Basic for java language

The for Statement

The for statement provides a compact way to iterate over a range of values. Programmers often refer to it as the «for loop» because of the way in which it repeatedly loops until a particular condition is satisfied. The general form of the for statement can be expressed as follows:

for (initialization; termination; increment) < statement(s) >

When using this version of the for statement, keep in mind that:

• The initialization expression initializes the loop; it’s executed once, as the loop begins.
• When the termination expression evaluates to false , the loop terminates.
• The increment expression is invoked after each iteration through the loop; it is perfectly acceptable for this expression to increment or decrement a value.

The following program, ForDemo , uses the general form of the for statement to print the numbers 1 through 10 to standard output:

The output of this program is:

Count is: 1 Count is: 2 Count is: 3 Count is: 4 Count is: 5 Count is: 6 Count is: 7 Count is: 8 Count is: 9 Count is: 10

Notice how the code declares a variable within the initialization expression. The scope of this variable extends from its declaration to the end of the block governed by the for statement, so it can be used in the termination and increment expressions as well. If the variable that controls a for statement is not needed outside of the loop, it’s best to declare the variable in the initialization expression. The names i , j , and k are often used to control for loops; declaring them within the initialization expression limits their life span and reduces errors.

The three expressions of the for loop are optional; an infinite loop can be created as follows:

The for statement also has another form designed for iteration through Collections and arrays This form is sometimes referred to as the enhanced for statement, and can be used to make your loops more compact and easy to read. To demonstrate, consider the following array, which holds the numbers 1 through 10:

The following program, EnhancedForDemo , uses the enhanced for to loop through the array:

class EnhancedForDemo < public static void main(String[] args)< int[] numbers = ; for (int item : numbers) < System.out.println("Count is: " + item); >> >

In this example, the variable item holds the current value from the numbers array. The output from this program is the same as before:

Count is: 1 Count is: 2 Count is: 3 Count is: 4 Count is: 5 Count is: 6 Count is: 7 Count is: 8 Count is: 9 Count is: 10

We recommend using this form of the for statement instead of the general form whenever possible.

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Java — Basic Syntax

When we consider a Java program, it can be defined as a collection of objects that communicate via invoking each other’s methods. Let us now briefly look into what do class, object, methods, and instance variables mean.

• Object − Objects have states and behaviors. Example: A dog has states — color, name, breed as well as behavior such as wagging their tail, barking, eating. An object is an instance of a class.
• Class − A class can be defined as a template/blueprint that describes the behavior/state that the object of its type supports.
• Methods − A method is basically a behavior. A class can contain many methods. It is in methods where the logics are written, data is manipulated and all the actions are executed.
• Instance Variables − Each object has its unique set of instance variables. An object’s state is created by the values assigned to these instance variables.

First Java Program

Let us look at a simple code that will print the words Hello World.

Example

public class MyFirstJavaProgram < /* This is my first java program. * This will print 'Hello World' as the output */ public static void main(String []args) < System.out.println("Hello World"); // prints Hello World >>

Let’s look at how to save the file, compile, and run the program. Please follow the subsequent steps −

• Open notepad and add the code as above.
• Save the file as: MyFirstJavaProgram.java.
• Open a command prompt window and go to the directory where you saved the class. Assume it’s C:\.
• Type ‘javac MyFirstJavaProgram.java’ and press enter to compile your code. If there are no errors in your code, the command prompt will take you to the next line (Assumption : The path variable is set).
• Now, type ‘ java MyFirstJavaProgram ‘ to run your program.
• You will be able to see ‘ Hello World ‘ printed on the window.

Output

C:\> javac MyFirstJavaProgram.java C:\> java MyFirstJavaProgram Hello World

Basic Syntax

About Java programs, it is very important to keep in mind the following points.

• Case Sensitivity − Java is case sensitive, which means identifier Hello and hello would have different meaning in Java.
• Class Names − For all class names the first letter should be in Upper Case. If several words are used to form a name of the class, each inner word’s first letter should be in Upper Case. Example:class MyFirstJavaClass
• Method Names − All method names should start with a Lower Case letter. If several words are used to form the name of the method, then each inner word’s first letter should be in Upper Case. Example:public void myMethodName()
• Program File Name − Name of the program file should exactly match the class name. When saving the file, you should save it using the class name (Remember Java is case sensitive) and append ‘.java’ to the end of the name (if the file name and the class name do not match, your program will not compile). But please make a note that in case you do not have a public class present in the file then file name can be different than class name. It is also not mandatory to have a public class in the file. Example: Assume ‘MyFirstJavaProgram’ is the class name. Then the file should be saved as ‘MyFirstJavaProgram.java’
• public static void main(String args[]) − Java program processing starts from the main() method which is a mandatory part of every Java program.

