Java While Loops: Structure and Validation

Looping Structures in Java

• Java offers three different looping structures for repetitive execution of code blocks.

The while Loop

• The syntax of a while loop is while (condition) { statements; }.
• Statements inside a while loop execute repeatedly as long as the specified condition remains true.
• The while loop is a pretest loop, meaning it checks the condition before executing the loop's statements.
• Care must be taken to ensure the condition becomes false at some point within the loop body, otherwise the loop becomes infinite
• A while loop can execute zero or more times, depending on initial condition.
• Loops that do not end because their condition never evaluates to false are called infinite loops.

Block Statements in Loops

• Curly braces ({}) are required to enclose block statements within while loops, similar to block if statements.

Input Validation with while Loop

• Input validation involves ensuring user-provided input is valid
• A while loop can validate the user input

The do-while Loop

• The do-while loop is a post-test loop, which means it executes the loop body before evaluating the condition.
• do-while loop syntax: do { statement(s); } while (condition);

The for Loop

• The for loop is a pre-test loop
• It allows initializing a control variable, testing a condition, and modifying the control variable in a single line of code
• The for loop has the following structure: for (initialization; test; update) { statement(s); }

Sections of the for Loop

• The initialization section lets the loop initialize its own control variable, and is executed once before the loop starts.
• The test section acts like the condition section of a while loop, determining when the loop should terminate.
• The update section is executed at the end of each loop iteration.
• The initialization section in a for loop is optional but typically provided.
• for loops often initialize a counter variable for use in the test section and update sections.
• Multiple variables can be initialized in the initialization section.
• Variables declared in the initialization section only have scope for the for loop.
• The update expression increments or decrements counter variables declared in the initialization section.
• The update section can modify multiple variables, with each variable's update treated as if on its own line.
• Avoid modifying the control variable of a for loop within the loop's body.
• The test section defaults to true if left out.
• Rely on the update section to modify the control variable, as modifying it in the loop body makes the code harder to maintain and debug.
• The only mandatory parts of a for loop are the semicolons.
• A for loop may initialize and update multiple variables.

Running Totals

• Running totals can be kept by programs using loops while evaluating data.

Sentinel Values

• Sentinel values can used to signal the end of input when the endpoint is unknown
• A user can enter some data that's not in the normal input range such as -1 when all normal data is positive
• End-of-file markers are often used by programs getting input files to signify that there is no more data

Nested Loops

• Loops can be nested within one another.
• With nested loops, the inner loop executes all its iterations for each single iteration of the outer loop.
• If the inner loop executes all its iterations 10 times, then the outer loop is executed 10 times, thus the loop statements will execute 100 times.

The break Statement

• The break statement abnormally terminates a loop
• Using break is considered bad practice as it bypasses normal mechanisms and makes the code hard to read and maintain.

The continue Statement

• The continue statement terminates the current iteration of a loop and begins the next iteration.
• continue causes the condition in while and for loops to be evaluated.
• Like break, the continue statement should be avoided because it makes the code harder to read and debug.

Deciding Which Loop to Use

• The while loop is a pretest loop and is appropriate when you might not want the statements to execute at all
• The do-while loop is a post-test loop, so it is appropriate to use when you want the statements to execute at least once.
• The for loop is a pretest loop which is appropriate where there is some type of counting variable that can be evaluated.

Generating Random Numbers with the Random Class

• Some applications, such as games and simulations, need randomly generated numbers.
• The Java API provides the Random class for generating random numbers, and can be imported: import java.util.Random;
• To use the Random class, create an instance: Random randomNumbers = new Random();

Methods of the Random Class

• nextDouble(): Returns the next random number as a double between 0.0 and 1.0.
• nextFloat(): Returns the next random number as a float between 0.0 and 1.0.
• nextInt(): Returns the next random number as an int which is between -2,147,483,648 and 2,147,483,648
• nextInt(int n): Returns a random number between 0 and n, where "n" is the integer argument.