Consider the fact Ubuntu 18.04's default repos come with a pretty ancient version of Java. And if you want a newer version, you have to use some third-party source. And the fact there are both OpenJDK and Oracle JDKs to choose from. Will that be JDK or JRE? Headless? Depending on the combination of JDK and software, maybe the fonts will go all funny. Got multiple JDKs? Some programs will use your default Java, some will invite you to select the JDK to use, some will bundle their own complete JDK. Oh, a program uses JavaFX? No, of course that isn't installed just because you installed Java.
These problems aren't inherent to Java, of course - it's 98% due to Oracle's attempts to inconvenience people into paying for licenses.