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Java basic grammar

A Java program can be considered as a collection of objects, and these objects through method calls to each other to work together. Introduced under the concept of classes, objects, methods and instance variables below.

  • Object: The object is an instance of a class, have state and behavior. For example, a dog is an object whose states are: color, name, breed; behavior: wagging its tail, called, eat and so on.
  • Class: The class is a template that describes the behavior of a class of objects and status.
  • Methods: The method is to conduct a class can have a number of ways. Logic operations, data modification, and all actions are completed in the method.
  • Examples of variables: Each object has a unique instance variables, state of the object instance variables from these values decision.

The first Java program

Let's look at a simple Java program, it will print the string Hello World

Examples

public class HelloWorld {
/ * The first Java program
* Prints the string Hello World
* /
public static void main (String [] args) {
System.out.println ( "Hello World"); // print Hello World
}
}

Running instance »

The following will be gradually describes how to save, compile and run this program:

  • Open Notepad, the above code added to it;
  • Save the file name: HelloWorld.java;
  • Open cmd command window, enter the location of the target file is located, assuming C: \
  • In the command window, type javac HelloWorld.java press the enter key to compile code. If the code is not an error, cmd command prompt, enter the next line. (Assuming that the environment variables are set up).
  • Then type java HelloWorld press the Enter key to run the program

You will see the Hello World in the window

C : > javac HelloWorld.java
C : > java HelloWorld 
Hello World

Gif presentation:


The basic syntax

When writing a Java program, you should note the following:

  • Case sensitive: Java is case-sensitive, which means that the identifier Hello and hello are different.
  • Class Name: For all classes, the class name first letter should be capitalized. If the class name consists of several-word, then the first letter of each word should be capitalized, for example MyFirstJavaClass.
  • Method name: All method names should be lowercase letter. If the method name contains several words, each word back initial capital letters.
  • Source filename: Source file name must be the same as the class name. When you save the file, you should use the class name as the file name to save (remember Java is case-sensitive), the file name suffix as .java. (If the file name and class name are not the same will result in a compilation error).
  • Main method entry: All Java programs by the public static void main (String [] args) method to begin.

Java identifier

All components of Java are required names. Class names, variable names and method names are called identifiers.

About Java identifiers, the following points should be noted:

  • All identifiers should begin with a letter (AZ or az), dollar sign ($), or an underscore (_)
  • After the first character can be any combination of characters
  • Keywords can not be used as an identifier
  • Identifiers are case sensitive
  • Legal identifier, for example: age, $ salary, _value, __ 1_value
  • Illegal identifier Example: 123abc, -salary

Java modifiers

Like other languages, Java can be modified using the modifier class methods and properties. There are two types of modifiers:

  • Access control modifiers: default, public, protected, private
  • Non-access control modifiers: final, abstract, strictfp

In the following sections we will discuss in-depth Java modifiers.


Java variables

Java, there are several types of variables are as follows
  • Local variables
  • Class variables (static variables)
  • Member variables (non-static variable)

Java arrays

Arrays are stored on the heap object, you can save multiple variables of the same type. In later chapters, we will learn how to declare and initialize an array structure.


Java enum

Java 5.0 introduced the enumeration, enumeration constraint variables can only be pre-set value. Using enumerations reduces code bug.

For example, we designed a program for the fruit juice shop, it will limit the juice of a small cup, medium cup, large cup. This means that it does not allow the customer in addition to the three point Dimensions juice.


Examples

class FreshJuice {
   enum FreshJuiceSize{ SMALL, MEDUIM, LARGE }
   FreshJuiceSize size;
}

public class FreshJuiceTest {
   public static void main(String []args){
      FreshJuice juice = new FreshJuice();
      juice.size = FreshJuice. FreshJuiceSize.MEDUIM ;
   }
}

Note: enum declaration may be used alone or inside the class declaration. Methods, variables, constructors can also be defined in the enumeration.


Java keywords

The following lists the Java reserved words. These reserved words can not be used to name constants, variables, and any identifiers.

Keyword description
abstract Abstract methods, abstract classes modifiers
assert Assertion conditions are met
boolean Boolean data type
break Out of the loop or label snippet
byte 8-bit signed data types
case A conditional switch statement
catch With the exception of information capture and try
char 16-bit Unicode character data type
class Class definitions
const Unused
continue The remaining portion of the loop is not executed
default switch statement default branch
do Loop, the loop body is executed at least once
double 64-bit double-precision floating-point number
else When the branch condition is not satisfied if executed
enum Enumerated type
extends It represents a class is a subclass of another class
final It represents a value after initialization can not change the representation can not be rewritten, or a class can not have subclasses
finally , Mainly to the completion of code execution and design for robustness and integrity of the program, whether there are code execution exception occurs.
float 32-bit single-precision floating-point number
for for loop
goto Unused
if Conditional statements
implements It represents a class implements an interface
import Importing classes
instanceof Test whether an object is an instance of a class
int 32-bit integer
interface Define an interface, a type of abstract methods and constants only
long 64-bit integer
native Representation of non-java code
new Assign a new instance of the class
package A package consisting of a series of related classes
private Etc. represents private field or method can be accessed only from within the class
protected It indicates that the field can only access the class or subclass or subclass other classes in the same package
public Represents total property or method
return Method returns a value
short 16 digits
static Represented at the class level definition, shared by all instances
strictfp Floating-point comparison using strict rules
super It represents the base class
switch Select statement
synchronized Represent the same time can only be accessed by one thread block of code
this It represents the current instance of the call or call another constructor
throw Throw an exception
throws Definition method may throw an exception
transient Do not modify the sequence of the field
try It represents a code block to do and finally with exception handling or throw an exception indicating whether the code is executed finally
void Marking method does not return any value
volatile Flag field may be multiple threads access, do not sync
while while loop

Java annotations

Similar to C / C ++, Java supports single-line and multi-line comments. Comment characters are ignored Java compiler.

public class HelloWorld {
   /* 这是第一个Java程序
    *它将打印Hello World
    * 这是一个多行注释的示例
    */
    public static void main(String []args){
       // 这是单行注释的示例
       /* 这个也是单行注释的示例 */
       System.out.println("Hello World"); 
    }
} 

Java blank line

Blank lines, or annotated line, Java compiler will be ignored.


inherit

In Java, a class can be derived from the other classes. If you want to create a class, and already has a class property or method you need, then you can be a newly created class inherited class.

Use inherited methods, you can reuse the existing class methods and properties, without rewriting the code. Inherited class is called a superclass (super class), the derived class is called a subclass (subclass).


interface

In Java, understood as the interface protocol between objects communicate with each other. The interface plays an important role in the succession.

Interface defines a method to use is derived, but the specific implementation of the method depends entirely on the derived class.

The next section describes the Java programming classes and objects. After you will have a clearer understanding of Java classes and objects.