Thread: A Question Regarding Abstract Classes & Packages

Hello all,

I'm just playing around with Java and trying to understand how abstract classes and methods work along with packages. I began writing some simple classes to understand how abstract classes and methods work :

Person Abstract Class :

public abstract class Person {
	protected String name;
	abstract void setName(String name);
	abstract void displayName();	
}

Dave Class implementing Person :

public class Dave extends Person {
 
	public Dave(){
		name = "Dave";
	}
 
	public void setName(String name){
		this.name = name;
	}
 
	public void displayName(){
		System.out.println("My name is " + name);
	}
}

And finally DaveTest class:

public class DaveTest {
 
	public static void main (String args[]){
		Person p = new Dave();
		p.displayName();
	}
}

Placing all three files in the same directory, they compile and execute. (See Dave1.zip for the files)

Now I wanted to put Person and Dave classes into a package of their own, so altered them to the following :
Person

package person;
public abstract class Person {
	protected String name;
	abstract void setName(String name);
	abstract void displayName();	
}

Dave

package person;
 
 
public class Dave extends Person {
 
	public Dave(){
		name = "Dave";
	}
 
	public void setName(String name){
		this.name = name;
	}
 
	public void displayName(){
		System.out.println("My name is " + name);
	}
}

DaveTest2

import person.Dave;
import person.Person;
 
public class DaveTest2 {
 
	public static void main (String args[]){
		Person p = new Dave();
		p.displayName();
	}
}

I put Person and Dave in a directory 'person' off the directory where DaveTest2 is located (see Dave2.zip for the location) but now when i try to compile DaveTest2, the compiler complains that displayName() method is not public ! :

DaveTest2.java:9: displayName() is not public in person.Person; cannot be accessed from outside package
p.displayName();
^
1 error

So I don't understand why DaveTest2 will not compile, all I have done is placed the classes into a package ! The only way round the error is to cast the object as (Dave) where it has been implemented but this seems ridiculous. What if i don't know the subclass I'm dealing with but I do know that it will have a displayName() implementation. I can't see the point of using abstract methods then if they are not available outside of a package.

Could someone possibly explain this or the error of my ways ?

thanks for that. I had actually tried to make the method displayName() public in the abstract class but the compiler complained that it was an illegal usage of the key word (when use in conjunction with abstract).

I tried it again and it compiled ! Weird !

But now I have another problem.

If i try compiling the implementing class Dave.java alone, the compiler complains it cannot find Person

javac Dave.java

Dave.java:2: cannot find symbol
symbol: class Person
public class Dave extends Person {
^
Dave.java:5: cannot find symbol
symbol : variable name
location: class person.Dave
name = "Dave1";
^
Dave.java:9: cannot find symbol
symbol : variable name
location: class person.Dave
this.name = name;
^
Dave.java:13: cannot find symbol
symbol : variable name
location: class person.Dave

But if I do javac *.java everything compiles ok !! That doesn't seem right to me !

When you have classes in different packages that need to be compiled together, you need to understand how Java handles packages. By default, the Java compiler treats packages as relative to where it is being run from - that is, it expects the packages it encounters to be in subdirectories of where it is running. When you run it from the 'person' package directory, it finds Dave.java because you told it to compile Dave.java. When it encounters person.Person in the Dave.java code, it looks for a subdirectory called 'person' containing class Person.java. It doesn't find it because it is already running in the person directory.

There are two ways to fix this - either run the compiler from the 'package root' directory - the directory that contains all the packages, or use the classpath option to tell javac where to find the package root directory.

For example, running javac from the package root directory:

javac person\Dave.java

In practice, for non-trivial applications, the classpath option is generally used, because it means you can run javac from anywhere and it will know where to find the other classes:

c:\>javac -classpath c:\projects\exercise1\src c:\projects\exercise1\src\person\Dave.java

Above, the compiler is running from the root of drive c: with a classpath to the package root (bolded) and it is being asked to compile the Dave.java file. It will now use the classpath to find all the files referred to by Dave.java. You can add other package root directories to the classpath by separating them with a semicolon ';'.