subclass won't compile - why?

[jDennis79]

Hi everybody,

I am trying to learn Java with a German book from 2010. In the chapter I am currently working on, the author introduces the creation and inheritance of classes and subclasses.

He gives a (very simple) example of a management software for car rental. So far, he created two classes. The first class is meant as an abstract class for vehicles in general. The code (I translated variable names to English):

package vehicle;

abstract class Vehicles {

	protected String name;
	protected short speed;
	protected boolean rented;

	abstract boolean newEntry();
	abstract boolean showEntry();
	
	public boolean rentCar() {

		if (rented) { return false; }
		else { rented = true; return true; }

	}

}

This class compiles correctly.

The second class is the first subclass of Vehicles, "Cars":

package vehicle;

import java.util.Scanner;

public class Cars extends Vehicles {

	public short numberOfSeats;
	
	public boolean newEntry() {
		Scanner entry = new Scanner(System.in);
		
		try {
			System.out.print("Description: ");
			name = entry.nextLine();
			System.out.print("Number of seats: ");
			numberOfSeats = entry.nextShort();
			System.out.print("Max. speed: ");
			speed = entry.nextShort();
			return true;
		} catch (Exception e) {
			System.out.println(e);
			return false;
		}
	}
	
	public boolean showEntry() {
		System.out.println("The car is a " + name);
		System.out.println("It has " + numberOfSeats + " seats.");
		System.out.println("Max. speed (mph): " + speed);
		
		if (rented) {
			System.out.println("It is not available.");
		} else {
			System.out.println("It is available.");
		}
		return true;
	}

}

If I try to compile this second class, I get the following error:

Cars.java:5: cannot find symbol
symbol: class Vehicles
public class Cars extends Vehicles {
                          ^
Cars.java:14: cannot find symbol
symbol  : variable name
location: class vehicle.Cars
			name = entry.nextLine();
			^
Cars.java:18: cannot find symbol
symbol  : variable speed
location: class vehicle.Cars
			speed = entry.nextShort();
			^
Cars.java:27: cannot find symbol
symbol  : variable name
location: class vehicle.Cars
		System.out.println("The car is a " + name);
		                                                 ^
Cars.java:29: cannot find symbol
symbol  : variable speed
location: class vehicle.Cars
		System.out.println("Max. speed (mph): " + speed);
		                                                           ^
Cars.java:31: cannot find symbol
symbol  : variable rented
location: class vehicle.Cars
		if (rented) {
		    ^
6 errors

My guess was that there is a problem with the class and variable specifiers, but the code listings match with the author's explanations in the text, so I don't see the problem here. Is there any mistake?

[Norm]

The problem is with packages.
The compiler needs the classpath set to the folder container the folder at the beginning of the package path.

For a quick test, comment out all the package statements and try compiling the code.

[PhHein]

Are you sure Norm? He should get the same if Vehicles isn't compiled, no? Both classes are in the same package.

[jDennis79]

@PhHein - yes, I did.

@Norm - you're right, after commenting out the package statements, both classes compile correctly. That raises two new questions: 1) Why did Vehicles.java compile nevertheless? Shouldn't there be an error too? 2) How do I set the classpath?

[Norm]

I'll try it.

For the compiler to find vehicle.Vehicles the classpath must point to where the vehicles folder is, not inside it where the . (current directory)

Here are two compile results screens. First with classpath only in the current folder:

Running: D:\Java\jdk1.6.0_02\bin\javac.exe -Xlint -g -deprecation -classpath D:\JavaDevelopment\;. Cars.java

Cars.java:5: cannot find symbol
symbol: class Vehicles
public class Cars extends Vehicles {
^
Cars.java:14: cannot find symbol
symbol : variable name
location: class vehicle.Cars
name = entry.nextLine();
^
Cars.java:18: cannot find symbol
symbol : variable speed
location: class vehicle.Cars
speed = entry.nextShort();
^
Cars.java:27: cannot find symbol
symbol : variable name
location: class vehicle.Cars
System.out.println("The car is a " + name);
^
Cars.java:29: cannot find symbol
symbol : variable speed
location: class vehicle.Cars
System.out.println("Max. speed (mph): " + speed);
^
Cars.java:31: cannot find symbol
symbol : variable rented
location: class vehicle.Cars
if (rented) {
^
6 errors

1 error(s)

************************************ HERE I changed the classpath to go up (..\.) one level: <<<<<<<<<

Running: D:\Java\jdk1.6.0_02\bin\javac.exe -Xlint -g -deprecation -classpath D:\JavaDevelopment\;.;..\. Cars.java


0 error(s)

[PhHein]

C:\whatever>javac -cp classpath Cars.java

classpath is the path to the folder containing the vehicle folder

[jDennis79]

This doesn't work. The result is the same error message as above. :-(

[Norm]

Can you run the commands in a command prompt?
Copy and paste the screen here.
To copy the contents of the command prompt window:
Click on Icon in upper left corner
Select Edit
Select 'Select All' - The selection will show
Click in upper left again
Select Edit and click 'Copy'

Paste here.

[jDennis79]

javac -classpath "Users/dennis/Entwicklung/Java/Einteigerseminar/Kap_05/fahrzeug/" Auto.java
Auto.java:5: cannot find symbol
symbol: class StrassenFzg
public class Auto extends StrassenFzg {
                          ^
Auto.java:14: cannot find symbol
symbol  : variable bez
location: class fahrzeug.Auto
			bez = eingabe.nextLine();
			^
Auto.java:18: cannot find symbol
symbol  : variable geschw
location: class fahrzeug.Auto
			geschw = eingabe.nextShort();
			^
Auto.java:27: cannot find symbol
symbol  : variable bez
location: class fahrzeug.Auto
		System.out.println("Das Auto ist ein " + bez);
		                                         ^
Auto.java:29: cannot find symbol
symbol  : variable geschw
location: class fahrzeug.Auto
		System.out.println("Höchstgeschwindigkeit (km/h): " + geschw);
		                                                       ^
Auto.java:31: cannot find symbol
symbol  : variable verliehen
location: class fahrzeug.Auto
		if (verliehen) {
		    ^
6 errors

[Norm]

Instead of the full absolute path for the classpath, try using a relative path that goes up one level: ..\. See bottom of post#5

[jDennis79]

Works. Thank you very much! Why doesn't the absolute path work?

[PhHein]

Because you have defined the package fahrzeug. So the compiler is looking for Users/dennis/Entwicklung/Java/Einteigerseminar/Kap_05/fahrzeug/fahrzeug/ which doesn't exist.

[jDennis79]

But shouldn't it work with

javac -classpath "Users/dennis/Entwicklung/Java/Einteigerseminar/Kap_05/" Auto.java

then? Because it doesn't.

[Norm]

Sorry, I don't know linux.

Have you tried it without the "s

[jDennis79]

OS X. :-) Okay, still Unix. - Yes, I did. Whatever. It works with relative path for now. I will try and find some internet sources about the whole classpath matter. I only wonder why the author doesn't explain it in his book - since it's already the 6th edition. :-)

Thanks guys!

[Norm]

Classpath is a mystery for many for a long time.
When using packages you have to understand it.