Java is a high-level, class-based, object-oriented programming language that is designed to have as few implementation dependencies as possible. It is a general-purpose programming language intended to let application developers write once, run anywhere (WORA),meaning that compiled Java code can run on all platforms that support Java without the need for recompilation.Java applications are typically compiled to bytecode that can run on any Java virtual machine (JVM) regardless of the underlying computer architecture. The syntax of Java is similar to C and C++, but has fewer low-level facilities than either of them. The Java runtime provides dynamic capabilities (such as reflection and runtime code modification) that are typically not available in traditional compiled languages. As of 2019, Java was one of the most popular programming languages in use according to GitHub,particularly for client-server web applications, with a reported 9 million developers.

• Mobile App Development.
• Desktop GUI Applications.
• Web-based Applications.
• Gaming Applications
• Big Data Technologies
• Cloud-based Applications

the fundamental forces that necessitated the invention of Java are portability and security, other factors also played an important role in molding the final form of the language. The key considerations were summed up by the Java team in the following list of buzzwords:

• Simple
• Secure
• Portable
• Object-oriented
• Robust
• Multithreaded
• Architecture-neutral
• Architecture-neutral
• Distributed
• High performance
• Dynamic

Java defines eight primitive types of data: byte, short, int, long, char, float, double, and boolean. The primitive types are also commonly referred to as simple types, and both terms will be used in this book. These can be put in four groups:

• Integers This group includes byte, short, int, and long, which are for whole-valued signed numbers.
• Floating-point numbers This group includes float and double, which represent numbers with fractional precision.
• Characters This group includes char, which represents symbols in a character set, like letters and numbers.
• Boolean This group includes boolean, which is a special type for representing true/false values.

The variable is the basic unit of storage in a Java program. A variable is defined by the combination of an identifier, a type, and an optional initializer. In addition, all variables have a scope, which defines their visibility, and a lifetime. These elements are examined next.

The identifiers that you choose have nothing intrinsic in their names that indicates their type. Java allows any properly formed identifier to have any declared type.

A programming language uses control statements to cause the flow of execution to advance and branch based on changes to the state of a program. Java’s program control statements can be put into the following categories: selection, iteration, and jump. Selection statements allow your program to choose different paths of execution based upon the outcome of an expression or the state of a variable. Iteration statements enable program execution to repeat one or more statements (that is, iteration statements form loops). Jump statements allow your program to execute in a nonlinear fashion. All of Java’s control statements are examined here.

The if statement is Java’s conditional branch statement. It can be used to route program execution through two different paths. Here is the general form of the if statement:

Here, each statement may be a single statement or a compound statement enclosed in curly braces (that is, a block). The condition is any expression that returns a boolean value. The else clause is optional.

The if works like this: If the condition is true, then statement1 is executed. Otherwise, statement2 (if it exists) is executed. In no case will both statements be executed

The switch statement is Java’s multiway branch statement. It provides an easy way to dispatch execution to different parts of your code based on the value of an expression. As such, it often provides a better alternative than a large series of if-else-if statements. Here is the general form of a switch statement:

switch (expression) {
  case value1:
    // statement sequence
    break; 
 case value2: 
   // statement sequence
    break;
.
.
  case valueN : 
    // statement sequence 
    break; 
 default: 
   // default statement sequence 
}

The class is at the core of Java. It is the logical construct upon which the entire Java language is built because it defines the shape and nature of an object. As such, the class forms the basis for object-oriented programming in Java. Any concept you wish to implement in a Java program must be encapsulated within a class.

Because the class is so fundamental to Java, this and the next few chapters will be devoted to it. Here, you will be introduced to the basic elements of a class and learn how a class can be used to create objects. You will also learn about methods, constructors, and the this keyword.

class classname  {
  type instance-variable1;
  // ...
  type instance-variable2;
  // ... 
  type instance-variableN;
  // ...
  type methodname1(parameter-list){
  // body of the method
  }
  type methodname2(parameter-list){
  // body of the method
  }
  type methodnameN(parameter-list){
  // body of the method
  }
}

Inheritance is one of the cornerstones of object-oriented programming because it allows the creation of hierarchical classifications. Using inheritance, you can create a general class that defines traits common to a set of related items. This class can then be inherited by other, more specific classes, each adding those things that are unique to it. In the terminology of Java, a class that is inherited is called a superclass. The class that does the inheriting is called a subclass. Therefore, a subclass is a specialized version of a superclass. It inherits all of the members defined by the superclass and adds its own, unique elements.

