Question

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("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);
}
}

包p2包含三个文件demo.java，OtherPackage.java和Protection2.java：

//Demo package p2.
package p2;

//Instantiate various classes in p2.
public class Demo {
public static void main(String[] args) {
    Protection2 ob1 = new Protection2();
    OtherPackage ob2 = new OtherPackage();
}
}

package p2;

OtherPackage.java

class OtherPackage {
OtherPackage() {
    p1.Protection p = new p1.Protection();
    System.out.println("Other package constructor");

    //class or package only
    //System.out.println("n = " + p.n);

    //class only
    //System.out.println("n_pri = " + p.n_pri);

    //class, subclass or package only
    //System.out.println("n_pro = " + p.n_pro)

    System.out.println("n_pub = " + p.n_pub);
}
}

和Protection2.java

package p2;

class Protection2 extends p1.Protection {
Protection2() {
    System.out.println("Derived other package constructor");

    //class or package only
    //System.out.println("n = " + n);

    //class only
    //System.out.println("n_pri = " + n_pri);

    System.out.println("n_pro = " + n_pro);
    System.out.println("n_pub = " + n_pub);
}
}

我尝试从包p2编译Demo.java：

:~/Desktop/Examples/p2$ javac *.java

我生成以下错误

Protection2.java:3: error: package p1 does not exist
class Protection2 extends p1.Protection {
                        ^
OtherPackage.java:5: error: package p1 does not exist
    p1.Protection p = new p1.Protection();
      ^
OtherPackage.java:5: error: package p1 does not exist
    p1.Protection p = new p1.Protection();
                            ^
Protection2.java:13: error: cannot find symbol
    System.out.println("n_pro = " + n_pro);
                                    ^
    symbol:   variable n_pro
    location: class Protection2
Protection2.java:14: error: cannot find symbol
    System.out.println("n_pub = " + n_pub);
                                    ^
    symbol:   variable n_pub
    location: class Protection2
5 errors

我尝试导入包p1，但它增加了另外一个错误以及上述5个错误：

Demo.java:4: error: package p1 does not exist
import p1.Protection;

Answer 1

我制作了这个目录结构：

                 package
                    |
       ----------------------------
       |                          |
    source                       bin
       |                          |
( All .java files )       ( Package folders after compilation )

Java文件：

Protection.java

package p1;

public class Protection {

    public int m_Pub;

    int m_N;
    private int m_Pri;
    protected int m_Pro;

    public Protection () {
        m_Pub = 4;
        m_N = 1;
        m_Pri = 2;
        m_Pro = 3;

        System.out.println ( "Base Constructor" );
        System.out.println ( "m_Pub = " + m_Pub );
        System.out.println ( "m_N = " + m_N );
        System.out.println ( "m_Pri = " + m_Pri );
        System.out.println ( "m_Pro = " + m_Pro );
    }
}

Demo.java

package p2;

public class Demo {

    public static void main ( String [] args ) {
        Protection2 protection2 = new Protection2 ();
        OtherPackage otherPackage = new OtherPackage ();
    }
}

OtherPackage.java

package p2;

import p1.Protection;

public class OtherPackage {

    public OtherPackage () {
        Protection protection = new Protection ();
        System.out.println ( "OtherPackage Constructor" );
    }
}

Protection2.java

package p2;

import p1.Protection;

public class Protection2 extends Protection {

    public Protection2 () {
        System.out.println ( "Derived Protection2 Constructor" );
        System.out.println ( "m_Pro = " + m_Pro );
        System.out.println ( "m_Pub = " + m_Pub );
    }
}

<强>汇编：

nipal_000@Lenovo-PC /cygdrive/c/mine/java/package
$ javac -d bin source/*.java

<强>执行：

nipal_000@Lenovo-PC /cygdrive/c/mine/java/package
$ cd bin

nipal_000@Lenovo-PC /cygdrive/c/mine/java/package/bin
$ java p2.Demo
Base Constructor
m_Pub = 4
m_N = 1
m_Pri = 2
m_Pro = 3
Derived Protection2 Constructor
m_Pro = 3
m_Pub = 4
Base Constructor
m_Pub = 4
m_N = 1
m_Pri = 2
m_Pro = 3
OtherPackage Constructor

nipal_000@Lenovo-PC /cygdrive/c/mine/java/package/bin

重点是，在编译时您未能指定，在哪里查找package p1，因为您位于文件夹p2内。在编译过程中，您需要放置所有.java个文件。

如果您尝试将每个.java文件保存在相应的package文件夹中，那么它会是这样的：

目录结构：

                     package
                        |
           ----------------------------
           |                          |
        source                       bin
           |                          |
     ------------     ( Package folders after compilation )
     |          |
     p1         p2
     |          | 
     |        Demo.java OtherPackage.java Protection2.java
Protection.java

<强>汇编：

nipal_000@Lenovo-PC /cygdrive/c/mine/java/package
$ javac -d bin source/p1/*.java source/p2/*.java

<强>执行：

nipal_000@Lenovo-PC /cygdrive/c/mine/java/package
$ cd bin

nipal_000@Lenovo-PC /cygdrive/c/mine/java/package/bin
$ java p2.Demo
Base Constructor
m_Pub = 4
m_N = 1
m_Pri = 2
m_Pro = 3
Derived Protection2 Constructor
m_Pro = 3
m_Pub = 4
Base Constructor
m_Pub = 4
m_N = 1
m_Pri = 2
m_Pro = 3
OtherPackage Constructor

nipal_000@Lenovo-PC /cygdrive/c/mine/java/package/bin

错误：从包p2编译java文件时，包p1不存在

1 个答案: