After getting used to the spoon-feeding IDE’s, writing some one-off classes might lead to this Exception.

Exception in thread “main” java.lang.NoClassDefFoundError: HelloWorld
Caused by: java.lang.ClassNotFoundException: HelloWorld
at java.net.URLClassLoader$1.run(URLClassLoader.java:200)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
at java.lang.ClassLoader.loadClass(ClassLoader.java:307)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
at java.lang.ClassLoader.loadClass(ClassLoader.java:252)
at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:320)
Could not find the main class: HelloWorld. Program will exit.

Very common problem. Your classpath needs a .: followed by the classpath, to include the class(es) in your current directory.

java -cp .:./lib/mysql.jar HelloWorld

Here’s a method that shows how to get all the static fields in a Java class and return them.

	public static List printFields(Class c) throws ClassNotFoundException, IllegalAccessException {
    		List fields = new ArrayList();
        java.lang.reflect.Field[] f = c.getDeclaredFields();
        for (java.lang.reflect.Field field : f) {
            int modifier = field.getModifiers();
            if (java.lang.reflect.Modifier.isStatic(modifier)){
          		String val = (String) field.get( null );
          		System.out.print(field.getName() +  ": " + val.toString() + ", ");
          		fields.add(val);
            }
        }
        return fields;
    }

I got an UnsatisfiedLinkError and wanted to find out whether our JDK upgrade had caused it and did some searching and found that javap -verbose lets you find out the version of a compiled class. So get the class (could be in a JAR file) and then follow this example where I find the version of MyCallback.class

C:>c:\jdk1.6.0_07\bin\javap -verbose MyCallback
Compiled from “MyCallback.java”
public class com.shivdev.MyCallback extends java.lang.Object implements com.shivdev.IFilter
SourceFile: “MyCallback.java”
minor version: 0
major version: 49

Then compare with the following:

major  minor  JDK
45       3    1.0
45       3    1.1
46       0    1.2
47       0    1.3
48       0    1.4
49       0    1.5
50       0    1.6

Alternatively, you can use this java class to find out the version:

public class FindVersion {

    private static int convert(byte upper, byte lower) {
        int ret =  upper << 8;
        ret |= lower;
        return ret;
    }

    public static void main(String[] args) throws Exception {
    		final String CLASS = "MyCallback.class";
        ClassLoader loader = FindVersion.class.getClassLoader();
        java.io.InputStream in = loader.getResourceAsStream(CLASS);
        try {
            byte[] buffer = new byte[8];
            in.read(buffer);
            int minor = convert(buffer[4], buffer[5]);
            int major = convert(buffer[6], buffer[7]);
            System.out.println(major + "." + minor);
        } finally {
            in.close();
        }
    }
}

I was trying to increase eclipse heap size and setting it to 1024MB and that wouldn’t work. Reducing it to 768MB did the trick for me.

C:\eclipse\eclipse.exe -vmargs -Xmx768M

Later, after installing Your Kit Profiler and trying to add a plug in for eclipse, I started seeing the same error. So I opened up the eclipse.ini and removed the –launcher.XXMaxPermSize 256m option.

findJAR.com is an excellent JAR search engine that helps to find JAR files containing required Java classes. Just enter the name of a class or the JAR file you’re looking and Search. findJAR.com can be used to easily resolve NoClassDefFoundError and ClassNotFoundException problems. Just awesome.

Here’s and excellent Hello World ANT Tutorial with which you could get started with a decent looking build.xml