Java While Loops and Do While Loops

Java Iteration: While Loops and Do While Loops
Definition of Iteration

  • Iteration is defined as the repetition of code for a specified number of times.

  • Both while loops and for loops fall under this definition of iteration.

While Loop

  • A while loop is a type of loop that executes code inside it when a specified condition is true.

  • The condition for a while loop is a logical test, akin to the conditions used in if statements.

  • Execution Flow:

    • If the logical test evaluates to true, the code inside the while loop executes.

    • If the test evaluates to false, the loop skips the code inside and continues; the execution moves past the loop.

Key Characteristics

  • It keeps executing as long as the condition remains true, which distinguishes it from if statements.

  • After executing the inner code, the loop will re-evaluate the condition; if still true, it will keep executing.

Example of a While Loop

  • An example of using a while loop could involve:

    • Asking the user for the number of items they purchased at a restaurant.

    • Requesting a tip from the user, subsequently calculating the tip and the final bill.

    • Code Example:

import javax.swing.JOptionPane;

double subtotal = 0.0;
String input = JOptionPane.showInputDialog("Enter item cost (or Cancel to finish):"); // Primer input

while (input != null && !input.isEmpty()) {
    try {
        double itemCost = Double.parseDouble(input);
        subtotal += itemCost;
        input = JOptionPane.showInputDialog("Enter item cost (or Cancel to finish):"); // Update input
    } catch (NumberFormatException e) {
        JOptionPane.showMessageDialog(null, "Invalid input. Please enter a number.");
        input = JOptionPane.showInputDialog("Enter item cost (or Cancel to finish):");
    }
}
JOptionPane.showMessageDialog(null, "Total subtotal: $" + String.format("%.2f", subtotal));

Primer Requirement

  • Some while loops require a primer: an initial input that allows the loop to start.

  • In the restaurant calculation example, the primer ensures that user input is received to prevent the loop from failing to execute.

Infinite Loops

  • Infinite loops can occur when the condition remains perpetually true, preventing subsequent code from executing.

  • Example:

    • If an integer variable x is initialized with a value of 5, and a condition checks if it is greater than 2, since 5 is always greater than 2, the loop will execute indefinitely.

  • Prevention: To avoid infinite loops, ensure the test condition is updated during each iteration. For example, decreasing x after each pass until it eventually becomes false.

Do While Loop

  • A do while loop is an alternative to the while loop where the code is executed first before the condition is tested.

  • Unlike the while loop, a do while loop does not require a primer, as execution starts prior to the test.

Structure of Do While Loop

  • Syntax:

    java do { // code to execute } while (condition);

  • The execution occurs first in the do section, and only after the code has run does it evaluate the condition stated in the while clause.

Example of Do While Loop

  • In the restaurant calculation example using a do while loop:

    • The loop prompts for the cost of a purchase and directly executes that code.

    • Immediately checks if the input is valid before performing calculations, thus avoiding the need for a primer.

    • Code Example:

import javax.swing.JOptionPane;

double subtotal = 0.0;
String input;

do {
    input = JOptionPane.showInputDialog("Enter item cost (or Cancel to finish):");
    if (input != null && !input.isEmpty()) {
        try {
            double itemCost = Double.parseDouble(input);
            subtotal += itemCost;
        } catch (NumberFormatException e) {
            JOptionPane.showMessageDialog(null, "Invalid input. Please enter a number.");
        }
    }
} while (input != null && !input.isEmpty()); // Condition checked after execution

JOptionPane.showMessageDialog(null, "Total subtotal: $" + String.format("%.2f", subtotal));

Differences Between While and Do While Loops

  • While Loop:

    • Requires a primer under certain conditions to begin execution.

    • Tests the condition before running the code block.

  • Do While Loop:

    • Executes the code before it checks the condition.

    • Does not require an initial input or primer, but still performs a necessary validation.

Importance of Validation

  • In scenarios where user input is deemed nullable, adding validation checks helps to avoid exceptions such as NullPointerException.

  • Without appropriate checks, if the input provided by users is null, code execution risks leading to runtime errors.

Practical Demonstrations

While Loop Demonstration

  • Initialization: Import necessary classes (e.g., java.text.DecimalFormat for formatting currency, javax.swing.JOptionPane for GUI inputs).

    • $decimalFormat dollar = new DecimalFormat("0.00")$

  • Structure:

    • Set up variables such as double subtotal = 0.0 for maintaining the running total of purchases, tips, taxes.

    • The while loop prompts for input, validating whether the input is null, and adds valid purchases to the subtotal until canceled.

Do While Loop Demonstration

  • Similar setup but executes the input request before checking if the value is valid, which simplifies user interaction and streamlines the portion of the code dedicated to acquiring inputs.

Summary

  • This tutorial comprehensively addresses how while loops and do while loops function within Java programming.

  • Focus is placed on understanding iteration, conditions, primers, and the practical application of loops through a restaurant billing example.

  • The next video will introduce the for loop, which possesses a more structured nature compared to the while loop.