Java Beginner Guide - Part 2: Control Flow

Table of Contents

• Introduction
• Conditional Statements
• Switch Statements
• Loops - For Loops
• Loops - While Loops
• Loops - Do-While Loops
• Loop Control - Break and Continue
• Nested Loops
• Practice Exercises

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Introduction

Control flow determines the order in which your code runs. By default, Java executes statements one after another. But what if you want to make decisions? What if you want to repeat something? That's where control flow comes in!

In this guide, you'll learn:

• How to make decisions with if/else
• How to choose between multiple options with switch
• How to repeat code with loops

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Conditional Statements

Conditional statements let your program make decisions based on conditions.

The if Statement

The if statement runs code only when a condition is true.

int age = 18;

if (age >= 18) {
    System.out.println("You are an adult!");
}

The if-else Statement

Add an else block to run code when the condition is false.

int age = 16;

if (age >= 18) {
    System.out.println("You are an adult!");
} else {
    System.out.println("You are a minor!");
}

The if-else-if Statement

Chain multiple conditions together.

int score = 85;

if (score >= 90) {
    System.out.println("Grade: A");
} else if (score >= 80) {
    System.out.println("Grade: B");
} else if (score >= 70) {
    System.out.println("Grade: C");
} else if (score >= 60) {
    System.out.println("Grade: D");
} else {
    System.out.println("Grade: F");
}

Comparison Operators

Remember these from Part 1?

Operator	Meaning	Example
==	Equal to	a == b
!=	Not equal to	a != b
>	Greater than	a > b
<	Less than	a < b
>=	Greater or equal	a >= b
<=	Less or equal	a <= b

Logical Operators

Combine multiple conditions:

int age = 25;
int income = 50000;

if (age >= 18 && income >= 30000) {
    System.out.println("You qualify for the loan!");
}

// OR example
boolean hasCard = false;
boolean hasCash = true;

if (hasCard || hasCash) {
    System.out.println("You can make a purchase!");
}

Operator	Meaning	Example
&&	AND (both must be true)	a && b
||	OR (at least one must be true)	a || b
!	NOT (reverses true/false)	!a

Ternary Operator (Shortcut)

A compact if-else in one line:

// Instead of:
String result;
if (age >= 18) {
    result = "Adult";
} else {
    result = "Minor";
}

// You can write:
String result = (age >= 18) ? "Adult" : "Minor";

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Switch Statements

When you have many conditions to check, switch can be cleaner than if-else.

Basic Switch

int day = 3;
String dayName;

switch (day) {
    case 1:
        dayName = "Monday";
        break;
    case 2:
        dayName = "Tuesday";
        break;
    case 3:
        dayName = "Wednesday";
        break;
    case 4:
        dayName = "Thursday";
        break;
    case 5:
        dayName = "Friday";
        break;
    case 6:
        dayName = "Saturday";
        break;
    case 7:
        dayName = "Sunday";
        break;
    default:
        dayName = "Invalid day";
}

System.out.println(dayName);  // Wednesday

Switch with Multiple Cases

Group cases together:

char grade = 'B';

switch (grade) {
    case 'A':
    case 'B':
    case 'C':
        System.out.println("You passed!");
        break;
    case 'D':
    case 'F':
        System.out.println("You failed.");
        break;
    default:
        System.out.println("Invalid grade.");
}

Switch Expression (Java 14+)

A shorter way to use switch:

int day = 3;
String dayName = switch (day) {
    case 1 -> "Monday";
    case 2 -> "Tuesday";
    case 3 -> "Wednesday";
    case 4 -> "Thursday";
    case 5 -> "Friday";
    case 6 -> "Saturday";
    case 7 -> "Sunday";
    default -> "Invalid";
};

Note: The -> syntax requires Java 14 or later. Use the traditional switch for older versions.

Why Use Switch?

• Cleaner than long if-else chains
• Good for menus and choices
• Can be used as an expression (Java 14+)

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Loops - For Loops

Loops let you repeat code multiple times. The for loop is perfect when you know how many times to repeat.

