I see Java 2 and Java 5.0, but was there a Java 3 and 4? And why is it Java 5.0 but not Java 2.0?

The joys of marketing...when the version of Java shifted from 1.1 to 1.2, the changes to Java were so dramatic that the marketers decided we needed a whole new "name", so they started calling it Java 2, even though the actual version of Java was 1.2. But versions 1.3 and 1.4 were still considered Java2. There never was a Java 3 or 4. Beginning with Java version 1.5, the marketers decided once again that the changes were so dramatic that a new name was needed (and most developers agreed), so they looked at the options. The next number In the name sequence would be 3: but calling Java 1.5 Java 3 seemed more confusing, so they decided to name It Java 5.0 to match the "5" in version "l.5" So, the original Java was versions 1.02 (the first official release) through 1.1 were just "Java" Versions 1,2,1.3, and 1.4 were "Java 2" And beginning with version 1.5,Java is called "Java 5.0" But you'll also see it called "Java 5" (without the ".0") and "Tiger" (its original code-name). We have no idea what will happen with the next release.. :-)

Source: HFJ