Chapter 1.  Introduction

The primitive types are defined to be the same on all machines and in all implementations, and are various sizes of two’s-complement integers, IEEE 754 floating-point numbers, a boolean type, and a Unicode character char type. Values of the primitive types do not share state.

Java proqramming language

Java is a class-based, object-oriented programming language that is designed to have as few implementation dependencies as possible. It is intended to let application developers write once, and run anywhere (WORA), meaning that compiled Java code can run on all platforms that support Java without the need for recompilation. Java was first released in 1995 and is widely used for developing applications for desktop, web, and mobile devices. Java is known for its simplicity, robustness, and security features, making it a popular choice for enterprise-level applications.

JAVA was developed by James Gosling at Sun Microsystems Inc in the year 1995 and later acquired by Oracle Corporation. It is a simple programming language. Java makes writing, compiling, and debugging programming easy. It helps to create reusable code and modular programs. Java is a class-based, object-oriented programming language and is designed to have as few implementation dependencies as possible. A general-purpose programming language made for developers to write once run anywhere that is compiled Java code can run on all platforms that support Java. Java applications are compiled to byte code that can run on any Java Virtual Machine. The syntax of Java is similar to c/c++.

History: Java’s history is very interesting. It is a programming language created in 1991. James Gosling, Mike Sheridan, and Patrick Naughton, a team of Sun engineers known as the Green team initiated the Java language in 1991. Sun Microsystems released its first public implementation in 1996 as Java 1.0. It provides no-cost -run-times on popular platforms. Java1.0 compiler was re-written in Java by Arthur Van Hoff to strictly comply with its specifications. With the arrival of Java 2, new versions had multiple configurations built for different types of platforms.

In 1997, Sun Microsystems approached the ISO standards body and later formalized Java, but it soon withdrew from the process. At one time, Sun made most of its Java implementations available without charge, despite their proprietary software status. Sun generated revenue from Java through the selling of licenses for specialized products such as the Java Enterprise System.

On November 13, 2006, Sun released much of its Java virtual machine as free, open-source software. On May 8, 2007, Sun finished the process, making all of its JVM’s core code available under open-source distribution terms.

The principles for creating java were simple, robust, secured, high-performance, portable, multi-threaded, interpreted, dynamic, etc. In 1995 Java was developed by James Gosling, who is known as the Father of Java. Currently, Java is used in mobile devices, internet programming, games, e-business, etc.

Implementation of a Java application program involves a following step. They include:
1. Creating the program
2. Compiling the program
3. Running the program

Remember that, before we begin creating the program, the Java Development Kit (JDK) must be properly installed on our system and also path will be set.
• Creating Program
We can create a program using Text Editor (Notepad) or IDE (NetBeans)
class Test
public static void main(String []args)
System.out.println(“My First Java Program.”);
>
>;

File -> Save -> d:\Test.java

• Compiling the program
To compile the program, we must run the Java compiler (javac), with the name of the source file on “command prompt” like as follows

If everything is OK, the “javac” compiler creates a file called “Test.class” containing byte code of the program.

• Running the program
We need to use the Java Interpreter to run a program.

Java programming language is named JAVA. Why?

After the name OAK, the team decided to give it a new name to it and the suggested words were Silk, Jolt, revolutionary, DNA, dynamic, etc. These all names were easy to spell and fun to say, but they all wanted the name to reflect the essence of technology. In accordance with James Gosling, Java the among the top names along with Silk, and since java was a unique name so most of them preferred it.

Java is the name of an island in Indonesia where the first coffee(named java coffee) was produced. And this name was chosen by James Gosling while having coffee near his office. Note that Java is just a name, not an acronym.

Java Terminology

Before learning Java, one must be familiar with these common terms of Java.

1. Java Virtual Machine(JVM): This is generally referred to as JVM. There are three execution phases of a program. They are written, compile and run the program.

• Writing a program is done by a java programmer like you and me.
• The compilation is done by the JAVAC compiler which is a primary Java compiler included in the Java development kit (JDK). It takes the Java program as input and generates bytecode as output.
• In the Running phase of a program, JVM executes the bytecode generated by the compiler.

