为什么有时候'包'是必要的，有时候是不必要的？ [英] How come sometimes 'package' is necessary, sometimes is unnecessary?

问题描述

我是Java新手。我按照一些教程使用Eclipse构建了一个简单的Java程序，如下所示：

I am new for Java. I followed some tutorial to build a simple Java program using Eclipse as below:

package samplelib;

public class samplelib {
    public static void main (String[] args) {
        System.out.println ("sample lib");
    }
}

我成功地在Eclipse中运行它。但是当我使用'javac'在命令行编译它然后使用'java'命令运行它时，我得到的消息如错误：无法找到或无法加载主类samplelib。

我删除了第一行package samplelib，再次使用了'javac'/'java'命令。它成功运行。

然后，我回到Eclipse并运行它，我收到以下错误消息：



线程中的异常 mainjava.lang.Error：未解决的编译问题：

at samplelib.samplelib.main（samplelib.java:4）



所以为什么有时我们需要'包装声明'，但有时我们不需要？谢谢。

I ran it inside Eclipse successfully. But when I compiled it at command line using 'javac' and then ran it using 'java' command, I got message like "Wrong: can't find or unable to load main class samplelib".
I deleted the first line "package samplelib", used 'javac'/'java' commands again. It rans successfully.
Then, I went back to Eclipse and ran it, I got the following error message:

Exception in thread "main" java.lang.Error: Unresolved compilation problem:
at samplelib.samplelib.main(samplelib.java:4)

So, how come sometimes we need 'package statement' but sometimes we don't? Thanks.

推荐答案

包有点像文件夹，其中一组公共文件保存在一起。包中的所有类都保存在一起，如 http://docs.oracle.com/所述javase / tutorial / java / package / index.html [ ^ ]。当您构建具有 package 语句的Java程序时，编译的类应该存储在具有相同名称的子目录中。然后调用 java 从其父级执行以下命令行：

A package is somewhat like a folder, where a set of common files are kept together. All classes in a package are kept together, as described at http://docs.oracle.com/javase/tutorial/java/package/index.html[^]. When you build a Java program that has a package statement, then the compiled class is expected to be stored in a subdirectory with the same name. You then call java to execute it from its parent with a command line of the form:

java packagename/classname

您还可以使用 -classpath 选项告诉Java存储类的位置。

You can also use the -classpath option to tell Java where your classes are stored.

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