correct way to write class inside a subfolder in java?-CodePudding

I am just starting to learn java.
I have made a project of java using vscode
The filePath looks like this project/src/own/test.java
I have just wrote a simple program :

package own;
import java.util.Random;
import java.util.Scanner;
import static java.lang.System.out;

public class test {
    public static void main (String[] args){
        int randomNumber = new Random().nextInt(7);
        System.out.println("Enter a number");
        Scanner keyboard = new Scanner(System.in);
        int inputNumber = keyboard.nextInt();
        if (randomNumber == inputNumber){
            System.out.println("You won!");
        }else{
            System.out.println("You loose!");
        }
        keyboard.close();
    }
}

But every time I run this is the vscode terminal is says:

Error: Could not find or load main class test

But it runs fine if package own; line is not there.
Vscode automatically included this line.
Can anyone tell me why that is so?
What is the use of package own.

CodePudding user response：

Every java class must always specify the "package ..." This reserved word is used for several important things, such as:

With package you identify in which zone of the project your class is located, in general, each java project has src/main/java/mypackage
- src, main, and java are directories, not packages (you can look up the difference between these). So the classes here will not have a package, you can execute a main method but this class will be invisible inside your project
- If you are at the "mypackage" level, from this path the class is already visible inside your project and the class will carry the "package mypackage";
By having your class with "package mypackage;" you can import this class to another class, and use the methods of the first class in the second class

package mypackage;

public class ClassOne {
   public static String goodMorning(){
       System.out.println(""good morning);
   }
}

And this class is in another file

package mypackage;

import mypackage.ClassOne;

public class ClassTwo{
    public static void main(String args...){
        ClassOne.goodMorning();
    }
}

If you know PHP it works similar to "require_once" (although they are similar import and require_once do not work the same)

I mentioned above about "visibility" and it is that packages are also used to know the scope of an access modifier, in java there are 4 of these: public, private, protected and default (by default it is when you do not put an access modifier )

//public access modifier
public class One{
    public String attribute;

    public void method(){
    }
}
//private access modifier
private class Two{
    private String attribute;

    private void method(){
    }
}

//protected access modifier
protected class Three{
    protected String attribute;

    protected void method(){
    }
}

//default access modifier
class Four{
    String attribute;

    void method(){
    }
}

Find out what each one is for.

And another thing it is useful for, is organizing classes by packages, you can organize a collection of classes in a better way, this is important for large projects.

There are still more things but as you say that you are learning I do not want to overload you with information, go easy, greetings.