Wildcard in Java classpath

One of the less known new features of JDK 6 is the ability to use wildcards in classpaths.
That means that instead of doing…
java -cp lib\x.jar;lib\y.jar;lib\z.jar; com.company.MyProggy
…you could just do…
java -cp lib\*.jar; com.company.MyProggy
That generally turns out to be quite a time saver. Especially so if you are trying out something at a remote box and vi is your only editor.
However, if you use the wild card, there is no way to predict the order in which the jars will be used. In some cases, the order is important – and in those cases, you will need to add in the jars like one normally does by enumerating each one explicitly.
Also, don’t forget the semi colon at the end of the classpath. Sometimes lots of time is lost due to this. It can be really frustrating!

java.lang.NoClassDefFoundError in java

This is normally a CLASSPATH problem. Try resetting the CLASSPATH, and/or make sure all third party JAR's are actually present in the correct locations. In some rare cases you will get this problem when the JVM has problems loading a third party jar due to version incompatibilities.

To set classpath in java, just see how to reset it. Following links may be useful to read:

• Path and classpath in java
• Determining current value of classpath
• Setting classpath in windows

Wildcards in Java Generics

What is covariance, contravariance and invariance?
As we've seen in the previous post, the subtyping relation of generic types is invariant. Sometimes, though, we'd like to use generic types in the same way we can use ordinary types:

• Narrowing a reference (covariance).
• Widening a reference (contravariance) 

Wildcard in Generics

Lower - bounded wildcards in Generics
Upper bounded wildcards in Generics
Using multiple bounds in Generics
The get and put principle of bounded wildcard
Restriction of Wildcards in generics