karan 111 Karan
35) What are the differences between the constructors
and methods?
There are many differences between constructors and methods. They are given below.
Java Constructor Java Method
A constructor is used to initialize the state of an object. A method is used to expose the behavior of an object. A constructor must not have a return type. A method must have a return type. The constructor is invoked implicitly. The method is invoked explicitly. The Java compiler provides a default constructor if you don't have any constructor in a class. The method is not provided by the compiler in any case. The constructor name must be same as the class name. The method name may or may not be same as class name.
- byte a = 10 ;
- byte b = 15 ;
- Test test = new Test(a,b);
- }
- } The output of the following program is: a = 10 b = 15 Here, the data type of the variables a and b, i.e., byte gets promoted to int, and the first parameterized constructor with the two integer parameters is called.
37) What is the output of the following Java program?
- class Test
- {
- int i;
- }
- public class Main
- {
- public static void main (String args[])
- {
- Test test = new Test();
- System.out.println(test.i);
- }
- } The output of the program is 0 because the variable i is initialized to 0 internally. As we know that a default constructor is invoked implicitly if there is no constructor in the class, the variable i is initialized to 0 since there is no constructor in the class.
38) What is the output of the following Java program?
- class Test
- {
- int test_a, test_b;
- Test(int a, int b)
- {
- test_a = a;
- test_b = b;
- }
- public static void main (String args[])
- {
- Test test = new Test();
- System.out.println(test.test_a+" "+test.test_b);
- }
- } There is a compiler error in the program because there is a call to the default constructor in the main method which is not present in the class. However, there is only one parameterized constructor in the class Test. Therefore, no default constructor is invoked by the constructor implicitly.
Core Java - OOPs Concepts: static keyword Interview Questions
39) What is the static variable?
The static variable is used to refer to the common property of all objects (that is not unique for each object), e.g., The company name of employees, college name of students, etc. Static variable gets memory only once in the class area at the time of class loading. Using a static variable makes your program more memory efficient (it saves memory). Static variable belongs to the class rather than the object.
- //Program of static variable
- class Student8{
- int rollno;
- String name;
- static String college ="ITS";
- Student8(int r,String n){
- rollno = r;
- name = n;
- }
- void display (){System.out.println(rollno+" "+name+" "+college);}
- public static void main(String args[]){
- Student8 s1 = new Student8( 111 ,"Karan");
- Student8 s2 = new Student8( 222 ,"Aryan");
- s1.display();
- s2.display();
- }
- }
