[JAVA] Null, empty string, blank

It seems that there was a sentence like this in a novel somewhere ... (laughs)

Well, I'm learning about Spring validation (input check), but I'm curious about it, so I'll leave it as a memorandum.

Reference: Explaining the difference between "NULL" and "Blank" The difference from "Space" and "Empty string"

• Null: Refers to a state that does not exist.
• Empty string: A character string with a length of 0.
• Blank: Half-width space "_" and full-width space "_" (don't make a mistake because the underscore is just easy to see).

  If you don't understand this, it may cause problems, so be careful. Especially when dealing with DB, it seems to be an error factor.

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