Final answer:
To store the sentence entered by the user using JOptionPane.showInputDialog(), use the code: String sentence = JOptionPane.showInputDialog(null, "Please enter a sentence:").
Step-by-step explanation:
The sentence generated by JOptionPane.showInputDialog() can be stored in a variable using the following code:
String sentence = JOptionPane.showInputDialog(null, "Please enter a sentence:");
This code will prompt the user to enter a sentence in a dialog box, and the sentence entered will be stored in the variable "sentence".