Thread: Cannot find symbol

hi,

I i'm writing some code code for a school assignment, but I get stuck at a cannot find symbol error:

javac *.java
Ecosysteem.java:34: cannot find symbol
symbol  : variable isKonijn
location: interface IDier
                                        Wereld[randomRij][randomKolom].isKonijn
= true;
                                                                      ^
Ecosysteem.java:53: cannot find symbol
symbol  : variable isVos
location: interface IDier
                                        Wereld[randomRij][randomKolom].isVos = t
rue;
                                                                      ^
Ecosysteem.java:86: cannot find symbol
symbol  : variable isKonijn
location: interface IDier
                                if (Wereld[rij][kolom].isKonijn) {
                                                      ^
Ecosysteem.java:88: cannot find symbol
symbol  : variable isVos
location: interface IDier
                                } else if (Wereld[rij][kolom].isVos) {
                                                             ^
4 errors

The assignment is about writing an ecosystem with konijnen (rabbits) and vossen (foxes). The files I've written so far:

Vos.java:
[Java] Vos.java - Pastebin.com
Konijn.java:
[Java] Konijn.java - Pastebin.com
Dier.java:
[Java] // abstracte klasse Dier welke de interface IDier.java implementeert // // // - Pastebin.com
IDier.java:
[Java] IDier.java - Pastebin.com
Ecosysteem.java
[Java] Ecosysteem.java - Pastebin.com
IEcosysteem.java
[Java] IEcosysteem.java - Pastebin.com

Can anybody explain to me why I get this error and how I can solve it?

Thanks in advance.

cannot find symbol

The error from the compiler says that the compile can NOT find a defintion for a symbol you have coded in the program. Look at the text of the error message, find the symbol that the compiler is talking about then check your code to see why that symbol is not defined in it.

Some common errors:
The name could be misspelled,
or it could be out of scope. Ie not defined within the enclosing {}s

Where are isKonijn and isVos defined?

isKonijn and isVos are defined in Konijn.java and Vos.java:

The beginning of Konijn.java:

class Konijn extends Dier {
 
        boolean isKonijn;
 
        /**
         * Deze methode returnt true als dit object een Konijn is, en anders returnt deze methode false;
         */
         public boolean isKonijn() {
                return true;
         }

The beginning of Vos.java:

class Vos extends Dier {
 
        boolean isVos;
        static int gegeten;
        static int[] eet;
 
        /**
         * Deze methode returnt true als dit object een Vos is, en anders returnt deze methode false
         */
        public boolean isVos() {
                return true;
        }

Why can't the compiler see those definitions when it is compiling the Ecosysteem.java file?
Are any of the class's in packages?

Some questions about the code:
Why are there method names with the same names as variables?
Why do the methods return true instead of the value of some variable?

Originally Posted by Norm

Why can't the compiler see those definitions when it is compiling the Ecosysteem.java file?
Are any of the class's in packages?

As far as I know, none of the class's are in a package.

Originally Posted by Norm

Some questions about the code:
Why are there method names with the same names as variables?
Why do the methods return true instead of the value of some variable?

The assignment gives the IDier.java and IEcosystem.java files (can't make any changes to them) and the other files must be created by the student. The methods must be included in Konijn.java and Vos.java (only the method 'loop' must be included in Dier.java).

Above the methods public boolean isKonijn() and public boolean isVos() stands:
/ **
* This method returns true if this object is a Rabbit, and otherwise false;
* /

Seems redundant for a method to be hardcoded to return true. Why would you call a method if you know it will always return true?

What data type is Wereld?
If it is a class, does that class have definitions for the two variables: (isKonijn and isVos) that the compiler can not find definitions for?

File structue:
Idier.java - This is the interface for the animals
Dier.java - implements the IDier interface (dier = animal)
Konijn.java - extends Dier.java (konijn = rabbit)
Vos.java - extends Dier.java (vos = fox)

IEcosysteem.java -This is the interface to create the world
Ecosysteem.java - implements IEcosysteem and is used to create the world. The world is a IDier[][].

Wereld is a IDier[][]

Does IDier have definitions for the variables that are not found?

You can only reference variables defined in IDier using the Wereld variable.

Is there a confusion between referencing a variable and calling a method? You have them with the SAME names.

Originally Posted by Norm

Does IDier have definitions for the variables that are not found?

You can only reference variables defined in IDier using the Wereld variable.