Java Identifiers

All Java components require names. Names used for classes, variables, and methods are called identifiers.

In Java, there are several points to remember about identifiers. They are as follows −

• All identifiers should begin with a letter (A to Z or a to z), currency character ($) or an underscore (_).
• After the first character, identifiers can have any combination of characters.
• A key word cannot be used as an identifier.
• Most importantly, identifiers are case sensitive.
• Examples of legal identifiers: age, $salary, _value, __1_value.
• Examples of illegal identifiers: 123abc, -salary.

Java Modifiers

Like other languages, it is possible to modify classes, methods, etc., by using modifiers. There are two categories of modifiers −

• Access Modifiers − default, public , protected, private
• Non-access Modifiers − final, abstract, strictfp

We will be looking into more details about modifiers in the next section.

Java Variables

Following are the types of variables in Java −

• Local Variables
• Class Variables (Static Variables)
• Instance Variables (Non-static Variables)

Java Arrays

Arrays are objects that store multiple variables of the same type. However, an array itself is an object on the heap. We will look into how to declare, construct, and initialize in the upcoming chapters.

Java Enums

Enums were introduced in Java 5.0. Enums restrict a variable to have one of only a few predefined values. The values in this enumerated list are called enums.

With the use of enums it is possible to reduce the number of bugs in your code.

For example, if we consider an application for a fresh juice shop, it would be possible to restrict the glass size to small, medium, and large. This would make sure that it would not allow anyone to order any size other than small, medium, or large.

Example

class FreshJuice < enum FreshJuiceSize< SMALL, MEDIUM, LARGE >FreshJuiceSize size; > public class FreshJuiceTest < public static void main(String args[]) < FreshJuice juice = new FreshJuice(); juice.size = FreshJuice.FreshJuiceSize.MEDIUM ; System.out.println("Size: " + juice.size); >>

The above example will produce the following result −

Output

Note − Enums can be declared as their own or inside a class. Methods, variables, constructors can be defined inside enums as well.

Java Keywords

The following list shows the reserved words in Java. These reserved words may not be used as constant or variable or any other identifier names.

abstract	assert	boolean	break
byte	case	catch	char
class	const	continue	default
do	double	else	enum
extends	final	finally	float
for	goto	if	implements
import	instanceof	int	interface
long	native	new	package
private	protected	public	return
short	static	strictfp	super
switch	synchronized	this	throw
throws	transient	try	void
volatile	while

Comments in Java

Java supports single-line and multi-line comments very similar to C and C++. All characters available inside any comment are ignored by Java compiler.

Example

public class MyFirstJavaProgram < /* This is my first java program. * This will print 'Hello World' as the output * This is an example of multi-line comments. */ public static void main(String []args) < // This is an example of single line comment /* This is also an example of single line comment. */ System.out.println("Hello World"); >>

Output

Using Blank Lines

A line containing only white space, possibly with a comment, is known as a blank line, and Java totally ignores it.

Inheritance

In Java, classes can be derived from classes. Basically, if you need to create a new class and here is already a class that has some of the code you require, then it is possible to derive your new class from the already existing code.

This concept allows you to reuse the fields and methods of the existing class without having to rewrite the code in a new class. In this scenario, the existing class is called the superclass and the derived class is called the subclass.

Interfaces

In Java language, an interface can be defined as a contract between objects on how to communicate with each other. Interfaces play a vital role when it comes to the concept of inheritance.

An interface defines the methods, a deriving class (subclass) should use. But the implementation of the methods is totally up to the subclass.

What is Next?

The next section explains about Objects and classes in Java programming. At the end of the session, you will be able to get a clear picture as to what are objects and what are classes in Java.

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Java Tutorial

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Basic for java language

Здесь мы описали шаблон будущего объекта – класс. У него есть две переменные состояния – speed и maxSpeed, а также две функции, которые описывают поведение объекта.

Теперь приложение нужно запустить, для этого Java надо подсказать, где находится точка входа в него. Для этого существует особое соглашение – нужно добавить в описание любого объекта метод:

Наше приложение запустилось, но ничего не произошло, потому что мы не добавили никакой логики в метод. Добавим простой вывод на консоль текста:

Создатели предложили называть эти папки как доменные имена в интернете, чтобы точно разграничить их, но это не обязательно, вы можете называть их как захотите.

Это так же позволяет легко импортировать чужие классы в свой код:

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