Q.1: What are the most important features of Java?
Java is object oriented, platform independent, secure, robust, simple, etc.

*Platform Independent
*Robust and Secure
*High performance
*Compiler and interpreter
*Object oriented

  Q.2: What do you understand by Java virtual machine?

Answer-1: VAMSI

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.


Java Virtual Machine(JVM) is a part of java runtime environment. which converts the byte code into the source code. JVM had both compiler and interpreter.

  Q.3: What is the difference between JDK, JRE, and JVM?

Answer-1: SANJAY

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.
JRE stands for Java Runtime Environment. It is the implementation of JVM. The Java Runtime Environment is a set of software tools which are used for developing Java applications. It is used to provide the runtime environment. It is the implementation of JVM. It physically exists. It contains a set of libraries + other files that JVM uses at runtime.
JDK is an acronym for Java Development Kit. It is a software development environment which is used to develop Java applications and applets. It physically exists. It contains JRE + development tools. JDK is an implementation of any one of the below given Java Platforms released by Oracle Corporation:
o Standard Edition Java Platform
o Enterprise Edition Java Platform
o Micro Edition Java Platform

  Q.4: How many types of memory areas are allocated by JVM?

Answer-1: RANJIT

Many types:
1. Class(Method) Area: Class Area stores per-class structures such as the runtime constant pool, field, method data, and the code for methods.
2. Heap: It is the runtime data area in which the memory is allocated to the objects
3. Stack: Java Stack stores frames. It holds local variables and partial results, and plays a part in method invocation and return. Each thread has a private JVM stack, created at the same time as the thread. A new frame is created each time a method is invoked. A frame is destroyed when its method invocation completes.
4. Program Counter Register: PC (program counter) register contains the address of the Java virtual machine instruction currently being executed.
5. Native Method Stack: It contains all the native methods used in the application.

  Q.5: List the features of Java Programming language.

Answer-1: JOSHUA

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 (32 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++.

  Q.6: What is JIT compiler?

Answer-1: VAMSI

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.

  Q.7: What is the platform?

Answer-1: SANJAY

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.

  Q.8: What are the main differences between the Java platform and other platforms?

Answer-1: RANJIT

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.

  Q.9: What gives Java its 'write once and run anywhere' nature?

Answer-1: JOSHUA

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.

  Q.10: What is classloader?

Answer-1: VAMSI

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.
3. 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.

  Q.11: Is Empty .java file name a valid source file name?

Answer-1: SANJAY

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:
1. //save by .java only
2. class A{
3. public static void main(String args[]){
4. System.out.println("Hello java");
5. }
6. }
7. //compile by javac .java
8. //run by java A
compile it by javac .java
run it by java A

  Q.12: Is delete, next, main, exit or null keyword in java?

Answer-1: RANJIT

No. No,these key words are not present in java,delet,next,exit are operations performed on java program,main is the predefind method,and null is the default value for the String type.

  Q.13: 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?

Answer-1: VAMSI

It is empty, but not null.

  Q.14: What if I write static public void instead of public static void?

Answer-1: JOSHUA

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

  Q.15: What is the default value of the local variables?

Answer-1: RANJIT

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


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