Java Loop Control Keywords

Overview

This lecture covers the break and continue keywords in Java, focusing on how they control the flow of loops. The notes include clear explanations, step-by-step code examples, and important points about variable scope. These notes are designed for a college-level course and are organized for easy reference, with ADHD-friendly formatting and thorough explanations so you can learn everything without needing to read the original material.

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The break Keyword

• The break keyword is used inside loops to exit the loop immediately, no matter what the loop’s condition is.
• It is especially useful for stopping infinite loops or exiting early when a specific condition is met.
• When break is executed, the program jumps to the first statement after the loop.

How break works:

• You can use break in any loop (for, while, or do-while).
• Once break is reached, the rest of the loop body is skipped, and the program continues after the loop.

Example 1: Exiting a for loop early

for (int i = 1; i <= 10; i++) {
    if (i == 5) {
        break; // Exit the loop when i is 5
    }
    System.out.print(i + " ");
}
// Output: 1 2 3 4

• The loop prints numbers 1 to 4. When i becomes 5, break stops the loop.

Example 2: Exiting an infinite while loop

while (true) {
    int n = scanner.nextInt();
    if (n == 0) {
        break; // Exit the loop if user enters 0
    }
    // Process n
}

• Even though the loop is infinite (while (true)), break allows you to exit when a certain condition is met.

Example 3: Using break after valid input

Scanner scanner = new Scanner(System.in);
int n;
while (true) {
    System.out.print("Enter a number between 1 and 10: ");
    n = scanner.nextInt();
    if (n >= 1 && n <= 10) {
        break; // Exit loop if input is valid
    }
    // If not valid, loop continues
}
System.out.println("You entered: " + n);

• The loop keeps asking until the user enters a valid number, then exits.

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The continue Keyword

• The continue keyword skips the rest of the current loop iteration and moves to the next iteration.
• In a for loop, after continue is executed, the update statement runs, then the loop condition is checked.
• In while or do-while loops, continue causes the loop condition to be checked again immediately.

How continue works:

• Use continue when you want to skip certain cases in a loop but keep looping.

Example 1: Skipping even numbers in a for loop

for (int i = 1; i <= 10; i++) {
    if (i % 2 == 0) {
        continue; // Skip even numbers
    }
    System.out.print(i + " ");
}
// Output: 1 3 5 7 9

• If i is even, continue skips the print statement, so only odd numbers are printed.

Example 2: Using continue in a while loop

int i = 0;
while (i < 5) {
    i++;
    if (i == 3) {
        continue; // Skip printing when i is 3
    }
    System.out.print(i + " ");
}
// Output: 1 2 4 5

• When i is 3, the print statement is skipped.

Example 3: Repeatedly asking for valid input

Scanner scanner = new Scanner(System.in);
int n;
while (true) {
    System.out.print("Enter a number between 1 and 10: ");
    n = scanner.nextInt();
    if (n < 1 || n > 10) {
        continue; // Ask again if input is invalid
    }
    System.out.println("n is between 1 and 10");
    break; // Exit loop if input is valid
}

• The loop keeps asking until the user enters a valid number.

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Example: Printing Odd Numbers with continue

• A for loop from 1 to 10 prints only odd numbers by skipping even numbers using continue.
• If the current number is even, continue skips the print statement.

Step-by-step code:

for (int i = 1; i <= 10; i++) {
    if (i % 2 == 0) {
        continue; // Skip even numbers
    }
    System.out.print(i + " ");
}
// Output: 1 3 5 7 9

How it works:

• Start with i = 1. If i is odd, print it.
• If i is even, skip printing and move to the next number.
• This continues until i is greater than 10.

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Example: Validating User Input with break and continue

• An infinite while (true) loop keeps asking the user to enter a number between 1 and 10.
• If the input is outside 1 to 10, continue restarts the loop to ask again.
• If the input is valid, break exits the loop.
• Declaring the input variable outside the loop allows its value to be used after breaking out.

Step-by-step code:

Scanner scanner = new Scanner(System.in);
int n;
while (true) {
    System.out.print("Enter a number between 1 and 10: ");
    n = scanner.nextInt();
    if (n < 1 || n > 10) {
        continue; // Ask again if input is invalid
    }
    System.out.println("n is between 1 and 10");
    break; // Exit loop if input is valid
}
// n can be used here
System.out.println("You entered: " + n);

How it works:

• The loop keeps asking until the user enters a valid number.
• Only then does it print confirmation and exit.

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Variable Scope in Loops

• A variable declared inside a loop is not accessible outside the loop.
• To use a variable after a loop (for example, after using break), declare it outside the loop.

Example 1: Variable declared outside the loop

int n; // Declared outside the loop
while (true) {
    n = scanner.nextInt();
    if (n == 5) {
        break;
    }
}
// n can be used here
System.out.println("Final value: " + n);

Example 2: Variable declared inside the loop

while (true) {
    int m = scanner.nextInt();
    if (m == 5) {
        break;
    }
}
// m cannot be used here (compilation error)

• Declaring the variable outside the loop makes it accessible after the loop ends.

Tip:

• If you need to use a value after the loop, always declare the variable before the loop starts.

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Key Terms & Definitions

• break — Keyword to immediately exit a loop’s execution.
• continue — Keyword to skip the rest of the loop body and go to the next iteration.
• loop — A programming construct for repeating code, such as for, while, or do-while loops.
• variable scope — The region in code where a variable can be accessed.

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Action Items / Next Steps

• Practice using break and continue with different types of loops in Java.
• Try writing loops that use both keywords to control flow.
• Experiment with variable scope by declaring variables inside and outside loops.
• Review loop structures (for, while, do-while) and see how break and continue affect their execution.
• If you get stuck, break the problem into small steps and focus on one part at a time. Take breaks if you need to refocus!

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Quick Reference: When to Use break and continue

• Use break when you want to exit a loop completely, even if the loop condition is still true.
• Use continue when you want to skip the rest of the current iteration and move to the next one.
• Always declare variables outside the loop if you need to use them after the loop ends.

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ADHD-Friendly Tips

• Focus on one example at a time—read the code, then the explanation.
• Use the code examples as templates for your own practice.
• Highlight or underline key words like break, continue, and variable names to help them stand out.
• Take short breaks between sections to help with focus and retention.
• If you feel overwhelmed, revisit the "Key Terms & Definitions" section for a quick refresher.