[CALCITE-6552] Enable CheckerFramework in 'server' module

[julianhyde]

In CALCITE-4199 we enabled CheckerFramework (via annotations and CI jobs) in the 'core' and 'linq4j' modules; this change further enables CheckerFramework in the 'server' module, and fixes all violations.

There are also a large number of 'cosmetic' modifications to improve code quality without changing behavior, including:

• Replace this.x = x; assert x != null; with this.x = requireNonNull(x); in constructors
• Replace assert in other code locations where it implements an invariant. We don't use requireNonNull because it throws NullPointerException; we would prefer to throw AssertionError or IllegalStateException
• Replace x.equals("") and x.size() == 0 with x.isEmpty()
• Make fields final where possible
• Make private methods and inner classes static where possible
• In class Pair<K, V> make the type variables K and V no longer nullable by default (you can make each of them nullable if you need)
• Always import Objects.requireNonNull, Integer.parseInt, Float.parseFloat, Double.parseDouble, Byte.parseByte, Short.parseShort, Boolean.parseBoolean, Long.parseLong via static import

This PR supersedes #3937

[mihaibudiu]

I imagine many of these conversions are automatically performed.
Would you like some of this code reviewed?
This may cause lots of conflicts if people have work under way.

[NobiGo]

LGTM.

[julianhyde]

@mihaibudiu

Would you like some of this code reviewed?

I see @NobiGo has reviewed and approved. I think we're good.

The vast majority of changes don't require review - there was only one choice to make. In one or two places I had to make a decision; e.g. in MongoEnumerator I added the 3 if ... throw lines, because the alternative, to change the return to @Nullable Object, was inferior:

  @Override public Object current() {
    if (current == null) {
      throw new IllegalStateException();
    }
    return current;
  }

This may cause lots of conflicts if people have work under way.

Agreed. I will minimize conflicts by merging this quickly, and avoiding changes that require discretion. I will probably merge today.

@NobiGo, Thanks for your review.

[sonarqubecloud]

Quality Gate passed

Issues
69 New issues
0 Accepted issues

Measures
0 Security Hotspots
72.1% Coverage on New Code
1.3% Duplication on New Code

See analysis details on SonarCloud