Unlike many popular programming languages, the popular Oracle Java package is only available using a PPA, not by the official repo.
Why isn't Oracle Java included in the standard Ubuntu repo?
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Ubuntu has OpenJDK in its repository. It's the fully open-sourced version of the Oracle JDK, which contains some closed-source parts.
It is a myth that OpenJDK is lower and less stable, but this was the past.
Today (since version 7), they're pretty equal. OpenJDK is even the reference implementation for Java 7 and Java 8. It's a race between those two implementations, as some new features appear first in OpenJDK and then later get included to OracleJDK, while some features get included the other way round.
The major differences are that OpenJDK has no Webstart included (but the open-source IcedTea does this job) and some other closed-source tools of Oracle missing, but nothing important.
So to sum up: As Ubuntu supports everything that is open-source, it supports OpenJDK, especially as it has no real disadvantages any more.