Q-1. What is Java?
ava is a programming language and a platform. Java is a high level, robust, object-oriented and secure programming language.
Java was developed by Sun Microsystems (which is now the subsidiary of Oracle) in the year 1995. James Gosling is known as the father of Java. Before Java, its name was Oak. Since Oak was already a registered company, so James Gosling and his team changed the name from Oak to Java.
Platform: Any hardware or software environment in which a program runs, is known as a platform. Since Java has a runtime environment (JRE) and API, it is called a platform.
Q-2. Why is Java not a pure object oriented language?
Java supports primitive data types - byte, boolean, char, short, int, float, long, and double and hence it is not a pure object oriented language.
Q.-3. What is JVM ?
JVM (Java Virtual Machine) is an abstract machine. It is called a virtual machine because it doesn't physically exist. It is a specification that provides a runtime environment in which Java bytecode can be executed.
JVMs are available for many hardware and software platforms. JVM, JRE, and JDK are platform dependent because the configuration of each OS is different from each other. However, Java is platform independent. There are three notions of the JVM: specification, implementation, and instance.
The JVM performs the following main tasks:
- Loads code
- Verifies code
- Executes code
- Provides runtime environment
Q-4.What is the difference between JDK, JRE, and JVM ?
JDK stands for Java Development Kit. It is a software development kit that includes tools for developing, compiling, and debugging Java programs. JRE stands for Java Runtime Environment. It is a software environment that provides the necessary runtime libraries and components for running Java programs. JVM is the virtual machine that executes the Java bytecode.
JRE
JRE is an acronym for Java Runtime Environment. It is also written as Java RTE. 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
(Java Development Kit) is a Kit that provides the environment to develop and execute(run) the Java program. JDK is a kit(or package) that includes two things
- Development Tools(to provide an environment to develop your java programs)
- JRE (to execute your java program).
JVM
JVM (Java Virtual Machine) is an abstract machine. It is called a virtual machine because it doesn't physically exist. It is a specification that provides a runtime environment in which Java bytecode can be executed. It can also run those programs which are written in other languages and compiled to Java bytecode.
JVMs are available for many hardware and software platforms. JVM, JRE, and JDK are platform dependent because the configuration of each OS is different from each other. However, Java is platform independent.
Q-5.What is JIT?
JIT stands for (Just-in-Time) compiler is a part of JRE(Java Runtime Environment), it is used for better performance of the Java applications during run-time. The use of JIT is mentioned in step by step process mentioned below:
- Source code is compiled with javac to form bytecode
- Bytecode is further passed on to JVM
- JIT is a part of JVM, JIT is responsible for compiling bytecode into native machine code at run time.
- The JIT compiler is enabled throughout, while it gets activated when a method is invoked. For a compiled method, the JVM directly calls the compiled code, instead of interpreting it.
- As JVM calls the compiled code that increases the performance and speed of the execution.
Q-6. What are Memory storages available with JVM?
JVM consists of a five type of memory storages as mentioned below:
- Class(Method) Area: stores class-level data of every class such as the runtime constant pool, field, and method data, and the code for methods.
- Heap: Objects are created or objects are stored. It is used to allocate memory to objects during run time.
- Stack: stores data and partial results which will be needed while returning value for method and performing dynamic linking
- Program Counter Register: stores the address of the Java virtual machine instruction currently being executed.
- Native Method Stack: stores all the native methods used in the application.
Q-7.What is a classloader?
Java ClassLoader is used to load the classes at run time. In other words, JVM performs the linking process at runtime. Classes are loaded into the JVM according to need.
Types of ClassLoader
Bootstrap Class Loader: It loads standard JDK class files from rt.jar and other core classes. It is a parent of all class loaders.
Extensions Class Loader: it loads classes from jre/lib/ext directory or any other directory as java.ext.dirs. It is implemented by sun.misc.Launcher$ExtClassLoader in JVM.
Application Class Loader: It loads application specific classes from the CLASSPATH environment variable. It can be set while invoking program using -cp or classpath command line options. It is a child of Extension ClassLoader. It is implemented by sun.misc.Launcher$AppClassLoader class. All Java ClassLoader implements java.lang.ClassLoader.
