NoClassDefFoundError

Question

I have an issue where NoClasDefFoundError is being thrown. It puzzles me since I am using interfaces, and no class definition should be available. I have read through some posts which point to Classpath, but I don't believe that to be the issue here (although I may be wrong). I am using NetBeans 6.9.1 IDE.

I have created a sample setup to reproduce the issue. Four projects: Interfaces, Objects, Locator and Consumer. Below you will find the implementations.

At runtime consumer coplains about missing SomeObject implementation, which it should not be aware of since it is accepting interface.

Exception in thread "main" java.lang.NoClassDefFoundError: objects/SomeObject

What am I missing?

package interfaces;
public interface ISomeInterface { }

package objects;
import interfaces.ISomeInterface;
public class SomeObject implements ISomeInterface{ }

package locator;
import interfaces.ISomeInterface;
import objects.SomeObject;
public class Locator { public static ISomeInterface LocateImplementation() { return new SomeObject(); }}

package consumer;
import interfaces.ISomeInterface;
import locator.Locator;
public class Main { public static void main(String[] args) { ISomeInterface object = Locator.LocateImplementation(); }}

Answer 1

You can get a NoClassDefFoundError exception with interfaces just as you can with classes. Consider the "Class" in the name of the exception to be the .class file that is generated from compiling a class or interface, not a Java class.

This is saying that the class/interface objects.SomeObject isn't visible on your classpath. Check the location of that .class file and ensure that it's on your classpath - if you're positive it's there, give us some screen shots or something that might help to debug the problem.

Answer 2

Think of NoClassDefFoundError as a runtime linkage problem. JRE loaded one class (or an interface) and it references another class (or an interface), but that referenced class isn't found.

The only way this can happen if you have packaging/classpath issues such that your runtime environment doesn't reflect how things are at build time.

If you are launching this from IDE, make sure that you aren't ignoring any errors and launching anyway. Some classes will not be generated that way.

Answer 3

Usually I run into these problems not when a class is missing, but when there is an error in the static initializers.

Try running your code in a debugger, and set the exception breakpoint to break when any exception is thrown, whether caught or not. I bet you have an uncaught exception in the static initializer for some reason.

Answer 4

Any Interface must be declared inside class

public class Calbacks {
    public interface IBaseFragmentInterface {
        void NotifyMainActivity();
    }
}

Answer 5

In the locateImplementation() method you are returning "new SomeObject()", JVM needs to have its definition when called. I think it is missing.

Answer 6

You should check if your SomeObject class is in class path because -

Well the JVM will be running the below code -

ISomeInterface object = Locator.LocateImplementation();

and when it does that it will call Locator.LocateImplementation(). This code internally tries to instantiate your SomeObject class which it does not find in the classpath.

So your below understanding

It puzzles me since I am using interfaces, and no class definition should be available.

Is not really valid.