Diff b/w ClassNotFoundException and NoClassDefFoundError

When i was googling about this I found that information in some of blogs were misleading. Hence an attempt to clear this confusion.

ClassNotFoundException

Java Specification for ClassNotFoundException says below:

Thrown when an application tries to load in a class through its string name using:

• The forName() method in class Class.
• The findSystemClass method() in class ClassLoader.
• The loadClass() method in class ClassLoader.

but no definition for the class with the specified name could be found.

So a ClassNotFoundException is thrown if an explicit attempt to load a class fails. ClassNotFoundException is thrown because the test attempts the load using an explicit call to loadClass().


NoClassDefFoundError

Java Specification for NoClassDefFoundError says:

Thrown if the Java virtual machine or a ClassLoader instance tries to load in the definition of a class (as part of a normal method call or as part of creating a new instance using the new expression) and no definition of the class could be found.

The searched-for class definition existed when the currently executing class was compiled, but the definition can no longer be found.

Essentially, this means that a NoClassDefFoundError is thrown as a result of a unsuccessful implicit class load.

public class MyNoClassDefFoundTest {
public static void main(String[] args) {
X x = new X();
}
}

public class X extends Y {
public void myShow(){
System.out.println("In X");
}
}

public class Y {
public void myShow(){
System.out.println("In Y");
}

Once you have compiled the code, remove the classfile of Y and execute the code. We got to see that it throws NoClassDefFound Error.

Please note that the same error would still occur if X referenced Y in any other way -- as a method parameter, for example, or as an instance field