: Do we have to introduce the character's name before using their names in a dialogue tag? Do we have to introduce the character's name before using their names in a dialogue tag? I am wondering

As others have said, no, you don't have introduce anything before a dialogue. Infact, you can have dialogues with totally unknown characters:

A low, menacing voice whispered behind his ear.

"I'd put that weapon down, kid."

or

She heard someone screaming in the distance.

"Fire!"

In both cases the character speaking is unknown to the PoV, is not being introduced, and could either be male or female.

Regarding the John example, I'd say it's lacking a bit of context. In a scene of a novel, if two characters are going to meet, I don't expect any of them to pop out of thin air and start talking.

The man was tall and wiry. The hair on the top of his head were receding, but he looked handsome. His squared jaw gave him a look of quiet confidence. As always, Matthew's attention was drawn in to the little details - the clean, recently ironed shirt, and the polished look of his belt. The stranger knew how to dress.

"My name's John," he said, ad he put his right hand forward.

"Happy to meet you" Matthew replied, shaking it.

Of course this is not always true. You can start a dialogue in medias res, with both character already in the middle of the action.

Chapter 24

"My name's John" the new intern said.

"Happy to meet you" Matthew answered.

Most names are usually gender specific, so most of the times you don't need to specify if the talker is male or female. If you haven't described a character before, the audience will take the underlying information to assume the gender.

Chapter 24

"My name's John"

Since John is typically used for males, you don't really need to say anything else. Unless, of course, the PoV tells us otherwise:

Chapter 24

"My name's John"

Matthew looked at the new intern, trying to wash away the dumbfounded expression on his face. He was probably failing at that.

"Yeah, I know." The girl chuckled at him. "You're not the first who gives me that look".

"I didn't mean to" He walked up to her and they shook hands. "It's just unusual."

The same can happen if you're using codenames, surnames, or names in a foreign language. But I'd call those exceptions, not the general case.

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No. Trust your reader's intuition for the obvious. If the first lines of your book read:

"Put that back!" Alicia scolded.
"No! Mine!" Richard said, defiant.
She grabbed the plastic bottle of cough syrup from him, and put it back on the grocery shelf. He started crying, and reached for it again, Alicia moved the cart to the center of the aisle so he couldn't.
"Mine!"
"No it isn't."

Reader's aren't stupid, and writing is not a mathematical or scientific proof. They will infer Alicia is an exasperated mother, Richard is a very young child, they are in a grocery store, and you don't have to tell them any of that.
You want to be clear, but you definitely can identify a new speaker without any description and just "blah blah blah," Charname said.
You can even give clues to their appearance by action, instead of telling.

"Let me get that for you," Michael said, easily reaching the top shelf, seeing the young woman was about to climb the shelving to reach it.
"Oh, thank you," Britney said. "Would you mind grabbing another one?"

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The only issue to worry about is that your reader knows who is speaking and can remember who the characters are scene to scene.

How you accomplish this is up to you.

That being said, it's a little weird to have "the man" and "John" so close together. So I'd shake it up slightly. An easy way would be to leave off the "said John" tag (that's easy because it's obvious to the reader he's the one talking there).

"My name is John!" the man said.

"Happy to meet you!" said Matthew.

"Likewise!"

You can also play around with the "the man" part.

Matthew walked up to a tall young man with a bright purple tie who
introduced himself as John.

"Happy to meet you!" Matthew said.

"Likewise!"

If you want to obscure the person's name and gender, you can do that too.

Matthew walked up to one of the new interns who was balancing a plate in one
hand and a cup of something steaming in the other.

"Happy to meet you!" Matthew said.

"Likewise! I'm John by the way."

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