NoClassDefFoundError-very strange behaiour

Question

While learning exceptions i tried the following steps and found a very strange behavior. Please find the steps below.

Created a Public Class A
Create a class B
Compiled A and B
Deleted the class B before running A

After doing above steps program runs absolutely fine and throws NoClassDefFoundError ,Now strange thing is when i declare the reference variable inside the try block and run the program again it throws an uncaught NoClassDefFoundError

public class A{
    public static void main(String[] args) {
           B m ;//Caught is getting printed if declare above try-catch()
        try {
            B m = new B();//Here Uncaught exception is throws
        } catch (java.lang.NoClassDefFoundError ex) {
            System.out.println("Caught!");
        }
    }
}

class B{

}

I really don't have any idea why this is happening please help me understand this

http://stackoverflow.com/questions/34413/why-am-i-getting-a-noclassdeffounderror-in-java — parlad, May 09 '17 at 10:29
Just declaring a reference to `B` should not throw the exception — TheLostMind, May 09 '17 at 10:37
No it won't but declaring outside try and use it inside catch will. — T-Bag, May 09 '17 at 10:38
@ShowStopper - Yes, where you declare it doesn't really matter. You need to ensure that the JVM tries to load the class — TheLostMind, May 09 '17 at 10:40
@ShowStopper - Use the `Java -v` option to see if the class `B` is loaded or not :) . Also, note that there is a difference between class loading and class initialization — TheLostMind, May 09 '17 at 10:43

score 2 · Accepted Answer · answered May 09 '17 at 10:36

You are deleting the class file of A after B has compiled and generated bytecode. So, code of B still references A and when your A starts running, the JVM will try to load and initialize B when the object is being created and since it cannot find the class in the class path of A, it throws NoClassDefinitionFoundException.

In your fist case you just declare a reference to B, the JVM will NOT try to load your class unless it really needs it, so you wont get any exception.

NoClassDefFoundError-very strange behaiour

1 Answers1