在java中使用包内的主类

Question

I am trying to to run the file Demo.java which is calling Protection class within the same package but it is giving error This is the main class.

package p1;
// Instantiate the various classes in p1.
class Demo {
  public static void main(String args[]) {
    Protection ob1 = new Protection();
    //Derived ob2 = new Derived();
    //SamePackage ob3 = new SamePackage();
  }
}

And this is the class that I want to use in the main class.

package p1;

public class Protection {

  public int n = 1;
  private int n_pri = 2;
  protected int n_pro = 3;
  public int n_pub = 4;

  public Protection() {
    System.out.println("base constructor");
    System.out.println("n = " + n);
    System.out.println("n_pri = " + n_pri);
    System.out.println("n_pro = " + n_pro);
    System.out.println("n_pub = " + n_pub);
  }
}

It is giving this error:

$ javac Demo.java
Demo.java:6: error: cannot find symbol
Protection ob1 = new Protection();
^
  symbol:   class Protection
  location: class Demo
Demo.java:6: error: cannot find symbol
Protection ob1 = new Protection();
                     ^
  symbol:   class Protection
  location: class Demo
2 errors
error: compilation failed

Answer 1

You should use javac , not java only

When you use the command java , you can execute a file, but only the classes in that file. Here you have several files, so you should compile them in order to use them.

Do the following:

$ mkdir p1
$ mv Demo.java Protection.java p1/
# edit p1/Demo.java to change `class Demo` to `public class Demo`
$ javac p1/*
$ java p1.Demo

This worked and resulted in the following:

base constructor
n = 1
n_pri = 2
n_pro = 3
n_pub = 4

Answer 2

You could try this:

1. open cmd in p1, use javac .\\Demo.java .\\Protection.java ; then you can see two .class files generated
2. use cd .. , then you can see your package p1
3. use java p1.Demo then you can see the expected output