Now, we understood that the function of Java Virtual Machine is to execute the bytecode produced by the compiler. Every Operating System has a different JVM but the output they produce after the execution of bytecode is the same across all the operating systems. This is why Java is known as a platform-independent language.

2. Bytecode in the Development Process: As discussed, the Javac compiler of JDK compiles the java source code into bytecode so that it can be executed by JVM. It is saved as .class file by the compiler. To view the bytecode, a disassembler like javap can be used.

3. Java Development Kit(JDK): While we were using the term JDK when we learn about bytecode and JVM. So, as the name suggests, it is a complete Java development kit that includes everything including compiler, Java Runtime Environment (JRE), java debuggers, java docs, etc. For the program to execute in java, we need to install JDK on our computer in order to create, compile and run the java program.

4. Java Runtime Environment (JRE): JDK includes JRE. JRE installation on our computers allows the java program to run, however, we cannot compile it. JRE includes a browser, JVM, applet support, and plugins. For running the java program, a computer needs JRE.

5. Garbage Collector: In Java, programmers can’t delete the objects. To delete or recollect that memory JVM has a program called Garbage Collector. Garbage Collectors can recollect the objects that are not referenced. So Java makes the life of a programmer easy by handling memory management. However, programmers should be careful about their code whether they are using objects that have been used for a long time. Because Garbage cannot recover the memory of objects being referenced.

6. ClassPath: The classpath is the file path where the java runtime and Java compiler look for .class files to load. By default, JDK provides many libraries. If you want to include external libraries they should be added to the classpath.

Primary/Main Features of Java

1. Platform Independent: Compiler converts source code to bytecode and then the JVM executes the bytecode generated by the compiler. This bytecode can run on any platform be it Windows, Linux, or macOS which means if we compile a program on Windows, then we can run it on Linux and vice versa. Each operating system has a different JVM, but the output produced by all the OS is the same after the execution of the bytecode. That is why we call java a platform-independent language.

2. Object-Oriented Programming Language: Organizing the program in the terms of a collection of objects is a way of object-oriented programming, each of which represents an instance of the class.

The four main concepts of Object-Oriented programming are:

• Abstraction
• Encapsulation
• Inheritance
• Polymorphism

3. Simple: Java is one of the simple languages as it does not have complex features like pointers, operator overloading, multiple inheritances, and Explicit memory allocation.

4. Robust: Java language is robust which means reliable. It is developed in such a way that it puts a lot of effort into checking errors as early as possible, that is why the java compiler is able to detect even those errors that are not easy to detect by another programming language. The main features of java that make it robust are garbage collection, Exception Handling, and memory allocation.

5. Secure: In java, we don’t have pointers, so we cannot access out-of-bound arrays i.e it shows ArrayIndexOutOfBound Exception if we try to do so. That’s why several security flaws like stack corruption or buffer overflow are impossible to exploit in Java. Also, java programs run in an environment that is independent of the os(operating system) environment which makes java programs more secure.

6. Distributed: We can create distributed applications using the java programming language. Remote Method Invocation and Enterprise Java Beans are used for creating distributed applications in java. The java programs can be easily distributed on one or more systems that are connected to each other through an internet connection.

7. Multithreading: Java supports multithreading. It is a Java feature that allows concurrent execution of two or more parts of a program for maximum utilization of the CPU.

8. Portable: As we know, java code written on one machine can be run on another machine. The platform-independent feature of java in which its platform-independent bytecode can be taken to any platform for execution makes java portable.

9. High Performance: Java architecture is defined in such a way that it reduces overhead during the runtime and at some times java uses Just In Time (JIT) compiler where the compiler compiles code on-demand basics where it only compiles those methods that are called making applications to execute faster.

10. Dynamic flexibility: Java being completely object-oriented gives us the flexibility to add classes, new methods to existing classes, and even create new classes through sub-classes. Java even supports functions written in other languages such as C, C++ which are referred to as native methods.

11. Sandbox Execution: Java programs run in a separate space that allows user to execute their applications without affecting the underlying system with help of a bytecode verifier. Bytecode verifier also provides additional security as its role is to check the code for any violation of access.

