Question

I write a source file and put that public class in a package :

package abc;
public class Employee
{
    // Constructor
    public Employee(String name, double salary)
    {
        this.name = name;
        this.salary = salary;
    }

    // Methods
    public String getName()
    {
        return this.name;
    }

    public double getSalary()
    {
        return this.salary;
    }

    // instance field
    private String name;
    private double salary;
}    

Then I try to compile it using command: javac Employee.java , it generates a .class file in the same directory as the source file

Now I try to use this package, so I write a source file :

import abc.*;
public class HelloWorld
{
    public static void main(String args[]){
        //System.out.println("hello world");

        Employee aEmployee = new Employee("David",1000);
        System.out.println(aEmployee.getName() + aEmployee.getSalary());
    }

}

I try to compile it using: javac HelloWorld.java , but it has a error says : package abc doesn’t exist

I have the following questions:
1) Why did this error happen ?
2) How to solve this problem ?
3) Each time when I package some classes, where can I find the package to use afterwards

I’ve read some docs about this, but that’s so complex, can somebody explain it simply ?

Answer 1

1) Why did this error happen ?

This is because the java compiler looks for a directory tree when it tries to load a package, either in the classpath or in a jar file. This means for a package called abc.foo.bar, it will look for the directory tree: /abc/foo/bar and expect classes that belong to that package to be there. You’ve compiled your Employee class but when you import it, the compiler looks for a directory abc in your classpath, and it’s not there.

2) How to solve this problem ?

You need to make sure when you compile the Employee class, its classfile is in a directory abc which is somewhere in your classpath. The simplest thing may be to create a directory called abc, then move the Employee.java file into the abc directory, then compile:

    javac abc/Employee.java

This will create a Employee.class file in the abc directory. Then you can compile your HelloWorld:

    javac HelloWorld.java

3) Each time when I package some classes, where can I find the package to use afterwards

In the directory tree that you’ve named your package. See the later part of the response to 1).