访问说明符错误（找不到符号）

Question

C:\Users\jaina_000\Desktop\learn_java\p1>javac Testp1.java
Testp1.java:6: error: cannot find symbol
                Protection ob = new Protection();
                ^
  symbol:   class Protection
  location: class Testp1
Testp1.java:6: error: cannot find symbol
                Protection ob = new Protection();
                                    ^
  symbol:   class Protection
  location: class Testp1
Testp1.java:7: error: cannot find symbol
                Derived ob1 = new Derived();
                ^
  symbol:   class Derived
  location: class Testp1
Testp1.java:7: error: cannot find symbol
                Derived ob1 = new Derived();
                                  ^
  symbol:   class Derived
  location: class Testp1
Testp1.java:8: error: cannot find symbol
                SamePackage ob2 = new SamePackage();
                ^
  symbol:   class SamePackage
  location: class Testp1
Testp1.java:8: error: cannot find symbol
                SamePackage ob2 = new SamePackage();
                                      ^
  symbol:   class SamePackage
  location: class Testp1
6 errors

package p1;
public class Testp1
    {
    public static void main(String a[])
        {
        Protection ob = new Protection();
        Derived ob1 = new Derived();
        SamePackage ob2 = new SamePackage();
        }
    }

package p1;

public class Protection
    {

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

    public Protection()
        {
        System.out.println("Inside 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);
        }
    }

package p1;

class Derived extends Protection
    {
    Derived()
        {
        System.out.println("Inside Derived 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);
        }
    }

package p1;

class SamePackage{

    SamePackage(){
        Protection p = new Protection();
        System.out.println("Inside SamePackage constructor.");
        System.out.println(" n = "+p.n);                
    //  System.out.println("n_pri = "+p.n_pri);     
        System.out.println("n_pro = "+p.n_pro);
        System.out.println("n_pub = "+p.n_pub);
    }
}

Answer 1

当你有包的时候，你不能像在类那样简单地编译或运行，因为它没有包。

编译时，需要导航到package文件夹的外部，然后使用命令javac[package]/[class]进行编译

javac p1/testp1.

运行时，请使用以下命令Java[package.class]。因此，在您的情况下，它应该是

Java P1.TestP1

确保您不在包内[p1]您应该在包外[通过cmd导航到learn_java文件夹后编译]。

最后，这是您的cmd看起来的样子

C:\Users\jaina_000\Desktop\learn_java>javac p1/Testp1.java

C:\Users\jaina_000\Desktop\learn_java>java p1.Testp1
Inside base constructor.
 n = 1
n_pri = 2
n_pro = 3
n_pub = 4
Inside base constructor.
 n = 1
n_pri = 2
n_pro = 3
n_pub = 4
Inside Derived constructor.
 n = 1
n_pro = 3
n_pub = 4
Inside base constructor.
 n = 1
n_pri = 2
n_pro = 3
n_pub = 4
Inside SamePackage constructor.
 n = 1
n_pro = 3
n_pub = 4
C:\Users\jaina_000\Desktop\learn_java>