Fix high severity issues reported by SonarCloud

[iluwatar]

SonarCloud is used for static code analysis. It currently reports 48 high severity issues.

See https://sonarcloud.io/project/issues?impactSeverities=HIGH&resolved=false&id=iluwatar_java-design-patterns

It is possible to run the same analysis on local machine using SonarLint plugin. See instructions at https://github.com/iluwatar/java-design-patterns/wiki/12.-IDE-instructions

Acceptance criteria

• High severity issues reported by SonarCloud have been fixed

[k1w1dev]

I can start having a go at these, can you assign this to me ?

[iluwatar]

Most of the high severity issues fixed by @k1w1dev, but still 18 remain. See https://sonarcloud.io/project/issues?impactSeverities=HIGH&resolved=false&id=iluwatar_java-design-patterns