Java 简明教程

Java - Basic Syntax

当我们考虑 Java 程序时，可以将它定义为通过调用彼此的方法而相互通信的对象集合。现在，让我们简单了解一下类、对象、方法和实例变量的含义。

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

让我们看一个简单的代码，它将打印单词 Hello World。

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
   }
}

让我们看看怎样来保存文件、编译和运行程序。请遵循以下步骤：

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

关于 Java 程序，牢记以下几点非常重要。

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

所有 Java 组件都需要名称。用于类、变量和方法的名称称为 identifiers。

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

在 Java 中，关于标识符，有几个要点需要记住。它们如下所示：

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

与其他语言一样，可以使用修饰符来修改类、方法等。有两类修饰符 -

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

我们将在下一部分详细了解修饰符。

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

Java Variables

以下是 Java 中的变量类型 -

Following are the types of variables in Java −

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

Java Arrays

数组是存储相同类型的多个变量的对象。但是，数组本身是堆上的对象。我们将在即将到来的章节中研究如何声明、构建和初始化。

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

枚举是在 Java 5.0 中引入的。枚举将变量限制为仅具有少数预定义值之一。此枚举列表中的值称为枚举。

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.

使用枚举可以减少代码中的错误数量。

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

例如，如果我们考虑一个鲜榨果汁店的应用程序，那么可以将玻璃杯的大小限制为小号、中号和大号。这将确保其他人无法订购除小号、中号或大号之外的任何尺寸。

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);
   }
}

Output

上面的示例将产生以下结果 -

The above example will produce the following result −

Size: MEDIUM

Note - 枚举可以自己声明，也可以在类内声明。方法、变量、构造函数也可以在枚举中定义。

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

Java Keywords

以下列表显示了 Java 中的保留字。这些保留字不能用作常量、变量或任何其他标识符名称。

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.

Sr.No	Reserved Words & Description
1	abstractAs per dictionary, abstraction is the quality of dealing with ideas rather than events.
2	assertassert keyword is used in Java to define assertion. An assertion is a statement in Java which ensures the correctness of any assumptions which have been done in the program.
3	booleanboolean datatype is one of the eight primitive datatype supported by Java. It provides means to create boolean type variables which can accept a boolean value as true or false.
4	breakThe break statement in Java programming language has the following two usages − When the break statement is encountered inside a loop, the loop is immediately terminated and the program control resumes at the next statement following the loop. It can be used to terminate a case in the switch statement.
5	bytebyte datatype is one of the eight primitive datatype supported by Java. It provides means to create byte type variables which can accept a byte value.
6	casecase keyword is part of switch statement which allows a variable to be tested for equality against a list of values.
7	catchAn exception (or exceptional event) is a problem that arises during the execution of a program.
8	charchar datatype is one of the eight primitive datatype supported by Java.
9	classJava is an Object-Oriented Language. As a language that has the Object-Oriented feature.
10	constfinal keyword is used to define constant value or final methods/classes in Java.
11	continueThe continue keyword can be used in any of the loop control structures.
12	defaultdefault keyword is part of switch statement which allows a variable to be tested for equality against a list of values.
13	doA do…while loop is similar to a while loop, except that a do…while loop is guaranteed to execute at least one time.
14	doubledouble datatype is one of the eight primitive datatype supported by Java.
15	ifAn if statement can be followed by an optional else statement, which executes when the Boolean expression is false.
16	enumThe Java Enum class is the common base class of all Java language enumeration types.
17	extendsextends is the keyword used to inherit the properties of a class. Following is the syntax of extends keyword.
18	finalfinal keyword is used to define constant value or final methods/classes in Java.
19	finallyfinally keyword is used to define a finally block. The finally block follows a try block or a catch block. A finally block of code always executes, irrespective of occurrence of an Exception.
20	floatfloat datatype is one of the eight primitive datatype supported by Java. It provides means to create float type variables which can accept a float value.
21	forA for loop is a repetition control structure that allows you to efficiently write a loop that needs to be executed a specific number of times.
22	gotogoto statement is not supported by Java currrenly. It is kept as a reserved keyword for future. As an alternative, Java supports labels with break and continue statement.
23	ifAn if statement consists of a Boolean expression followed by one or more statements.
24	implementsGenerally, the implements keyword is used with classes to inherit the properties of an interface.
25	importimport keyboard is used in context of packages.
26	instanceofinstanceof keyword is an operator which is used only for object reference variables.
27	intint datatype is one of the eight primitive datatype supported by Java.
28	interfaceAn interface is a reference type in Java. It is similar to class. It is a collection of abstract methods.
29	longlong datatype is one of the eight primitive datatype supported by Java.
30	native
31	new
32	packagePackages are used in Java in order to prevent naming conflicts, to control access, to make searching/locating and usage of classes, interfaces, enumerations and annotations easier, etc.
33	privateMethods, variables, and constructors that are declared private can only be accessed within the declared class itself.
34	protectedThe protected access modifier cannot be applied to class and interfaces.
35	publicA class, method, constructor, interface, etc. declared public can be accessed from any other class.
36	return
37	shortBy assigning different data types to variables, you can store integers, decimals, or characters in these variables.
38	staticThe static keyword is used to create variables that will exist independently of any instances created for the class.
39	strictfp
40	superThe super keyword is similar to this keyword.
41	switchA switch statement allows a variable to be tested for equality against a list of values.
42	synchronized
43	thisthis keyword is a very important keyword to identify an object. Following are the usage of this keyword.
44	throwIf a method does not handle a checked exception, the method must declare it using the throws keyword.
45	transientSerialization is a concept using which we can write the state of an object into a byte stream so that we can transfer it over the network (using technologies like JPA and RMI).
46	tryA method catches an exception using a combination of the try and catch keywords.
47	void
48	volatile
49	whileA while loop statement in Java programming language repeatedly executes a target statement as long as a given condition is true.

Comments in Java

Java 使用非常类似于 C 和 C++ 的单行和多行注释。Java 编译器忽略任何注释中可用的所有字符。

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

Hello World

Using Blank Lines

仅包含空白字符（可能带有注释）的行称为空白行，Java 会完全忽略它。

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

Inheritance

在 Java 中，类可从类派生。基本上，如果需要创建一个新类，并且此处已有的类包含一些所需的代码，那么可以从已有的代码派生新类。

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.

此概念使你可以重复使用现有类的字段和方法，而无需在新建的类中重写代码。在这种情况下，现有类称为 superclass ，派生类称为 subclass 。

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

在 Java 语言中，接口可定义为对象之间如何进行通信的契约。在继承概念中，接口起着至关重要的作用。

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.

接口定义派生类（子类）应使用的方法。但方法的实现完全取决于子类。

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?

下一部分解释 Java 编程中的对象和类。在本部分结束时，你将清楚 Java 中的对象和类是什么。

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.