Some basic concepts in Java

Thứ Năm, 27/02/2020

Tram Ho

Overview

Java was originally developed by Sun Microsystem, started by James Gosling and launched in 1995. Currently, Java is a popular programming language, used for many technology products. We can highlight some key features of Java as follows:

High-level programming language
An object-oriented programming language (Object oriented). In Java, all are objects.
Not dependent on the platform (Platform independent). Unlike many other languages such as C and C ++, when compiling a Java program it will not compile into the platform specified on the machine but compile into byte code regardless of the platform. Byte code can be run on all devices with JVM – Java Virtual Machine installed.
It is a multithreaded language. Java programmers can write so that the program runs multiple threads simultaneously.
High performance

Basic Syntax

The basic components in a Java program

Object – Objects have states and behaviors.
Class – As a framework for creating Objects, defining Object states and behaviors
Methods – A behavior, a class has many behaviors. In methods, it is the place to process logic and manipulate data.
Instance variables – Object has a set of duplicate instance variables. The value assigned to the entity variables creates the state of the Object

The first show

<span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">MyFirstJavaProgram</span> <span class="token punctuation">{</span>

   <span class="token comment">/* This is my first java program.
    * This will print 'Hello World' as the output
    */</span>

   <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span> <span class="token punctuation">(</span> String <span class="token punctuation">[</span> <span class="token punctuation">]</span> args <span class="token punctuation">)</span> <span class="token punctuation">{</span>
      System <span class="token punctuation">.</span> out <span class="token punctuation">.</span> <span class="token function">println</span> <span class="token punctuation">(</span> <span class="token string">"Hello World"</span> <span class="token punctuation">)</span> <span class="token punctuation">;</span> <span class="token comment">// prints Hello World</span>
   <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

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

}

Basic syntax rules

Differentiate between ordinary flowers. In java, when defining Hello and hello, it will be different.
The class name needs to be capitalized with the first letter and every word in the name needs to capitalize the first letter. EXAMPLE: MyFirstJavaProgram.class
The method name lowercase the first letter, and the first letters of the words after it are capitalized. For example: myFirstJavaMethod ()
Program File Name: The name of the file should exactly match the class name in the file. For example: class MyFirstJavaProgram, the file name is also MyFirstJavaProgram.java
The Java program process starts with the main (String [] args) function, which is a must for every Java program.

Definition in Java (Java identifiers)

All components of a Java program need a name. Like class, method, variable that called definition (Identifiers). There are a few guidelines when creating definitions like this:

All definitions must start with a letter, the $ character or the underscore (_), then may include other characters.
The definition must not coincide with key words in Java
Use case sensitive rules. For example: FirstJavaProgram.class, firstMethod ()
Examples of valid definitions: doSomeThing (), abc, $ 123, _abc
Examples of invalid definitions: 123, if, -salary

Java variables

Local variables
Class variables (Static variables)
Instance variables (Non-static variables)

Array

Arrays are objects that store variables of the same type. The array itself is an object in the heap

Java Enums

Enums restrict a variable to only one value within a few predefined values. Example:

<span class="token keyword">class</span> <span class="token class-name">Phone</span> <span class="token punctuation">{</span>
    <span class="token keyword">enum</span> Type <span class="token punctuation">{</span> SAMSUNG <span class="token punctuation">,</span> APPLE <span class="token punctuation">}</span> <span class="token punctuation">;</span>
    Type type <span class="token punctuation">;</span>

    <span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">PhoneTest</span> <span class="token punctuation">{</span>
        <span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">void</span> <span class="token function">main</span> <span class="token punctuation">(</span> String <span class="token punctuation">[</span> <span class="token punctuation">]</span> args <span class="token punctuation">)</span> <span class="token punctuation">{</span>
            Phone myPhone <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Phone</span> <span class="token punctuation">(</span> <span class="token punctuation">)</span> <span class="token punctuation">;</span>
            myPhone <span class="token punctuation">.</span> type <span class="token operator">=</span> Phone <span class="token punctuation">.</span> Type <span class="token punctuation">.</span> SAMSUNG <span class="token punctuation">;</span>
            System <span class="token punctuation">.</span> out <span class="token punctuation">.</span> <span class="token function">println</span> <span class="token punctuation">(</span> <span class="token string">"My phone type: "</span> <span class="token operator">+</span> myPhone <span class="token punctuation">.</span> type <span class="token punctuation">)</span> <span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

class Phone {

enum Type { SAMSUNG , APPLE } ;

Type type ;

public class PhoneTest {

public class void main ( String [ ] args ) {

Phone myPhone = new Phone ( ) ;

myPhone . type = Phone . Type . SAMSUNG ;

System . out . println ( "My phone type: " + myPhone . type ) ;

}

Java keyword

The keywords listed below are not used in Java definitions

abstract	assert	boolean	break
bytes	case	catch	char
class	const	tiếp tục	default
by	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

Comment

Java supports single-line and multi-line comments. All contents in the commet will be ignored by the compiler Example:

<span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">MyFirstJavaProgram</span> <span class="token punctuation">{</span>

   <span class="token comment">/* This is my first java program.
    * This will print 'Hello World' as the output
    * This is an example of multi-line comments.
    */</span>

   <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span> <span class="token punctuation">(</span> String <span class="token punctuation">[</span> <span class="token punctuation">]</span> args <span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token comment">// This is an example of single line comment</span>
      <span class="token comment">/* This is also an example of single line comment. */</span>
      System <span class="token punctuation">.</span> out <span class="token punctuation">.</span> <span class="token function">println</span> <span class="token punctuation">(</span> <span class="token string">"Hello World"</span> <span class="token punctuation">)</span> <span class="token punctuation">;</span>
   <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

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.

*/

// This is an example of single line comment

/* This is also an example of single line comment. */

}

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