Project4

Introduction

JAVA was developed by James Gosling at Sun Microsystems Inc in the year 1991, 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++.

The Java Class Library (JCL) is a set of dynamically loadable libraries that Java Virtual Machine (JVM) languages can call at run time. Because the Java Platform is not dependent on a specific operating system, applications cannot rely on any of the platform-native libraries. Instead, the Java Platform provides a comprehensive set of standard class libraries, containing the functions common to modern operating systems. JCL serves three purposes within the JVM:

Like other standard code libraries, they provide the programmer a well-known set of useful facilities, such as container classes and regular expression processing.
The library provides an abstract interface to tasks that would normally depend heavily on the hardware and operating system, such as network access and file access.
Some underlying platforms may not support all of the features a Java application expects. In these cases, the library implementation can either emulate those features or provide a consistent way to check for the presence of a specific feature.

What you should already know

This guide assumes you have the following basic background:

A general understanding of the Internet and the World Wide Web (WWW).
Good working knowledge of C and C++.
Java is also a platform, which means that Java code can run on any machine that has a Java Virtual Machine (JVM) on it.

Java

In contrast to Java's compile-time system of classes built by declarations, JavaScript supports a runtime system based on a small number of data types representing numeric, Boolean, and string values. JavaScript has a prototype-based object model instead of the more common class-based object model. The prototype-based model provides dynamic inheritance; that is, what is inherited can vary for individual objects. JavaScript also supports functions without any special declarative requirements. Functions can be properties of objects, executing as loosely typed methods.

ava is a general purpose programming language, much like Python or JavaScript. The language itself is specifically an object oriented programming language, so bears similarities to C++, C#. Java is also a platform, which means that Java code can run on any machine that has a Java Virtual Machine (JVM) on it. Originally those two things were interchangeable, and the only thing that ran on the JVM was Java. But since that time, a number of languages have been written which can run on the Java platform, languages like Scala, Groovy, an implementation of Ruby called jRuby, and an implementation of Python called Jython.

Hello world

To get started with writing Java, open the Visual Studio Code code and write your first "Hello world" Java code:

class HelloWorld { // Your program begins with a call to main(). //
            Prints "Hello, World" to the terminal window. public static void
            main(String args[]) { System.out.println("Hello, World"); } }

Select the code in the pad and hit Ctrl+R to watch it unfold in your browser!

Variables

Variable in Java is a data container that saves the data values during Java program execution. Every variable is assigned a data type that designates the type and quantity of value it can hold. Variable is a memory location name of the data.

Variable in Java is a data container that saves the data values during Java program execution. Every variable is assigned a data type that designates the type and quantity of value it can hold. Variable is a memory location name of the data. A variable is a name given to a memory location. For More On Variables please check Variables in Java.

A variable name can consist of Capital letters A-Z, lowercase letters a-z digits 0-9, and two special characters such as _ underscore and $ dollar sign. The first character must be a letter. Blank spaces cannot be used in variable names. Java keywords cannot be used as variable names. Variable names are case-sensitive. The maximum length of the variable is 64 characters. Variable names always should exist on the left-hand side of assignment operators.

Declaring variables

You can declare a variable in three ways:

With the keyword var. For example, var x = 42. This syntax can be used to declare both local and global variables.

By simply assigning it a value. For example, x = 42. This always declares a global variable. It generates a strict JavaScript warning. You shouldn't use this variant.

With the keyword let. For example, let y = 13. This syntax can be used to declare a block scope local variable. See Variable scope below.

Variable scope

Scope of a variable is the part of the program where the variable is accessible. Like C/C++, in Java, all identifiers are lexically (or statically) scoped, i.e.scope of a variable can determined at compile time and independent of function call stack. Java programs are organized in the form of classes. Every class is part of some package. Java scope rules can be covered under following categories.

These variables must be declared inside class (outside any function). They can be directly accessed anywhere in class.We can declare class variables anywhere in class, but outside methods. Access specified of member variables doesn’t affect scope of them within a class. Member variables can be accessed outside a class with following rules

if (true) { var x = 5; } System.out(x); // 5

This behavior changes, when using the let declaration introduced in ECMAScript 2015.

if (true) { let y = 5; } System.out(y); // ReferenceError: y is not
            defined

Global variables

To define a Global variable in java, the keyword static is used. Java actually doesn’t have the concept of Global variable, it is known as class variable ( static field ). These are the variables that can be used by the entire class.

To define the Global Variable, you can just use the static Keyword like this: public class Example { public static int a; public static int b; }

Constants

You can create a read-only, named constant with the const keyword. The syntax of a constant identifier is the same as for a variable identifier: it must start with a letter, underscore or dollar sign and can contain alphabetic, numeric, or underscore characters.

const PI = 3.14;

A constant cannot change value through assignment or be re-declared while the script is running. It has to be initialized to a value.