Basic For Loop

for (int i = 1; i <= 5; i++) {
    System.out.println("Count: " + i);
}

// Output:
// Count: 1
// Count: 2
// Count: 3
// Count: 4
// Count: 5

The three parts:

1. int i = 1 - Start: where to begin
2. i <= 5 - Condition: when to stop
3. i++ - Update: what to do each time

For Loop Breakdown

for (int i = 0; i < 10; i++) {
    // Code runs 10 times (i = 0,1,2,3,4,5,6,7,8,9)
}

for (int i = 10; i > 0; i--) {
    // Countdown from 10 to 1
}

for (int i = 0; i <= 20; i += 2) {
    // Even numbers from 0 to 20
}

For-Each Loop (Enhanced For)

Loop through arrays and collections without an index:

String[] fruits = {"Apple", "Banana", "Cherry"};

for (String fruit : fruits) {
    System.out.println(fruit);
}

// Output:
// Apple
// Banana
// Cherry

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Loops - While Loops

The while loop repeats as long as a condition is true. Use it when you don't know how many times to repeat.

Basic While Loop

int count = 1;

while (count <= 5) {
    System.out.println("Count: " + count);
    count++;  // Don't forget this or it loops forever!
}

// Output:
// Count: 1
// Count: 2
// Count: 3
// Count: 4
// Count: 5

While vs For Loop

// For loop - when you know the count
for (int i = 0; i < 5; i++) {
    System.out.println(i);
}

// While loop - when count is unknown
Scanner scanner = new Scanner(System.in);
int number;

System.out.println("Enter a positive number:");
number = scanner.nextInt();

while (number <= 0) {
    System.out.println("Invalid! Enter positive:");
    number = scanner.nextInt();
}
System.out.println("Thanks!");

Infinite Loop Warning

Be careful! If the condition never becomes false, the loop runs forever:

// BAD - infinite loop!
// while (true) {
//     System.out.println("This runs forever!");
// }

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Loops - Do-While Loops

The do-while loop runs the code at least once before checking the condition.

Basic Do-While

int count = 1;

do {
    System.out.println("Count: " + count);
    count++;
} while (count <= 5);

// Output:
// Count: 1
// Count: 2
// Count: 3
// Count: 4
// Count: 5

While vs Do-While

// While - might not run at all
int num = 10;
while (num < 5) {
    System.out.println("This won't print");
}

// Do-While - runs at least once
do {
    System.out.println("This WILL print once");
} while (num < 5);

Practical Example: User Input

Scanner scanner = new Scanner(System.in);
String choice;

do {
    System.out.println("Menu: 1. Play  2. Score  3. Exit");
    System.out.print("Enter choice: ");
    choice = scanner.nextLine();
    
    switch (choice) {
        case "1": System.out.println("Playing..."); break;
        case "2": System.out.println("Score: 100"); break;
        case "3": System.out.println("Goodbye!"); break;
        default: System.out.println("Invalid choice!");
    }
} while (!choice.equals("3"));

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Loop Control - Break and Continue

Break - Exit the Loop Immediately

for (int i = 1; i <= 10; i++) {
    if (i == 5) {
        break;  // Exit the loop when i is 5
    }
    System.out.println(i);
}

// Output: 1, 2, 3, 4

Continue - Skip Current Iteration

for (int i = 1; i <= 5; i++) {
    if (i == 3) {
        continue;  // Skip when i is 3
    }
    System.out.println(i);
}

// Output: 1, 2, 4, 5 (3 is skipped)

Practical Examples

Break - Finding an item:

String[] names = {"Alice", "Bob", "Charlie", "Diana"};

for (String name : names) {
    if (name.equals("Charlie")) {
        System.out.println("Found Charlie!");
        break;  // No need to keep searching
    }
}

Continue - Filtering:

// Print only even numbers
for (int i = 1; i <= 10; i++) {
    if (i % 2 != 0) {
        continue;  // Skip odd numbers
    }
    System.out.println(i);
}

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Nested Loops

A loop inside another loop! This is useful for patterns and grids.

