构建 Java 源文件和/或声明/导入包以确保无错误编译的规则是什么？

Question

I have several files kept in a package, customQ . This package is imported into a test client called QWithExceptions . The source tree is arranged as follows:

/tryThis09_01/+
              |
              -QWithExceptions.java
              |
              /customQ/+
                       |
                       -circularQ.java
                       -staticQ.java
                       -emptyQException.java
                       -fullQException.java

The test client compiles without error when I cd to tryThis09_01/ and run $javac QWithExceptions.java .

However, when I decided to experiment a little I find that my intuition regarding packages and their structure is completely off. The actual package customQ and the test client QWithExceptions is buried more deeply in a source tree like so:

~/learningJava/beginnersGuide/chapter09/tryThis09_01/+
                                                     |
                                                     -QWithExceptions.java
                                                     |
                                                     /customQ/+
                                                              |
                                                              -circularQ.java
                                                              -staticQ.java
                                                              -emptyQException.java
                                                              -fullQException.java

The linter in my development environment suggested that I change the package declaration in customQ/*.java from package customQ; to package beginnersGuide.chapter09.tryThis09_01.customQ; . After doing so, I then changed the import statement in the test client to reflect the new package name and moved QWithExceptions.java to ~/learningJava/ and executed $javac QWithExceptions.java , which resulted in a flurry of compilation errors.

I've also tried $javac -cp.:$CLASSPATH QWithExcpetions.java and $javac -d. -cp.:$CLASSPATH QWithExcpetions.java $javac -d. -cp.:$CLASSPATH QWithExcpetions.java , as well as moving QWithExceptions.java further up and further down the source tree one directory at a time, all with various errors.

At the moment, I'm grappling with a bad class file error. While I understand that the compiler wants me to move the offending file somewhere else in the file tree (or perhaps modify the package declarations or import statements) I'm not sure how to do this without invoking a package does not exist error.

I'm surely missing some essential rule of package writing that isn't (to me) immediately obvious from the trivial implementations found on the broader web or in my Java texts. If you know what it is, please share your wisdom.

Answer 1

I cannot reproduce your issue because you forgot to show the error messages and file content. But I can show you how it works:

My folder structure:

stefan@stefanpc:~/learningjava$ tree
.
├── beginnersGuide
│   └── chapter09
│       └── tryThis09_01
│           └── customQ
│               └── Test.java
└── Main.java

File Test.java:

package beginnersGuide.chapter09.tryThis09_01.customQ;

public class Test
{
   public int i;
}

File Main.java:

import beginnersGuide.chapter09.tryThis09_01.customQ.Test;

public class Main {
    
    Test test;

    public static void main(String[] args){
        System.out.println("Hello");
    }
}

Compile:

stefan@stefanpc:~/learningjava$ javac Main.java 
stefan@stefanpc:~/learningjava$ 

Classes and the related filenames should start with an uppercase character. But I assume that this is not your problem cause.

Recommendations of Oracle: https://docs.oracle.com/javase/tutorial/java/package/namingpkgs.html

Answer 2

Java requires a package declaration align with the folder structures. For example if you have a package abc' declared at the top of a source file, you need to put that path in an a/b/c folder structure. Note that the folder structure is relative to the source root during compilation. So if you have your code in a 'src' folder, and try to compile from the context the parent of that 'src', you will get errors because the folder structure looks like src/a/b/c to the compiler. Likewise, if your compilation context is set to folder 'a', you would get errors because now the folder looks like b/c to the compiler.

This is a common issue for people new to Java. It's meant to make things easy to find but can be an annoyance.