We should declare a class as abstract in the following situations:

1. When the class is inherited from an abstract class, but not all the abstract methods have been overridden.
2. In the case when minimum one of the methods in the class is declared as an abstract.

Polymorphism is used when there is a need for override functionality when inheriting class. It’s about shared classes and shared contracts.

Polymorphism is used when there is a need for override functionality when inheriting class. It’s about shared classes and shared contracts.

There are the following features in Java Programming Language.

• Simple: Java is easy to learn. The syntax of Java is based on C++ which makes it easier to write the program in it.
• Object-Oriented: Java follows the object-oriented paradigm which allows us to maintain our code as a combination of different types of objects that incorporates both data and behavior.
• Portable: Java supports the 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.
• 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 a platform on which its code is executed. Java doesn't depend upon the operating system to be executed.
• 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 secure.
• 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.
• 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.
• Interpreted: Java uses the Just-in-time (JIT) interpreter along with the compiler for the program execution.
• 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++).
• 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.
• 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.
• Dynamic: Java is a dynamic language. It supports the dynamic loading of classes. It means classes are loaded on demand. It also supports functions from its native languages, i.e., C and C++.

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.

ble> 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 Windows, web-based, enterprise, and mobile applications. Design Goal C++ was designed for systems and application programming. It was an extension of the 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 the 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 inheritances C++ supports multiple inheritances. Java doesn't support multiple inheritances through the 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 a pointer program in C++. Java supports pointers 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 compilers only. C++ is compiled and run using the compiler which converts source code into machine code so, C++ is platform dependent. Java uses a compiler and an interpreter. Java source code is converted into bytecode at compilation time. The interpreter executes this bytecode at runtime and produces output. Java is interpreted which 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 comments. Java supports documentation comment (/** ... */) to create documentation for java source code. Virtual Keyword C++ supports virtual keywords so that we can decide whether or not to override a function. Java has no virtual keyword. We can override all non-static methods by default. In other words, non-static methods are virtual by default. unsigned right shift >>> C++ doesn't support >>> operator. Java supports an unsigned right shift >>> operator that fills zero at the top for the negative numbers. For positive numbers, it works the same as the >> operator. Inheritance Tree C++ creates a new inheritance tree always. Java uses a single inheritance tree always because all classes are the child of the Object class in java. The object class is the root of the inheritance tree in java. Hardware C++ is nearer to hardware. Java is not so interactive with hardware. Object-oriented C++ is an object-oriented language. However, in C language, a single root hierarchy is not possible. Java is also an object-oriented language. However, everything (except fundamental types) is an object in Java. It is a single root hierarchy as everything gets derived from java. lang.Object.

>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.

JRE

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

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: