使用多个主类

客官°小女子只卖身不卖艺 2022-12-06 15:41 112阅读 0赞

Normally at the outset of learning the Java programming language, there will be a number of code examples that are useful to compile and run to fully understand them. When using an IDE like NetBeans it's easy to fall into the trap of creating a new project every time for each new piece of code. However, it can all happen in one project.

通常，在学习Java编程语言之初，就会有许多代码示例可用于编译和运行以充分理解它们。 当使用NetBeans之类的IDE时，很容易陷入每次为每段新代码创建一个新项目的陷阱。 但是，所有这些都可能在一个项目中发生。

## 创建代码示例项目 **(** Creating a Code Example Project **)** ##

A NetBeans project contains the classes needed to build a Java application. The application uses the [main class][] as the starting point for the execution of the Java code. In fact, in a new [Java application][] project created by NetBeans only one class included - the main class contained within the *Main.java* file. Go ahead and make a new project in NetBeans and called it *CodeExamples*.

NetBeans项目包含构建Java应用程序所需的类。 该应用程序将[主类][main class]用作执行Java代码的起点。 实际上，在由NetBeans创建的新[Java应用程序][Java application]项目中，仅包含一个类*-Main.java*文件中包含的主类。 继续，在NetBeans中创建一个新项目，并将其称为*CodeExamples* 。

Let's say I want to try programming some [Java code][] to output the result of adding 2 + 2. Put the following code into the main method:

假设我想尝试编写一些[Java代码][Java code]来输出加2 + 2的结果。将以下代码放入main方法中：

>   
> int result = 2 + 2;  
> int结果= 2 + 2;  
> System.out.println(result);  
> System.out.println(结果);  
> \}  
> \}

When the application is compiled and executed the output printed is "4". Now, if I want to try out another piece of Java code I have two choices, I can either overwrite the code in the main class or I can put it in another main class.

编译并执行应用程序时，输出的输出为“ 4”。 现在，如果我想尝试另一段Java代码，我有两种选择，我可以覆盖主类中的代码，也可以将其放在另一个主类中。

## 多个主要班级 **(** Multiple Main Classes **)** ##

NetBeans projects can have more than one main class and it's easy to specify the main class an application should run. This allows a programmer to switch between any number of main classes within the same application. Only the code in one of the main classes will be executed, effectively making each class independent of each other.

NetBeans项目可以具有多个主类，并且很容易指定应用程序应运行的主类。 这允许程序员在同一应用程序中的任意数量的主类之间切换。 仅执行主类之一中的代码，从而有效地使每个类彼此独立。

**Note:** This is not usual in a standard Java application. All it needs is one main class as a starting point for the execution of the code. Remember this is a tip for running multiple code examples within one project.

**注意：**这在标准Java应用程序中并不常见。 它所需要的只是一个主类，作为执行代码的起点。 请记住，这是在一个项目中运行多个代码示例的技巧。

Let's add a new main class to the *CodeSnippets* project. From the **File** menu choose **New File**. In the **New File** wizard pick the **Java Main Class** file type (it's in the Java category). Click **Next**. Name the file *example1* and click **Finish**.

让我们向*CodeSnippets*项目添加一个新的主类。 从“ **文件”**菜单中选择“ **新建文件”** 。 在“ **新建文件”**向导中，选择**Java Main Class**文件类型(在Java类别中)。 单击**下一步** 。 将文件命名为*example1* ，然后单击**Finish** 。

In the *example1* class add the following code to the [main method][]:

在*example1*类中，将以下代码添加到[main方法中][main method] ：

>   
> System.out.println("Four");  
> System.out.println(“ Four”);  
> \}  
> \}

Now, compile and run the application. The output will still be "4". This is because the project is still set up to use the *Main* class as it's main class.

现在，编译并运行该应用程序。 输出仍将为“ 4”。 这是因为该项目仍设置为使用*Main*类作为其主类。

To change the main class being used, go to the **File** menu and choose **Project Properties**. This dialog gives all the options that can be changed in a NetBeans project. Click on the **Run** category. On this page, there is a **Main-Class** option. Currently, it is set to *codeexamples.Main* (i.e., the Main.java class). By clicking the **Browse** button to the right, a pop-up window will appear with all the main classes that are in the *CodeExamples* project. Choose *codeexamples.example1* and click **Select Main Class**. Click **OK** on the **Project Properties** dialog.

要更改正在使用的主类，请转到“ **文件”**菜单，然后选择“ **项目属性”** 。 此对话框提供了可以在NetBeans项目中更改的所有选项。 单击**运行**类别。 在此页面上，有一个**Main-Class**选项。 当前，它设置为*codeexamples.Main* (即Main.java类)。 通过单击右侧的“ **浏览”**按钮，将出现一个弹出窗口，其中*包含CodeExamples*项目中的所有主要类。 选择*codeexamples.example1* ，然后点击**Select Main Class** 。 在“ **项目属性”**对话框上，单击“ **确定** ”。

Compile and run the application again. The output will now be "four" because the main class being used is now *example1.java*.

编译并再次运行该应用程序。 现在的输出为“四个”，因为正在使用的主类现在为*example1.java* 。

Using this approach it's easy to try out lots of different Java code examples and keep them all in one NetBeans project. but still be able to compile and run them independent of each other.

使用这种方法，可以很容易地尝试许多不同的Java代码示例，并将它们全部保留在一个NetBeans项目中。 但仍然能够相互独立地编译和运行它们。

> 翻译自: [https://www.thoughtco.com/using-multiple-main-classes-2034250][https_www.thoughtco.com_using-multiple-main-classes-2034250]

[main class]: https://www.thoughtco.com/main-class-2034233
[Java application]: https://www.thoughtco.com/coding-a-simple-graphical-user-interface-2034064
[Java code]: https://www.thoughtco.com/creating-your-first-java-program-2034124
[main method]: https://www.thoughtco.com/definition-of-control-statements-958050
[https_www.thoughtco.com_using-multiple-main-classes-2034250]: https://www.thoughtco.com/using-multiple-main-classes-2034250