12. Write Once Run Anywhere: As discussed above java application generates a ‘.class’ file that corresponds to our applications(program) but contains code in binary format. It provides ease t architecture-neutral ease as bytecode is not dependent on any machine architecture. It is the primary reason java is used in the enterprising IT industry globally worldwide.

13. Power of compilation and interpretation: Most languages are designed with the purpose of either they are compiled language or they are interpreted language. But java integrates arising enormous power as Java compiler compiles the source code to bytecode and JVM executes this bytecode to machine OS-dependent executable code.

class : class keyword is used to declare classes in Java
public : It is an access specifier. Public means this function is visible to all.
static : static is again a keyword used to make a function static. To execute a static function you do not have to create an Object of the class. The main() method here is called by JVM, without creating any object for class.
void : It is the return type, meaning this function will not return anything.
main : main() method is the most important method in a Java program. This is the method which is executed, hence all the logic must be inside the main() method. If a java class is not having a main() method, it causes compilation error.
String[] args : This is used to signify that the user may opt to enter parameters to the Java Program at command line. We can use both String[] args or String args[]. Java compiler would accept both forms.
System.out.println : This is used to print anything on the console like “printf” in C language.

Example

Chapter 1. Introduction

The Java ® programming language is a general-purpose, concurrent, class-based, object-oriented language. It is designed to be simple enough that many programmers can achieve fluency in the language. The Java programming language is related to C and C++ but is organized rather differently, with a number of aspects of C and C++ omitted and a few ideas from other languages included. It is intended to be a production language, not a research language, and so, as C. A. R. Hoare suggested in his classic paper on language design, the design has avoided including new and untested features.

The Java programming language is strongly and statically typed. This specification clearly distinguishes between the compile-time errors that can and must be detected at compile time, and those that occur at run time. Compile time normally consists of translating programs into a machine-independent byte code representation. Run-time activities include loading and linking of the classes needed to execute a program, optional machine code generation and dynamic optimization of the program, and actual program execution.

The Java programming language is a relatively high-level language, in that details of the machine representation are not available through the language. It includes automatic storage management, typically using a garbage collector, to avoid the safety problems of explicit deallocation (as in C’s free or C++’s delete ). High-performance garbage-collected implementations can have bounded pauses to support systems programming and real-time applications. The language does not include any unsafe constructs, such as array accesses without index checking, since such unsafe constructs would cause a program to behave in an unspecified way.

The Java programming language is normally compiled to the bytecode instruction set and binary format defined in The Java Virtual Machine Specification, Java SE 17 Edition.

1.1. Organization of the Specification

Chapter 2 describes grammars and the notation used to present the lexical and syntactic grammars for the language.

Chapter 3 describes the lexical structure of the Java programming language, which is based on C and C++. The language is written in the Unicode character set. It supports the writing of Unicode characters on systems that support only ASCII.

Chapter 4 describes types, values, and variables. Types are subdivided into primitive types and reference types.

The primitive types are defined to be the same on all machines and in all implementations, and are various sizes of two’s-complement integers, IEEE 754 floating-point numbers, a boolean type, and a Unicode character char type. Values of the primitive types do not share state.

Reference types are the class types, the interface types, and the array types. The reference types are implemented by dynamically created objects that are either instances of classes or arrays. Many references to each object can exist. All objects (including arrays) support the methods of the class Object , which is the (single) root of the class hierarchy. A predefined String class supports Unicode character strings. Classes exist for wrapping primitive values inside of objects. In many cases, wrapping and unwrapping is performed automatically by the compiler (in which case, wrapping is called boxing, and unwrapping is called unboxing). Classes and interfaces may be generic, that is, they may be parameterized by reference types. Parameterized types of such classes and interfaces may then be invoked with specific type arguments.

Variables are typed storage locations. A variable of a primitive type holds a value of that exact primitive type. A variable of a class type can hold a null reference or a reference to an object that is an instance of the named class or any subclass of that class. A variable of an interface type can hold a null reference or a reference to an instance of any class that implements the named interface. A variable of an array type can hold a null reference or a reference to an array. A variable of class type Object can hold a null reference or a reference to any object, whether class instance or array.

Chapter 5 describes conversions and numeric promotions. Conversions change the compile-time type and, sometimes, the value of an expression. These conversions include the boxing and unboxing conversions between primitive types and reference types. Numeric promotions are used to convert the operands of a numeric operator to a common type where an operation can be performed. There are no loopholes in the language; casts on reference types are checked at run time to ensure type safety.

Chapter 6 describes declarations and names, and how to determine what names mean (that is, which declaration a name denotes). The Java programming language does not require classes and interfaces, or their members, to be declared before they are used. Declaration order is significant only for local variables, local classes, local interfaces, and the order of field initializers in a class or interface. Recommended naming conventions that make for more readable programs are described here.

Chapter 7 describes the structure of a program, which is organized into packages. The members of a package are classes, interfaces, and subpackages. Packages, and consequently their members, have names in a hierarchical name space; the Internet domain name system can usually be used to form unique package names. Compilation units contain declarations of the classes and interfaces that are members of a given package, and may import classes and interfaces from other packages to give them short names.

Packages may be grouped into modules that serve as building blocks in the construction of very large programs. The declaration of a module specifies which other modules (and thus packages, and thus classes and interfaces) are required in order to compile and run code in its own packages.

The Java programming language supports limitations on external access to the members of packages, classes, and interfaces. The members of a package may be accessible solely by other members in the same package, or by members in other packages of the same module, or by members of packages in different modules. Similar constraints apply to the members of classes and interfaces.

Chapter 8 describes classes. The members of classes are classes, interfaces, fields (variables) and methods. Class variables exist once per class. Class methods operate without reference to a specific object. Instance variables are dynamically created in objects that are instances of classes. Instance methods are invoked on instances of classes; such instances become the current object this during their execution, supporting the object-oriented programming style.

Classes support single inheritance, in which each class has a single superclass. Each class inherits members from its superclass, and ultimately from the class Object . Variables of a class type can reference an instance of the named class or any subclass of that class, allowing new classes to be used with existing methods, polymorphically.

Classes support concurrent programming with synchronized methods. Methods declare the checked exceptions that can arise from their execution, which allows compile-time checking to ensure that exceptional conditions are handled. Objects can declare a finalize method that will be invoked before the objects are discarded by the garbage collector, allowing the objects to clean up their state.

For simplicity, the language has neither declaration “headers” separate from the implementation of a class nor separate type and class hierarchies.

A restricted kind of class, enum classes, supports the definition of small sets of values and their manipulation in a type safe manner. Unlike enumerations in other languages, enum constants are objects and may have their own methods.

Another restricted kind of class, record classes, supports the compact expression of simple objects that serve as aggregates of values.

Chapter 9 describes interfaces. The members of interfaces are classes, interfaces, constant fields, and methods. Classes that are otherwise unrelated can implement the same interface. A variable of an interface type can contain a reference to any object that implements the interface.

Classes and interfaces support multiple inheritance from interfaces. A class that implements one or more interfaces may inherit instance methods from both its superclass and its superinterfaces.

Annotations are metadata that may be applied to declarations in a program, as well as to the uses of types in declarations and expressions. The form of an annotation is defined by an annotation interface, a specialized kind of interface. Annotations are not permitted to affect the semantics of programs in the Java programming language in any way. However, they provide useful input to various tools.

Chapter 10 describes arrays. Array accesses include bounds checking. Arrays are dynamically created objects and may be assigned to variables of type Object . The language supports arrays of arrays, rather than multidimensional arrays.

Chapter 11 describes exceptions, which are nonresuming and fully integrated with the language semantics and concurrency mechanisms. There are three kinds of exceptions: checked exceptions, run-time exceptions, and errors. The compiler ensures that checked exceptions are properly handled by requiring that a method or constructor can result in a checked exception only if the method or constructor declares it. This provides compile-time checking that exception handlers exist, and aids programming in the large. Most user-defined exceptions should be checked exceptions. Invalid operations in the program detected by the Java Virtual Machine result in run-time exceptions, such as NullPointerException . Errors result from failures detected by the Java Virtual Machine, such as OutOfMemoryError . Most simple programs do not try to handle errors.

Chapter 12 describes activities that occur during execution of a program. A program is normally stored as binary files representing compiled classes and interfaces. These binary files can be loaded into a Java Virtual Machine, linked to other classes and interfaces, and initialized.

After initialization, class methods and class variables may be used. Some classes may be instantiated to create new objects of the class type. Objects that are class instances also contain an instance of each superclass of the class, and object creation involves recursive creation of these superclass instances.

When an object is no longer referenced, it may be reclaimed by the garbage collector. If an object declares a finalizer, the finalizer is executed before the object is reclaimed to give the object a last chance to clean up resources that would not otherwise be released. When a class is no longer needed, it may be unloaded.

Chapter 13 describes binary compatibility, specifying the impact of changes to classes and interfaces on other classes and interfaces that use the changed classes and interfaces but have not been recompiled. These considerations are of interest to developers of classes and interfaces that are to be widely distributed, in a continuing series of versions, often through the Internet. Good program development environments automatically recompile dependent code whenever a class or interface is changed, so most programmers need not be concerned about these details.

Chapter 14 describes blocks and statements, which are based on C and C++. The language has no goto statement, but includes labeled break and continue statements. Unlike C, the Java programming language requires boolean (or Boolean ) expressions in control-flow statements, and does not convert types to boolean implicitly (except through unboxing), in the hope of catching more errors at compile time. A synchronized statement provides basic object-level monitor locking. A try statement can include catch and finally clauses to protect against non-local control transfers. Chapter 14 also describes patterns, which are used within statements (and expressions) to conditionally declare and initialize local variables.

Chapter 15 describes expressions. This document fully specifies the (apparent) order of evaluation of expressions, for increased determinism and portability. Overloaded methods and constructors are resolved at compile time by picking the most specific method or constructor from those which are applicable.

Chapter 16 describes the precise way in which the language ensures that local variables are definitely set before use. While all other variables are automatically initialized to a default value, the Java programming language does not automatically initialize local variables in order to avoid masking programming errors.

Chapter 17 describes the semantics of threads and locks, which are based on the monitor-based concurrency originally introduced with the Mesa programming language. The Java programming language specifies a memory model for shared-memory multiprocessors that supports high-performance implementations.

Chapter 18 describes a variety of type inference algorithms used to test applicability of generic methods and to infer types in a generic method invocation.

Chapter 19 presents a syntactic grammar for the language.

Java proqramming language

Learn why Java remains such a popular development platform and how it accelerates projects and supports a wide range of emerging technologies.

What is Java?

Java is a widely used object-oriented programming language and software platform that runs on billions of devices, including notebook computers, mobile devices, gaming consoles, medical devices and many others. The rules and syntax of Java are based on the C and C++ languages.

One major advantage of developing software with Java is its portability. Once you have written code for a Java program on a notebook computer, it is very easy to move the code to a mobile device. When the language was invented in 1991 by James Gosling of Sun Microsystems (later acquired by Oracle), the primary goal was to be able to “write once, run anywhere.”

It’s also important to understand that Java is much different from JavaScript. Javascript does not need to be compiled, while Java code does need to be compiled. Also, Javascript only runs on web browsers while Java can be run anywhere.

New and improved software development tools are coming to market at a remarkable pace, displacing incumbent products once thought to be indispensable. In light of this continual turnover, Java’s longevity is impressive; more than two decades after its creation, Java is still the most popular language for application software development—developers continue to choose it over languages such as Python, Ruby, PHP, Swift, C++, and others. As a result, Java remains an important requirement for competing in the job market.

How Java works

Before exploring the reasons for Java’s enduring popularity, let’s review what Java is in more detail and its importance for enterprise application development.

Java is a technology consisting of both a programming language and a software platform. To create an application using Java, you need to download the Java Development Kit (JDK), which is available for Windows, macOS, and Linux. You write the program in the Java programming language, then a compiler turns the program into Java bytecode—the instruction set for the Java Virtual Machine (JVM) that is a part of the Java runtime environment (JRE). Java bytecode runs without modification on any system that supports JVMs, allowing your Java code to be run anywhere.

