在显示它之前，你必须操纵 fullName 变量。

一些事情：

 fullName = JOptionPane.showInputDialog（null， 你的全名是什么？）; 

 fullName = fullName.trim（）。replaceFirst（  \\\\ +，System.lineSeparator（））;  //  替换行尾字符遇到的第一个空格 

 JOptionPane.showMessageDialog（null， 您输入了： + System.lineSeparator（）+全名）; 

For Example this is my code:

String fullName; // Stores fullName as a String variable

fullName = JOptionPane.showInputDialog(null,"What is your full name?");
// Creates input dialog box containing "What is your full name?"

JOptionPane.showMessageDialog(null, "You entered: " +
fullName);

I want to display the first name in its own line the last name in another line.

What I have tried:

I've tried using "\n" inside the message box, but it just takes the whole full name to a new line.

You have to manipulate the fullName variable before displaying it.
Something along:

fullName = JOptionPane.showInputDialog(null,"What is your full name?");

fullName = fullName.trim().replaceFirst("\\s+", System.lineSeparator()); // Replace the first whitespace(s) encountered by an end-of-line character

JOptionPane.showMessageDialog(null, "You entered:" + System.lineSeparator() + fullName);