Is there a confusion between referencing a variable and calling a method? You have them with the SAME names.

This was exactly what was going on, thanks for helping.

I've done a lot of work on it and now I'm getting my next error:

Exception in thread "main" java.lang.NullPointerException
        at Ecosysteem.run(Ecosysteem.java:88)
        at Ecosysteem.main(Ecosysteem.java:190)

Line 190 is: w.run();

	public static void main(String[] args) {
		Ecosysteem w = new Ecosysteem();
   		w.Ecosysteem(3,3,1,0);
		w.afdrukken();
 
		for (int i = 0; i < 2; i++) {
			w.run();
			w.afdrukken();
		}
	}

Line 88 is: if (Wereld[rij][kolom].isVos()) {

	public void run() {
 
		IDier[][] omgeving = {{},{}};
		Vos vos = new Vos();
		Konijn konijn = new Konijn();
 
		// vossen kunnen konijnen opeten
		for (int rij=0; rij<nrrijen; rij++) {
			for (int kolom=0; kolom<nrkolommen; kolom++) {
				// is deze cel een vos?
				if (Wereld[rij][kolom].isVos()) {
 
... more code

The run method needs to change the Ecosysteem w, but I didn't got it working...

Does Wereld[rij][kolom] have a null value?

Originally Posted by Norm

Does Wereld[rij][kolom] have a null value?

It could. I created a field (nrrijen by nrkolommen) were a number of rabbits (konijn) and foxes (vos) are randomly placed, so some cels stay empty.

Then you must test if null before using it.

Fixed it with adding Wereld[rij][kolom] != null to the if statement.

Get another error:

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 0
        at Ecosysteem.run(Ecosysteem.java:169)
        at Ecosysteem.main(Ecosysteem.java:273)

about line [I]omgeving[j] = Wereld[rijB][kolomB];

public void run() {
 
		IDier[][] omgeving = {{}};
		Vos vos = new Vos();
		Konijn konijn = new Konijn();
 
		// vossen kunnen konijnen opeten
		for (int rij=0; rij<nrrijen; rij++) {
			for (int kolom=0; kolom<nrkolommen; kolom++) {
				if (Wereld[rij][kolom] != null) {
					// is deze cel een vos?
					if (Wereld[rij][kolom].isVos()) {
						// vierkant Dier-grid (3x3), met het dier zelf in het midden aanmaken
						for (int i = 0; i < 3; i++) {
							for (int j = 0; j < 3; j++) {
								int rijA = -1 + i;
								int kolomA = -1 + j;
								int rijB;
								int kolomB;
 
								// onder is boven, boven is onder, rechts is links en links is rechts
								// ofwel raster is een bol
 
								// als rij + rijA = -1 dan nrrijen
								if (rij + rijA == -1) {
									rijB = nrrijen;
								// als rij + rijA = nrrijen +1 dan 0
								} else if (rij + rijA == nrrijen + 1) {
									rijB = 0;
								// anders rij + rijA
								} else {
									rijB = rij + rijA;
								}
								// als kolom + kolomA = -1 dan nrkolommen
								if (kolom + kolomA == -1) {
									kolomB = nrkolommen;
								// als kolom + kolomA = nrkolommen +1 dan 0
								} else if (kolom + kolomA == nrkolommen + 1) {
									kolomB = 0;
								// anders kolom + kolomA
								} else {
									kolomB = kolom + kolomA;
								}
 
								omgeving[i][j] = Wereld[rijB][kolomB];
							}
						}
						// eet, eet, eet
						vos.eet(omgeving,pvek);
					}
				}
			}
		}

As far as i know i've taken care of illegal indexes for the array, or am i wrong?

What is the size of the array being used on line 169? What is the value of the index used there?

Originally Posted by Norm

What is the size of the array being used on line 169? What is the value of the index used there?

omgeving[i][j] where i and j are 0, 1 or 2 (so size 3?)
it's empty till the cel's of Wereld[][] are linked to it.

Wereld[][] is has the size of nrrijen (number of rows) and nrkolommen (number of columns)

Print out the .length of the array to see its size.
You get an ArrayIndexOutOfBoundsException because your index is too big.
You need to know the size of an array to be able to use it.

it's empty

Where do you give it a value? for example with the new statement

Add a println just before the statement where the exception occurs and print out the values of all of the indexes being used. That will show you what the code is doing.

What statement gives a size to omgeving? Can you post it here?