java包只有一个公共类吗？

Question

I tired creating a package with one public class and one with multiple public classes and then import them through another package and both times the program worked. I understand the difference, in the first i can only use the methods of the one public class, but that class can use other classes in its package, and when more classes are public i can directly access all those classes through the alien package.

Now when we import java.util.Scanner, and we import java.util.ArrayList isn't this the same package? When we import our package with one public class it only shows the one public. That means that java.util has multiple public classes, or?

Also what does that java. mean, does it make the difference between my package and java library packages?

Answer 1

Yes, a package can have multiple public classes.

Creating a new package for every public class would be extremely cumbersome and provide no overview at all.

java. are just the standard libraries that come with the JDK/JRE.

Packages are really just like folders on your system. You can have multiple files in a folder without a problem.

Typically packages are structured in a way that they combine related classes to make it easier for the developer. If you want to have no packages at all, or you want a new package for every class: it doesn't matter. You'll be your own worst enemy, but it is surely possible.

Answer 2

import java.util.ArrayList;

means: let me use the class ArrayList , which is in the package java.util , by only typing ArrayList and not java.util.ArrayList .

java.util is the name of the package, nothing else. It's where the Java developers chose to store the ArrayList class. It's not different from any other package name, except you're not allowed to use the java package for your own classes, because it's reserved for standard Java classes.

import java.util.*;

means: let me use all the classes of the package java.util ( ArrayList , LinkedList , Collection , etc.) by typing their simple name ( ArrayList , LinkedList , Collection , etc.) instead of their fully qualified name ( java.util.ArrayList , java.util.LinkedList , java.util.Collection , etc.)

All these classes are public (as documented in their javadoc). If they weren't public, you wouldn't be able to use them in your own classes.