RFR: 8298567: Make field in RandomAccessFile final [v3]

[Per Minborg]

> This PR proposes making a field in `RandomAccessFile` final. Also, it modernises a switch statement.

Per Minborg has updated the pull request incrementally with one additional commit since the last revision:

  Remove redundant check

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Changes:
  - all: https://git.openjdk.org/jdk/pull/11632/files
  - new: https://git.openjdk.org/jdk/pull/11632/files/20c9d49a..99c147a3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11632&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11632&range=01-02

  Stats: 4 lines in 1 file changed: 0 ins; 3 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/11632.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11632/head:pull/11632

PR: https://git.openjdk.org/jdk/pull/11632