[Java Beginner] Problem to "Could not find the main class"

We are beginners of Java programming. We study Java development with JDK 1.6.0_35 and its shipped tools. We have already added the directory of Java utilities "Java\jdk1.6.0_35\bin" to the PATH system environment variable of Windows. Then we began the study to Java.

We study Java with the book "First Head Java Second Edition". When we tried to compile and run a demo program in the book, we encountered a problem. The code is listed in the following:

Extracted from the book:

-------------------------------

public class Dog {
    String name;
    public static void main(String[] args) {
        Dog dog1 = new Dog();
        dog1.bark();
        dog1.name = "Bart";
        
        Dog[] myDogs = new Dog[3];
        myDogs[0] = new Dog();
        myDogs[1] = new Dog();
        myDogs[2] = dog1;
        
        myDogs[0].name = "Fred";
        myDogs[1].name = "Marge";
        
        System.out.print("last don't name is ");
        System.out.println(myDogs[2].name);
        
        int x = 0;
        while (x < myDogs.length) {
            myDogs[x].bark();
            x = x+1;
        }
    }
    public void bark() {
        System.out.println(name + " says Ruff!");
    }
    
    public void eat() { }
    
    public void chaseCat() { }
}

-------------------------------------

We typed the above code with Notepad, and then saved it as Dog.java. Then we compile the code with the javac in prompt:

F:\Programming\Java\Dog\javac Dog.java

The code is compiled successfully. But when we tried to run it, an exception raised:

The JVM told us that "Could not find the main class: dog." 

Oops, the code could pass through the compiler, so the code should be able to run.  What's the problem? Have no main class? We have already declared that!!! And the Dog.class file is existed in the program directory.

After checked the code again over again, about half an hour after, we finally found that the class name we want to run is wrong, it should be "Dog" not "dog". If we want to run the demo program, we should type in:

java Dog

Then, the miracle happened:

The Class name in Java is case-sensitive!!!!

We have searched for the above problem on the Internet, someone who told us that the problem is due to the JVM version is difference than the compiler version. Now we knew that a lilttle typos could also cause such errors.

By:

Ma Xiaoguang and Ma Xiaoming