The scope rules for constants are the same as those for let block scope variables. If the const keyword is omitted, the identifier is assumed to represent a variable.

You cannot declare a constant with the same name as a function or variable in the same scope. For example:

// THIS WILL CAUSE AN ERROR function f() {}; const f = 5; // THIS
            WILL CAUSE AN ERROR ALSO function f() { const g = 5; var g;
            //statements }

However, object attributes are not protected, so the following statement is executed without problems.

const MY_OBJECT = {"key": "value"}; MY_OBJECT.key =
            "otherValue";

Data types

The latest ECMAScript standard defines Eight data types:

Eight data types that are primitives:
- Boolean. true and false.
- byte: The byte data type is an 8-bit signed two’s complement integer. The byte data type is useful for saving memory in large arrays. Syntax: byte byteVar;
- short: The short data type is a 16-bit signed two’s complement integer. Similar to byte, use a short to save memory in large arrays, in situations where the memory savings actually matters. Syntax: short shortVar;
- int: It is a 32-bit signed two’s complement integer. Syntax: int intVar;
- long: The long data type is a 64-bit two’s complement integer. Syntax: long longVar;
- float: The float data type is a single-precision 32-bit IEEE 754 floating-point. Use a float (instead of double) if you need to save memory in large arrays of floating-point numbers. Syntax: float floatVar;
double: The double data type is a double-precision 64-bit IEEE 754 floating-point. For decimal values, this data type is generally the default choice. Syntax: double doubleVar;
char: The char data type is a single 16-bit Unicode character. Syntax: char charVar;

The Reference Data Types will contain a memory address of variable values because the reference types won’t store the variable value directly in memory. They are strings, objects, arrays, etc.

if...else statement

Use the if statement to execute a statement if a logical condition is true. Use the optional else clause to execute a statement if the condition is false. An if statement looks as follows: if (condition) { statement_1; } else { statement_2; } condition can be any expression that evaluates to true or false. See Boolean for an explanation of what evaluates to true and false. If condition evaluates to true, statement_1 is executed; otherwise, statement_2 is executed. statement_1 and statement_2 can be any statement, including further nested if statements.

You may also compound the statements using else if to have multiple conditions tested in sequence, as follows:

if (condition_1) { statement_1; } else if (condition_2) {
            statement_2; } else if (condition_n) { statement_n; } else {
            statement_last; }

In the case of multiple conditions only the first logical condition which evaluates to true will be executed. To execute multiple statements, group them within a block statement ({ ... }) . In general, it's good practice to always use block statements, especially when nesting if statements:

if (condition) { statement_1_runs_if_condition_is_true;
            statement_2_runs_if_condition_is_true; } else {
            statement_3_runs_if_condition_is_false;
            statement_4_runs_if_condition_is_false; }

It is advisable to not use simple assignments in a conditional expression, because the assignment can be confused with equality when glancing over the code. For example, do not use the following code: if (x = y) { /* statements here */ } If you need to use an assignment in a conditional expression, a common practice is to put additional parentheses around the assignment. For example: if ((x = y)) { /* statements here */ }

while statement

A while statement executes its statements as long as a specified condition evaluates to true. A while statement looks as follows: while (condition) statement If the condition becomes false, statement within the loop stops executing and control passes to the statement following the loop.

The condition test occurs before statement in the loop is executed. If the condition returns true, statement is executed and the condition is tested again. If the condition returns false, execution stops and control is passed to the statement following while.

To execute multiple statements, use a block statement ({ ... }) to group those statements.

Example:

The following while loop iterates as long as n is less than three:

var n = 0; var x = 0; while (n < 3) { n++; x += n; }

With each iteration, the loop increments n and adds that value to x. Therefore, x and n take on the following values:

After the first pass: n = 1 and x = 1
After the second pass: n = 2 and x = 3
After the third pass: n = 3 and x = 6

After completing the third pass, the condition n < 3 is no longer true, so the loop terminates.

Function declarations

A function definition (also called a function declaration, or function statement) consists of the function keyword, followed by:

The name of the function.
A list of arguments to the function, enclosed in parentheses and separated by commas.
The Java statements that define the function, enclosed in curly brackets, { }.

For example, the following code defines a simple function named square:

function square(number) { return number * number; }

The function square takes one argument, called number. The function consists of one statement that says to return the argument of the function (that is, number) multiplied by itself. The return statement specifies the value returned by the function.

return number * number;

Primitive parameters (such as a number) are passed to functions by value; the value is passed to the function, but if the function changes the value of the parameter, this change is not reflected globally or in the calling function.

Reference

All the documentation in this page is taken from MDN