compilation - Why do java source files require package declarations?

Question

I think I am failing to understand java package structure, it seemed redundant to me that java files have a package declaration within, and then are also required to be present in a directory that matches the package name. For example, if I have a MyClass.java file:

package com.example;

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

Then I would be required to have this file located in com/example, relative to the base directory, and I would execute java com.example.MyClass from the base directory to run it.

Why wouldn't the compiler be able to infer the package name by looking at the directory structure? For example, if I compiled the file from the base directory javac comexampleMyClass.java, I am not understanding why the MyClass.java wouldn't implicity belong to the com.example package.

I understand there is a default package, but it still seems that the package declaration in the source file is redundant information?

Answer 1

As you (implicitly) acknowledged, you are not required to declare the name of a package in the case if the default package. Let us put that quibble aside ...

The reason for this seeming redundancy is that without a package declaration, the meaning of Java¹ source code would be ambiguous. For example, a source file whose pathname was "/home/steve/project/src/com/example/Main.java" could have 7 different fully qualified names, depending on how you compiled the code. Most likely, only one of those will be the "correct" one. But you wouldn't be able to tell which FQN is correct by looking at (just) the one source file.

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It should also be noted that the Java language specification does not require you to organize the source code tree according to the packages. That is a requirement of a (large) family of Java compilers, but a conformant compiler could be written that did not require this. For example:

• The source code could be held in a database.
• The source code could be held in a file tree with random file names².

In such eventualities, the package declaration would not be duplicative of file pathnames, or (necessarily) of anything. However, unless there was some redundancy, finding the correct source "file" for a class would be expensive for the compiler ... and problematic for the programmer.

Considerations like the above are the practical reason that most Java tool chains rely on file tree structure to locate source and compiled classes.

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^{1 - By this, I mean hypothetical dialect of Java which didn't require package declarations.
2 - The compiler would need to scan the file tree to find all Java files, and parse them to work out which file defined which class. Possible, but not very practical.}