ClassNotFoundException Vs NoClassDefFoundError in Java

Both of the exceptions that are ClassNotFoundException and NoClassDefFoundError occur when the class is not found at runtime. They are related to the Java classpath. 

ClassNotFoundException

ClassNotFoundException occurs when you try to load a class at runtime using Class.forName() or loadClass() methods and requested classes are not found in classpath. Most of the time this exception will occur when you try to run an application without updating the classpath with JAR files. This exception is a checked Exception derived from java.lang.Exception class and you need to provide explicit handling for it. This exception also occurs when you have two class loaders and if a ClassLoader tries to access a class that is loaded by another classloader in Java. You must be wondering that what actually is classloader in Java. Java ClassLoader is a part of the Java Runtime Environment that dynamically loads Java classes in JVM(Java Virtual Machine). The Java Runtime System does not need to know about files and files systems because of classloaders. 

Example

// Java Program to Illustrate ClassNotFoundException
public class GFG {

    // Main driver method
    public static void main(String args[])
    {

        // Try block to check for exceptions
        try {

            Class.forName("GeeksForGeeks");
        }

        // Catch block to handle exceptions
        catch (ClassNotFoundException ex) {

            // Displaying exceptions on console along with
            // line number using printStackTrace() method
            ex.printStackTrace();
        }
    }
}

Output:

ClassNotFoundException is raised in the above program as class "GeeksForGeeks" is not found in the classpath.

NoClassDefFoundError

Now dwelling on the next exception that is NoClassDefFoundError occurs when the class was present during compile time and the program was compiled and linked successfully but the class was not present during runtime. It is an error that is derived from LinkageError. Linkage error occurs when a class has some dependencies on another class and the latter class changes after compilation of the former class. NoClassFoundError is the result of implicit loading of class because of calling a method or accessing a variable from that class. This error is more difficult to debug and find the reason why this error occurred. So in this case you should always check the classes which are dependent on this class. In order to illustrate let us first make any two classes for a java program and link them. 

Example

// Java Program to Illustrate NoClassDefFoundError Exception

// Class 1
// Helper class
class GeeksForGeeks {

    // Method
    void greeting()
    {
        // Print statement whenever method is called
        System.out.println("hello!");
    }
}

// Class 2
// Main class
class GFG {

    // Main driver method
    public static void main(String args[])
    {
        // Creating object of class 1
        // inside main() in class2
        GeeksForGeeks geeks = new GeeksForGeeks();

        // Calling method of above class
        geeks.greeting();
    }
}

hello!

Output explanation: 

Above program will be successfully compiled and generated two classes 'GeeksForGeeks.class' and 'GFG.class.' Now remove GeeksForGeeks.class file and run GFG.class when we saw that at Java runtime NoClassDefFoundError will be thrown.

ClassNotFoundException Vs NoClassDefFoundError

• As the name suggests, ClassNotFoundException is an exception while NoClassDefFoundError is an error.
• ClassNotFoundException occurs when classpath does not get updated with required JAR files while error occurs when the required class definition is not present at runtime.