I've never specifically installed Java on OS/2, it always somehow got installed automagically when installing Netscape...
Now, when I want to install TCPAPPS, I'm told I can only do this through Java.
I don't know why Netscape hasn't installed Java. Can anyone tell me how to do it manually?
Java is not installed as part of Netscape. Perhaps you're thinking of the fact that the full SDK version of Java 1.1 and/or 1.3 used Netscape for its guided install.
Since WSeB, Java 1.1.8 is installed along with the OS as part of the FIBASE component, assuming you select the right options. That's the version that TCPAPPS works with best.
You can install later by running CLIFI /A:T to create an Install object, then load the install data using "response file -> read" from its context menu and selecting FIBASE.RSP from \OS2\INSTALL. Then double click the object, deselect everything except the Java items, and choose Install. It will go to \JAVA11 on the boot drive (it's possible to change that, but it requires rather tricky advanced techniques).