Basic Nested Loop

for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 3; j++) {
        System.out.println("i=" + i + ", j=" + j);
    }
}

// Output:
// i=1, j=1
// i=1, j=2
// i=1, j=3
// i=2, j=1
// ... and so on

Print a Pattern

// Print a square of stars
for (int row = 1; row <= 4; row++) {
    for (int col = 1; col <= 4; col++) {
        System.out.print("* ");
    }
    System.out.println();  // New line after each row
}

// Output:
// * * * * 
// * * * * 
// * * * * 
// * * * * 

Multiplication Table

// Print 1-5 times table
for (int i = 1; i <= 5; i++) {
    for (int j = 1; j <= 10; j++) {
        System.out.printf("%2d x %2d = %2d  ", i, j, i*j);
    }
    System.out.println();
}

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Practice Exercises

Exercise 1: Even or Odd

Write a program that asks for a number and tells if it's even or odd.

import java.util.Scanner;

public class EvenOrOdd {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter a number: ");
        int num = scanner.nextInt();
        
        if (num % 2 == 0) {
            System.out.println("Even!");
        } else {
            System.out.println("Odd!");
        }
    }
}

Exercise 2: Grade Calculator

Convert a numeric score to a letter grade.

public class GradeCalculator {
    public static void main(String[] args) {
        int score = 87;
        char grade;
        
        if (score >= 90) {
            grade = 'A';
        } else if (score >= 80) {
            grade = 'B';
        } else if (score >= 70) {
            grade = 'C';
        } else if (score >= 60) {
            grade = 'D';
        } else {
            grade = 'F';
        }
        
        System.out.println("Grade: " + grade);
    }
}

Exercise 3: Count to Ten

Use a for loop to print numbers 1 to 10.

public class CountToTen {
    public static void main(String[] args) {
        for (int i = 1; i <= 10; i++) {
            System.out.println(i);
        }
    }
}

Exercise 4: Sum of Numbers

Calculate the sum of 1 to 100.

public class SumNumbers {
    public static void main(String[] args) {
        int sum = 0;
        
        for (int i = 1; i <= 100; i++) {
            sum += i;
        }
        
        System.out.println("Sum: " + sum);  // 5050
    }
}

Exercise 5: Multiplication Table

Print the 5 times table using a loop.

public class TimesTable {
    public static void main(String[] args) {
        int num = 5;
        
        for (int i = 1; i <= 10; i++) {
            System.out.println(num + " x " + i + " = " + (num * i));
        }
    }
}

Exercise 6: Find the Maximum

Find the largest number in an array.

public class FindMax {
    public static void main(String[] args) {
        int[] numbers = {5, 12, 3, 8, 99, 1};
        int max = numbers[0];
        
        for (int i = 1; i < numbers.length; i++) {
            if (numbers[i] > max) {
                max = numbers[i];
            }
        }
        
        System.out.println("Maximum: " + max);  // 99
    }
}

Exercise 7: FizzBuzz

Print numbers 1 to 20, but:

• Print "Fizz" for multiples of 3
• Print "Buzz" for multiples of 5
• Print "FizzBuzz" for multiples of both

public class FizzBuzz {
    public static void main(String[] args) {
        for (int i = 1; i <= 20; i++) {
            if (i % 3 == 0 && i % 5 == 0) {
                System.out.println("FizzBuzz");
            } else if (i % 3 == 0) {
                System.out.println("Fizz");
            } else if (i % 5 == 0) {
                System.out.println("Buzz");
            } else {
                System.out.println(i);
            }
        }
    }
}

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Summary

In this guide, you learned:

• ✅ How to make decisions with if, else if, and else
• ✅ How to use comparison and logical operators
• ✅ How to use the ternary operator for short conditions
• ✅ How to use switch statements for multiple choices
• ✅ How to use for loops when you know the count
• ✅ How to use while loops when you don't know the count
• ✅ How to use do-while loops for at-least-once execution
• ✅ How to control loops with break and continue
• ✅ How to use nested loops for patterns

Next Steps: Move on to Java Beginner Part 3 to learn about methods and arrays!

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Great job! Keep practicing! 🎉