Unable to Compile Application

I'm new to these forums so I just say hi before I make my post. :(hi):

I have just installed Java and Notepad++ and here is my application and program codes respectively:

Code:

---

public class MyApplication {

        public static void main(String[] args) {
       
                MyFirstProgram p = new MyFirstProgram();
                p.start();
               
        }
       
}

---

and

Code:

---

public class MyFirstProgram {

        public static void main(String[] args) {
       
                System.out.println("Hello world!");
               
        }
       
}

---

These two files are saved in "C:\MyFirstProgram". First I tried to compile and run the program using command prompt. The program comiled but the application did not:

Code:

---

C:\MyFirstProgram>javac MyFirstProgram.java

C:\MyFirstProgram>javac MyApplication.java
MyApplication.java:6: error: cannot find symbo
                p.start();
                ^
  symbol:  method start()
  location: variable p of type MyFirstProgram
1 error

---

I got my codes straight from my textbook so I don't understand what's wrong here. I thought maybe the CLASSPATH is incorrect, so I tried the following which also didn't work:

Code:

---

C:\MyFirstProgram>javac -cp . MyFirstProgram.java
C:\MyFirstProgram>javac -cp . MyApplication.java
MyApplication.java:6: error: cannot find symbol
                p.start();
                ^
  symbol:  method start()
  location: variable p of type MyFirstProgram
1 error

---

So I left command prompt and tried to compile and run the program using Notepad++ but I got the following error:

Code:

---

NPP_EXEC: "Java - Run"
CD: C:\MyFirstProgram
Current directory: C:\MyFirstProgram
java MyApplication
Process started >>>
java.lang.NoClassDefFoundError: MyApplication
Caused by: java.lang.ClassNotFoundException: MyApplication
        at java.net.URLClassLoader$1.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
Could not find the main class: MyApplication.  Program will exit.
Exception in thread "main" <<< Process finished.
================ READY ================

---

So, what's causing this problem? Any help and guidance is greatly appreciated.

Hi Roam, welcome to the forums.

Code:

---

C:\MyFirstProgram>javac MyApplication.java
MyApplication.java:6: error: cannot find symbo
                p.start();
                ^
  symbol:  method start()
  location: variable p of type MyFirstProgram

---

The error is saying that p is of type MyFirstProgram, because that's how you declare it ("MyFirstProgram p = ..."). But the MyFirstProgram class does not define a start() method. You could create a start method and see what happens:

Code:

---

public class MyFirstProgram {

    public static void main(String[] args) {
       
        System.out.println("Hello world!");
               
    }

    [color=green]public void start() {
        System.out.println("Hello from the start() method!");
    }[/color]
}

---

-----

Code:

---

C:\MyFirstProgram>javac -cp . MyFirstProgram.java
C:\MyFirstProgram>javac -cp . MyApplication.java

---

The value of the CLASSPATH variable is not the problem (if, indeed, it has a value). But what you have there is a good way of compiling the two programs. I say two programs because they both have static void main() methods and they will both run. After compiling both you can see this with:

Code:

---

C:\MyFirstProgram>java -cp . MyFirstProgram
C:\MyFirstProgram>java -cp . MyApplication

---

-----

The Notepad++ error ("java.lang.NoClassDefFoundError: MyApplication") is happening because there is no MyApplication class yet. That's a result of the compile error and will go away once you have MyApplication compiled correctly.

I might add that I have no clue what your book was getting at. *Possibly* they meant you to change the MyFirstProgram class so that "public static void main(..." became "public void start(...". Ie

Code:

---

public class MyFirstProgram {

    [color=green]public static void start()[/color] {
       
        System.out.println("Hello world!");
               
    }
}

---

The idea would then be that MyApplication's main() method creates an instance of MyFirstProgram and then calls that instance's start() method to actually run the program.

btw, why it always start with "static" doesn't mean: "contain dynamic value along the program lifetime"? but the start() doesn't it just print line-ing...

Sorry, in my post #3 the code I suggested you try and think about should not have had the static in it.

Especially when you start the only thing that should be static is the main() method. "static" means something like "shared by all instances" and is rarely used because most things (methods and variables) are specific to a particular instance. As an example consider the MyApplication class (which is compiling now?):

Code:

---

public class MyApplication {
    public static void main(String[] args) {        // 3
        MyFirstProgram p = new MyFirstProgram();    // 1
        p.start();                                  // 2
    }
}

---

(1) When you use "new" you are creating a new instance (of MyFirstProgram)
(2) The start() method is called with respect to that new instance. Ie start() is not static.
(3) main() is static because it is not associated with any particular instance of the class.