Java Basic Syntax

A Java program can be considered as a collection of objects that work together by calling each other's methods. The following briefly introduces the concepts of classes, objects, methods, and instance variables.

  • Object: An object is an instance of a class, with state and behavior. For example, a dog is an object; its states include color, name, breed; its behaviors include wagging its tail, barking, eating, etc.
  • Class: A class is a template that describes the behavior and state of a class of objects.
  • Method: Methods are behaviors. A class can have many methods. Logical operations, data modifications, and all actions are completed within methods.
  • Instance Variables: Each object has unique instance variables. The state of an object is determined by the values of these instance variables.

First Java Program

Below is a simple Java program that will output the stringHello World

Example

public class HelloWorld { /*First Java program * It will output the string Hello World*/ public static void main(String[] args) { System.out.println("Hello World"); //Output Hello World } }

Run Example »

Below is a step-by-step guide on how to save, compile, and run this program:

  • Open a code editor, and add the above code to it;
  • Save the file name as: HelloWorld.java;
  • Open the cmd command window, and navigate to the location of the target file, for example C:\
  • In the command line window, typejavac HelloWorld.javaPress Enter to compile the code. If there are no errors in the code, the cmd command prompt will move to the next line (assuming the environment variables are set up correctly).
  • Then typejava HelloWorldPress Enter to run the program.

You will see Hello World in the window.

$ javac HelloWorld.java
$ java HelloWorld 
Hello World

If you encounter encoding problems, we can use-encodingthe option settingutf-8to compile:

javac -encoding UTF-8 HelloWorld.java 
java HelloWorld

Gif illustration:

Basic Syntax

When writing Java programs, you should pay attention to the following points:

  • Case sensitivity: Java is case-sensitive, which means the identifiers Hello and hello are different.
  • Class name: For all classes, the first letter of the class name should be capitalized. If the class name consists of several words, the first letter of each word should be capitalized, for exampleMyFirstJavaClass 。
  • Method name: All method names should start with a lowercase letter. If a method name contains several words, the first letter of each subsequent word should be capitalized.
  • Source file name: The source file name must be the same as the class name. When saving the file, you should use the class name as the file name (remember that Java is case-sensitive). The file extension is.java. (If the file name and the class name are not the same, it will cause a compilation error).
  • Main method entry: All Java programs start executing from thepublic static void main(String[] args)method.

Java Identifiers

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

Regarding Java identifiers, there are several points to note:

  • All identifiers should begin with a letter (A-Z or a-z), a dollar sign ($), or an underscore (_)
  • After the first character, it can be any combination of letters (A-Z or a-z), dollar signs ($), underscores (_), or digits
  • Keywords cannot be used as identifiers
  • Identifiers are case-sensitive
  • Examples of legal identifiers: age, $salary, _value, __1_value
  • Examples of illegal identifiers: 123abc, -salary

Java Modifiers

Like other languages, Java can use modifiers to modify methods and attributes in classes. There are mainly two types of modifiers:

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

We will discuss Java modifiers in depth in later chapters.

Java Variables

Java mainly has the following types of variables
  • Local variables
  • Class variables (static variables)
  • Member variables (non-static variables)

Java Arrays

Arrays are objects stored on the heap that can hold multiple variables of the same type. In later chapters, we will learn how to declare, construct, and initialize an array.

Java Enums

Java 5.0 introduced enums. Enums restrict variables to only pre-set values. Using enums can reduce bugs in code.

For example, if we design a program for a juice shop, it will restrict juice to small, medium, and large cups. This means it does not allow customers to order juice in any size other than these three.

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 ; } }

Note:Enums can be declared separately or within a class. Methods, variables, and constructors can also be defined in enums.

Java Keywords

The following lists the Java keywords. These reserved words cannot be used for constants, variables, or the names of any identifiers.

Category Keyword Description
Access control private Private
protected Protected
public Public
default Default
Class, method, and variable modifiers abstract Declare abstract
class Class
extends Extend, inherit
final Final value, unchangeable
implements Implement (interface)
interface Interface
native Local, native method (non-Java implementation)
new Create
static Static
strictfp Strict floating point, precise floating point
synchronized Thread, synchronization
transient Transient
volatile Volatile
Program control statements break Break out of loop
case Define a value for switch selection
continue Continue
do Run
else Otherwise
for Loop
if If
instanceof Instance
return Return
switch Execute according to value selection
while Loop
Error handling assert Assert whether an expression is true
catch Catch exception
finally Execute regardless of whether an exception occurs
throw Throw an exception object
throws Declare that an exception may be thrown
try Catch exception
Package-related import Import
package Package
Primitive types boolean Boolean
byte Byte
char Character
double Double-precision floating point
float Single-precision floating point
int Integer
long Long integer
short Short integer
Variable references super Parent class, superclass
this Current class
void No return value
Reserved keywords goto Is a keyword, but cannot be used
const Is a keyword, but cannot be used

Note:Java's null is not a keyword; similar to true and false, it is a literal constant and is not allowed to be used as an identifier.

Java Comments

Similar to C/C++, Java also supports single-line and multi-line comments.

Characters in comments will be ignored by the Java compiler.

public class HelloWorld { /*This is the first Java program * It will output Hello World * This is an example of a multi-line comment*/ public static void main(String[] args){ //This is an example of a single-line comment /*This is also an example of a single-line comment*/ System.out.println("Hello World"); } }

For more content, you can refer to:Java Comments。

Java Blank Lines

Blank lines or lines with comments will be ignored by the Java compiler.

Inheritance

In Java, a class can be derived from other classes. If you want to create a class and there is already a class that has the properties or methods you need, you can have the newly created class inherit that class.

Using inheritance, you can reuse the methods and properties of existing classes without rewriting the code. The inherited class is called the super class, and the derived class is called the subclass.

Interface

In Java, an interface can be understood as a protocol for communication between objects. Interfaces play a very important role in inheritance.

An interface only defines the methods to be used by derived classes, but the specific implementation of the methods depends entirely on the derived class.

Difference Between Java Source Programs and Compiled Execution

As shown in the figure below:

The next section introduces classes and objects in Java programming. After that, you will have a clearer understanding of classes and objects in Java.

Other extensions