Java-软件包问题

Question

I'm not new to Java but I never learned Packages. Anyways, let's say I create a folder called maina .

I put in file main.java in folder maina :

package maina;

import other.Dice;

public class main
{


    public static void main(String[] args)
    {

        System.out.println("Hello world!");
        System.out.println(Dice.test);
    }
}

Then I create a new folder called other inside the folder maina . In folder other I put file Dice.java :

package other;

public class Dice
{

public Dice() {

        String test = "Testing!";
    }

}

OK, now Dice.java compiles fine. However when I compile main.java I get this error:

C:\Users\tekaffi\Documents\ask\maina\main.java:13: cannot find symbol
symbol  : variable test
location: class other.Dice
        System.out.println(Dice.test);
                               ^
1 error

Tool completed with exit code 1

What am I doing wrong?

Here's the error I get when I compile:

C:\Users\wekaffi\Documents\ask\maina\myMain.java:3: package maina.other does not exist
import maina.other.Dice;
                  ^
C:\Users\wekaffi\Documents\ask\maina\myMain.java:13: cannot find symbol
symbol  : class Dice
location: class maina.myMain
        Dice myDice = new Dice();
        ^
C:\Users\wekaffi\Documents\ask\maina\myMain.java:13: cannot find symbol
symbol  : class Dice
location: class maina.myMain
        Dice myDice = new Dice();
                          ^
3 errors

Tool completed with exit code 1

Answer 1

It has nothing to do with packages.

Your code is seriously messed up, you're trying to call a "test" member on the "Dice" class but you haven't created that member. besides that, you can't have a class named "main" and then have a static main method in it beacuse the compiler will think the main method is the constructor you need to rename your class to something else.

For your code to work your Dice class needs to look like this:

package maina.other;

public class Dice
{

public String test;

public Dice() {

        this.test = "Testing!";
    }

}

And for the print to work you need to create a new instance of Dice before you print Either that or you make Dice static. So your main needs to be renamed to myMain and then the class should look like this:

package maina;

import maina.other.Dice;

public class myMain
{


    public static void main(String[] args)
    {

        System.out.println("Hello world!");
        Dice myDice = new Dice();
        System.out.println(myDice.test);
    }
}

If you're placing stuff where you said you are, it should work fine package-wise

Answer 2

Your Dice class must have a package declaration like so

package maina.other;

Your main class should import Dice like so

import maina.other.*;

Answer 3

如果在/ maina / other中，它将是包maina.other

Answer 4

Dice needs a public static string test . The current test is a non-static local variable to the constructor. Or you can make test non-static and then have the constructor set the test member and then do new Dice().test in the main.java

And the package name doesn't matter, foldering is only a convention and is IIRC ignored by the compiler. So thats not the issue here!

Answer 5

当您将包“ other”放入“ maina”内部时，新包为

package maina.other;

Answer 6

package maina.other;

As a side note, if you're using an IDE like Eclipse, you don't have to make the directories manually - it does it for you. Also navigating your packages with the package viewer is easy.

Answer 7

This one is tough to explain, and you've mentioned that you're new to Java, so please don't let me confuse you.

The package of a top-level type, such as main or Dice is whatever is listed in the package declaration. The package for Dice could just be other , even though the corresponding directory is nested inside the directory that corresponds to package amain .

The key is resource discovery when compiling and running. When you compile, you can specify a sourcepath and a classpath that helps the compiler resolve dependencies. Likewise, when you run, you can specify a classpath that helps the JVM resolve dependencies. There is no restriction that root packages not be nested inside one another. So, both amain and other could be root packages, like so:

% cd <directory-that-contains-amain>
% javac -sourcepath .:amain amain/main.java amain/other/Dice.java
% java -classpath .:amain amain.main

This is considered abnormal (and consequently poor) practice, however. You shouldn't do it, but you could.