Refactor monitor patterns example code

[bakazhou]

Refactor

• Refactoring the code in monitor pattern that is not reasonable enough

Pull request description

• Modify amount random value origin and bound
• Add getBalance method in Bank class
• Add test for new getBalance method
• Some constants are extracted

It is my first pull request
Have a nice day😁

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[sonarcloud]

SonarCloud Quality Gate failed.   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

72.7% Coverage
0.0% Duplication

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[iluwatar]

Looks good! Thank you for the contribution 🎉

@all-contributors please add @bakazhou for code

[allcontributors]

@iluwatar

I couldn't determine any contributions to add, did you specify any contributions?
Please make sure to use valid contribution names.

I've put up a pull request to add @bakazhou! 🎉