Behaviour of return() ???

[achana chan]

Hi , I am new to this forum and to JAVA.
I need to understand a few basic behaviour JAVA class codes :
(1) I have a piece of code with both public and private methods : do I assume right that the public methods can be called from other JAVA applications through "import" ?
(2) In this class-file , there is this method :
public String toString(){
return "Name : " + v_name + "\n" + ... ;
}
Even though this method was not explicitly "called" from the JAVA application which inherited them, it displayed the results of the db queries. Do I assume right that this is due to the "return" JAVA command ?
(3) Will a JAVA application e.g.

(public static void main(String args[]){...})

work through all its methods (as distinct from a class-file, which will only use the method() called upon ?
(4) Using package, I seem to inherit everything in the subdirectory specified in the package statement e.g. package thisDemo seems to allow me to pull in all the classes in the subdirectory .../myDemo
(5) Similar to (4), I can "import myDemo.*" can I can call on all public methods, so what is the fine differences.
Finally, sorry for these basic questions, I grew up with 3GL's
[ August 23, 2002: Message edited by: achana chan ]

[Dirk Schreckmann]

I am new to this forum and to JAVA.
Welcome to JavaRanch!
Here is a list of free Java tutorials that I have found useful:

Sun's Java Tutorial

Introduction to Computer Science using Java by Bradley Kjell

Introduction to Programming Using Java by David J. Eck

Dick Baldwin's Java Programming Tutorials

(1) I have a piece of code with both public and private methods : do I assume right that the public methods can be called from other JAVA applications through "import" ?
No. Import statements tell the compiler where to find classes and interfaces, not methods of classes or object.
(2) ... and (3)
I don't think that I understand the questions.
(4) Using package, I seem to inherit everything in the subdirectory specified in the package statement e.g. package thisDemo seems to allow me to pull in all the classes in the subdirectory .../myDemo
Perhaps there is a typo in the question - "thisDemo" versus "myDemo". And I don't think that I understand the question.
(5) ... I can "import myDemo.*" can I can call on all public methods, so what is the fine differences.
You cannot call the methods of classes or objects by importing a package. See the answer to question number one.

[Anthony Villanueva]

Originally posted by achana chan:
(1) I have a piece of code with both public and private methods : do I assume right that the public methods can be called from other JAVA applications through "import" ?
(2) In this class-file , there is this method :
public String toString(){
return "Name : " + v_name + "\n" + ... ;
}
Even though this method was not explicitly "called" from the JAVA application which inherited them, it displayed the results of the db queries. Do I assume right that this is due to the "return" JAVA command ?
(3) Will a JAVA application e.g.

(public static void main(String args[]){...})

work through all its methods (as distinct from a class-file, which will only use the method() called upon ?
(4) Using package, I seem to inherit everything in the subdirectory specified in the package statement e.g. package thisDemo seems to allow me to pull in all the classes in the subdirectory .../myDemo
(5) Similar to (4), I can "import myDemo.*" can I can call on all public methods, so what is the fine differences.

(1) Yes.
(2) Any Java object inside System.out.println() will call its toString() method. That's probably what happened.
(3) Code inside main() will be executed unless it programmatically exits or encounters a runtime error ("exception").
(4) No. Importing a package will not import its subpackages. You must import each subpackage explicitly.
(5) Can you clarify? Fine differences between what and what?
-------
Domine absolvi

[achana chan]

I mean the difference between "package..." and "import ..."
In one class file I have this (which is actually a sub-directory .../myDemo):
...
package myDemo;
In another , I do following :
...
import myDemo.*;
(seems more like a maledictus domini at the moment)

[Anthony Villanueva]

Packaging is basically used in Java to make class names unique, and to impose a certain order among your .class files. It's analogous to namespaces. For example, your application may be using two different Date classes so a naming conflict is possible.
Let's define two sample packages. Suppose my classpath root is at C:\anthony\java\applications. I create a folder named "drink" and inside I have:

// Java.java
package drink;
public class Java
{
private String kind;
public Java(String k)
{
kind = k;
}
public String getKind()
{
return kind;
} 
}

I create another folder C:\anthony\java\applications\language and inside I have:

// Java.java
package language;
public class Java
{
private final boolean portable = true;
public boolean isPortable()
{
return portable;
}
}

Note the package declarations at the top of each. Finally I need a client class at C:\anthony\java\applications:

import language.Java;
import drink.Java;
public class Test29
{
public static void main(String args[])
{
language.Java j = new language.Java();
System.out.println("Is Java portable? " + j.isPortable());
drink.Java starbucks = new drink.Java("black");
System.out.println("My coffee is " + starbucks.getKind());
}
}

Note the import declarations at the top. Also, I am using the fully qualified names of both Java classes to disambiguate them. I could have used
import language.*;
import drink.*;
which means "import all classes inside the language and drink packages". Finally, the classes I defined the package had access modifiers like public and private, which specifies if they are accessible outside the package, accessible between classes with an inheritance relationship, accessible between classes inside the package, or being inaccessible to any other class at all.

[Anthony Villanueva]

Okay, let's do some tweaking. Let's change the class drink.Java a bit:

package drink;
public class Java
{
private String kind;
public Java(String k)
{
kind = k;
}
public String getKind()
{
return kind;
} 
[B]
public String toString()
{
return "This coffee is really " + kind;
}
}
[/B]

I added the toString() method here. This method is overriding the toString() method of class Object, which is a superclass of drink.Java. I didn't have to do anything -- all classes have class Object as a superclass. Finally I change the client class as well:

import language.Java;
import drink.Java;
public class Test29
{
public static void main(String args[])
{
language.Java j = new language.Java();
System.out.println("Is Java portable? " + j.isPortable());
drink.Java starbucks = new drink.Java("black");
System.out.println("My coffee is " + starbucks.getKind());
System.out.println(starbucks);
}
}

[achana chan]

Hi !
Thanks .

[achana chan]

Hi !
One more question on this thread. I noticed that you have classes within classes, socalled subclasses. This is vcommon in the stack of JAVA books infornt on me. However I still have problems (in the olden days, I would say it is "scoping" and "visibility" ):

public class AllEmpDS extends HttpServlet
{
   ...
public void doGet(HttpServletRequest req ,       HttpServletResponse resp){
  ...
  PrintWriter out = resp.getWriter();
  ...
}//end of method doGet()public boolean showAllEmp(){
  boolean result=false;
  try {
   stmt = getStatement();
   rset = stmt.executeQuery( ... :wink: 
   ...
   [B]out.println(sb.toString());[/B]
   ...     
  }catch (Exception exc){exc.printStackTrace();}
   ...
}//end of method showAllEmp()
...
}//end of class AllEmpDS

Javac has this complain :
AllEmpDS.java:71: cannot resolve symbol
symbol : variable out
location: class connections.AllEmpDS
out.println(sb.toString());
and yet, I have declare the variable out in the public method doGet().

[Anthony Villanueva]

out is a local variable of doGet() so it only has scope inside doGet(). Referring to out inside showAllEmp() causes this error. Why not use public boolean showAllEmp(PrintWriter out) instead?

[achana chan]

Hi !
Once again, thanks.
I was doing things like
public boolean showAllEmp(out)
etc
Do you work for JavaRanch ?

[Anthony Villanueva]

Originally posted by achana chan:
Do you work for JavaRanch ?

We're all volunteers here.