The Java software platform consists of the JVM, the Java API, and a complete development environment. The JVM parses and runs (interprets) the Java bytecode. The Java API consists of an extensive set of libraries including basic objects, networking and security functions; Extensible Markup Language (XML) generation; and web services. Taken together, the Java language and the Java software platform create a powerful, proven technology for enterprise software development.

Why Java matters

If you are an enterprise application developer, you already know what Java is, and it’s likely that your organization already has thousands, even millions, of lines of production code written in Java. You will likely need some level of Java expertise to allow you to troubleshoot, maintain, and upgrade your existing codebase.

However, it would be a mistake to view Java only in terms of legacy applications. The Java language forms the heart of the Android operating system, which powers by far the largest share of the world’s smartphones. Java is also among the most popular languages for machine learning and data science applications. Its robustness, ease of use, cross-platform capabilities and security make Java the language of choice for internet solutions in many enterprise shops.

In particular, Java technology is an ideal framework for developing web applications, the foundation for a digital business in any industry. Java application servers are web containers for Java components, XML, and web services, which interact with databases and provide dynamic web content. Java application servers form a stable deployment environment for enterprise applications with capabilities such as transaction management, security, clustering, performance, availability, connectivity, and scalability.

Technical benefits

When it comes to choosing a programming language and environment for your next enterprise application, there are solid technical reasons to consider Java, including interoperability, scalability, and adaptability.

The core philosophy behind its creation—interoperability across disparate devices—remains the strongest argument for favoring Java for new enterprise applications. Java’s object-oriented architecture allows you to create modular programs and reusable code, shortening development cycles and extending the longevity of enterprise applications.

Platform scalability is a key attribute of Java. With Java, you can use one single system across a broad range of use cases. Existing desktop applications can be easily adapted to run on smaller devices that have limited resources. You can also migrate applications from mobile to desktop, developing business apps for the Android platform and then integrating them into your current desktop software, bypassing lengthy and expensive development cycles.

Java also wins points with strategic planners for its ability to adapt to new use cases. For example, Java is widely considered to be an ideal platform for the Internet of Things (IoT). The typical IoT application interconnects a large number of disparate devices, a task that is greatly simplified by the fact that billions of devices run Java. Furthermore, Java’s extensive ecosystem of developers is constantly developing and sharing new libraries with functionality specifically targeted at IoT application development.

Business benefits

The technical arguments for Java are compelling, but the business reasons to choose Java are equally strong: a large talent pool, a short learning curve, and a wide range of integrated development environments (IDEs).

As more companies use connected devices, machine learning algorithms, and cloud solutions, the demand for skilled developers continues to grow. Many analysts foresee a scarcity of senior-level programmers in the near future, making it difficult to staff new software initiatives. Demand for mobile app developers could soon easily exceed the available supply.

The large talent pool of Java developers constitutes a compelling reason to base major software initiatives on Java. When staffing managers post job openings for Java developers, they can expect to receive many qualified resumes and fill those positions relatively quickly. Managers can also draw on contract resources to supplement in-house staff for specific tasks without adding headcount.

In addition to senior-level developers, major software initiatives also require large numbers of junior contributors. While Java remains a popular introductory programming language in university computer science curriculums, many graduates lack the proficiency to be productive on day one. Java is easier to learn and master than many other programming languages, leading to a shorter learning curve and faster ramp-up to productivity. Java’s extensive online community of developer forums, tutorials, and user groups helps beginners get up to speed rapidly and provides seasoned programmers with effective, proven problem-solving tools.

In the area of programming tools, Java offers a range of IDEs. Experienced Java developers can quickly ramp up on a new environment, which frees development managers to choose the IDE that best fits the type of project, budget, development methodology and programmer skill level. Many seasoned Java programmers think of NetBeans, Eclipse, and IntelliJ IDEA as the top three IDEs for enterprise application development. But there are cases where a more lightweight IDE such as DrJava, BlueJ, JCreator, or Eclipse Che is the best choice.