The do-while loop in Java

General Structure

The do-while loop executes the statements at least once before checking the condition.

The general form is:

do {
    instruções;
} while(condição);

Difference between for and while loops

• In for and while, the condition is checked before executing the loop.
• In do-while, the condition is checked after the loop is executed, ensuring that it executes at least once.

Simple Example

A program that loops until the user enters the letter 'q':

// Demonstra o laço do-while.
class DWDemo {
    public static void main(String args[]) throws java.io.IOException {
        char ch;
        do {
            System.out.print("Press a key followed by ENTER: ");
            ch = (char) System.in.read(); // obtém um char
        } while(ch != 'q');
    }
}

Improved Guessing Game

A guessing program that loops until the user guesses the correct letter:
see Guess4.java

Explanation of the Divination Program

• The program reads characters from the user until the correct letter is guessed.
• There are two do-while loops:
• The first loop continues until the user guesses the correct letter.
• The second loop discards extra characters from the input buffer until it finds an n (newline).

Execution Example

Typical execution of the guessing program

I'm thinking of a letter between A and Z.
Can you guess it: A
...Sorry, you're too low
Try again!
I'm thinking of a letter between A and Z.
Can you guess it: Z
...Sorry, you're too high
Try again!
I'm thinking of a letter between A and Z.
Can you guess it: K
** Right **

Important Detail

• Console input is stored in a line buffer, so pressing ENTER adds a newline sequence that needs to be discarded.
• The second do-while loop removes extra characters from the buffer to avoid unwanted readings.

Conclusion

• The do-while loop is useful when you need to ensure that the block of statements is executed at least once.
• It is especially practical for user interactions where initial action is required before checking the condition.

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