Question

This seems like a simple problem, but somehow I’m having issues with this code. I’m getting back into Java after a few years, and I’m making a 2D game. In it, I have a main driver class called SnakeGame that loads a new instance of the class GameBoard.

SnakeGame.java

package snake2;
import javax.swing.JFrame;

public class SnakeGame extends JFrame {

public SnakeGame() {
    add(new GameBoard());

    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setSize(320, 340);
    setLocationRelativeTo(null);
    setTitle("Snake Game");

    setResizable(false);
    setVisible(true);
}

public static void main(String[] args) {
    new SnakeGame();
}
}

In the same directory lies GameBoard.java, which has a constructer with no required parameters:

public GameBoard() {
    [... more code ...]
}

Edit

Both GameBoard.java and SnakeGame.java have package snake2; at the first line of their files.

However, I keep receiving the following error:

SnakeGame.java:7: cannot find symbol
symbol  : class GameBoard
location: class snake2.SnakeGame
    add(new GameBoard());
            ^
1 error

Edit #2

I’ve tried to add it to my class path, using java -cp . GameBoard after javac. Here’s the first line of the terminal’s scary looking and unnecessarily verbose response:

Exception in thread "main" java.lang.NoClassDefFoundError: GameBoard (wrong name: snake2/GameBoard)

This is as if I’ve misspelled the class, or misspelled a file name. Although, to my knowledge, I haven’t done either. Is there some other problem with my code that I haven’t noticed?

Thanks for any help in advance.

Answer 1

Besides being in the same directory, do you have a package statement in GameBoard.java?

---

Edit for running the program —

@Stephen: I’m running java, and all the files are in the same directory.

From @kbolino

You must put your source files in a snake2 directory and run javac from the parent directory […]

You must also invoke java, not just javac, with the correct package and classpath.

Examples that worked for me, after creating SnakeGame and GameBoard stubs:

1. Current dir is projects which is the parent of the snake2 dir where the files are:
   java snake2.SnakeGame

2. Current dir is snake2 where I was editing the files:
   java -cp .. snake2.SnakeGame

For case #2 since you’re in the package dir you have to put the parent dir in the classpath.

.

Adding a ridiculous amount of information showing my session with both the javac compile command and java run. You don’t need -cp for compiling as suggested by some.

---

[~/tests/java/snake2]$ pwd
/home/stephenp/tests/java/snake2

[~/tests/java/snake2]$ cat GameBoard.java

package snake2;

class GameBoard {

    private static int instanceCount = 0;

    GameBoard() {
        GameBoard.instanceCount++;
    }

    void howMany() {
        System.out.println(instanceCount + " GameBoards have been created.");
    }
}

[~/tests/java/snake2]$ cat SnakeGame.java

package snake2;

public class SnakeGame {

    private GameBoard board = null;

    public SnakeGame() {
        this.board = new GameBoard();
    }

    void report(String msg) {
        System.out.println(msg);
    }

    public static void main(String[] args) {
        SnakeGame game = new SnakeGame();
        game.report("I exist");
        game.board.howMany();
    }
}


[~/tests/java/snake2]$ ls -1
GameBoard.java
SnakeGame.java

[~/tests/java/snake2]$ javac *.java

[~/tests/java/snake2]$ ls -1
GameBoard.class
GameBoard.java
SnakeGame.class
SnakeGame.java

[~/tests/java/snake2]$ java SnakeGame
Exception in thread "main" java.lang.NoClassDefFoundError: SnakeGame (wrong name: snake2/SnakeGame)
        at java.lang.ClassLoader.defineClass1(Native Method)
        at java.lang.ClassLoader.defineClass(ClassLoader.java:620)
        at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:124)
        at java.net.URLClassLoader.defineClass(URLClassLoader.java:260)
        at java.net.URLClassLoader.access$000(URLClassLoader.java:56)
        at java.net.URLClassLoader$1.run(URLClassLoader.java:195)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
        at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:276)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:251)
        at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:319)

[~/tests/java/snake2]$ java snake2.SnakeGame
Exception in thread "main" java.lang.NoClassDefFoundError: snake2/SnakeGame
Caused by: java.lang.ClassNotFoundException: snake2.SnakeGame
        at java.net.URLClassLoader$1.run(URLClassLoader.java:200)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
        at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:276)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:251)
        at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:319)

[~/tests/java/snake2]$ java -cp .. snake2.SnakeGame
I exist
1 GameBoards have been created.

---

Packaging

Package up your application in a .jar file along with a Manifest that specifies the main class to run, then you can run your program just using

java -jar myprogram.jar

To do that, create a file Manifest.txt in the directory above your package directory

[~/tests/java/snake2]$ cd ..

[~/tests/java]$ ls -1F
Manifest.txt
snake2/

[~/tests/java]$ cat Manifest.txt
Main-Class: snake2.SnakeGame

[~/tests/java]$ jar cfm myprogram.jar Manifest.txt snake2/*.class

[~/tests/java]$ jar tf myprogram.jar
META-INF/
META-INF/MANIFEST.MF
snake2/GameBoard.class
snake2/SnakeGame.class

You can also wrap a script (shell script, batch file, etc.) around that so you just run myprogram and it runs java -jar myprogram.jar

This would all be part of your build process.