// Create a superclass. 
class A {
   int i, j;
    void showij() {
     System.out.println("i and j: " + i + " " + j);
   }
 }  
// Create a subclass by extending class A. 
class B extends A {
   int k;
    void showk() {
     System.out.println("k: " + k);
   }
} 

Packages are containers for classes. They are used to keep the class name space compartmentalized. For example, a package allows you to create a class named List, which you can store in your own package without concern that it will collide with some other class named List stored elsewhere. Packages are stored in a hierarchical manner and are explicitly imported into new class definitions.

To create a package is quite easy: simply include a package command as the first statement in a Java source file. Any classes declared within that file will belong to the specified package. The package statement defines a name space in which classes are stored. If you omit the package statement, the class names are put into the default package, which has no name. (This is why you haven’t had to worry about packages before now.) While the default package is fine for short, sample programs, it is inadequate for real applications.

Using interface, you can specify a set of methods that can be implemented by one or more classes. In its traditional form, the interface, itself, does not actually define any implementation. Although they are similar to abstract classes, interfaces have an additional capability: A class can implement more than one interface. By contrast, a class can only inherit a single superclass (abstract or otherwise).

access interface name { 
        return-type method-name1(parameter-list);
         return-type method-name2(parameter-list);
          type final-varname1 = value;
         type final-varname2 = value;
         //...
         return-type method-nameN(parameter-list);
         type final-varnameN = value; 
}

A Java exception is an object that describes an exceptional (that is, error) condition that has occurred in a piece of code. When an exceptional condition arises, an object representing that exception is created and thrown in the method that caused the error. That method may choose to handle the exception itself, or pass it on. Either way, at some point, the exception is caught and processed. Exceptions can be generated by the Java run-time system, or they can be manually generated by your code. Exceptions thrown by Java relate to fundamental errors that violate the rules of the Java language or the constraints of the Java execution environment. Manually generated exceptions are typically used to report some error condition to the caller of a method.

Java exception handling is managed via five keywords: try, catch, throw, throws, and finally. Briefly, here is how they work. Program statements that you want to monitor for exceptions are contained within a try block. If an exception occurs within the try block, it is thrown. Your code can catch this exception (using catch) and handle it in some rational manner. System-generated exceptions are automatically thrown by the Java run-time system. To manually throw an exception, use the keyword throw. Any exception that is thrown out of a method must be specified as such by a throws clause. Any code that absolutely must be executed after a try block completes is put in a finally block.

try {
           // block of code to monitor for errors      
}       
catch (ExceptionType1 exOb) {
           // exception handler for ExceptionType1      
}       
catch (ExceptionType2 exOb) {
           // exception handler for ExceptionType2      
}      
// ...      
finally {
           // block of code to be executed after try block ends      
}

Java provides built-in support for multithreaded programming. A multithreaded program contains two or more parts that can run concurrently. Each part of such a program is called a thread, and each thread defines a separate path of execution. Thus, multithreading is a specialized form of multitasking

Process-based multitasking is the feature that allows your computer to run two or more programs concurrently. For example, process-based multitasking enables you to run the Java compiler at the same time that you are using a text editor or visiting a web site. In process-based multitasking, a program is the smallest unit of code that can be dispatched by the scheduler.

In a Thread-based multitasking environment, the thread is the smallest unit of dispatchable code. This means that a single program can perform two or more tasks simultaneously. For instance, a text editor can format text at the same time that it is printing, as long as these two actions are being performed by two separate threads. Thus, Process-based multitasking deals with the “big picture,” and thread-based multitasking handles the details.

Swing is a framework that provides more powerful and flexible GUI components than does the AWT. As a result, it is the GUI that has been widely used by Java programmers for more than a decade.

Swing did not exist in the early days of Java. Rather, it was a response to deficiencies present in Java’s original GUI subsystem: the Abstract Window Toolkit. The AWT defines a basic set of controls, windows, and dialog boxes that support a usable, but limited graphical interface. One reason for the limited nature of the AWT is that it translates its various visual components into their corresponding, platform-specific equivalents, or peers. This means that the look and feel of a component is defined by the platform, not by Java. Because the AWT components use native code resources, they are referred to as heavyweight.

• Swing Components Are Lightweight
• Swing Supports a Pluggable Look and Feel

• All the documentation in this page is taken from JAVA the complete Reference 9th Edition by Herbert Schildt