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Basics

Basics · Prerequisite: previous lecture

04. Control Flow

**Control flow** statements direct how code branches or repeats. Java's control flow looks like C's, but with modern additions like JDK 14+'s **switch expression**.

JavaJDKbasicscontrol flow

Duration

⏱ ~1-1.5 hours

Level

📊 Beginner

Prerequisite

🎯 Previous lecture or equivalent knowledge

OUTCOME

**Control flow** statements direct how code branches or repeats. Java's control flow looks like C's, but with modern additions like JDK 14+'s **switch expression**.

What you'll learn

1Write `if` / `else if` / `else` chains
2Use the new **switch expression** (`->` / `yield`)
3Know the differences between `for`, `while`, `do-while`, and the **enhanced for**
4Use `break`, `continue`, and labeled jumps

Overview

**Control flow** statements direct how code branches or repeats. Java's control flow looks like C's, but with modern additions like JDK 14+'s **switch expression**.

Core Concepts

1) `if` / `else if` / `else`

java

int score = 85;
if (score >= 90) {
    System.out.println("A");
} else if (score >= 80) {
    System.out.println("B");
} else {
    System.out.println("C or below");
}

2) Switch expression (JDK 14+)

java

int day = 3;
String name = switch (day) {
    case 1, 2, 3, 4, 5 -> "weekday";
    case 6, 7 -> "weekend";
    default -> "error";
};
System.out.println(name);

Group several values on one arm: `case 1, 2, 3 ->`
Use a block + `yield` for multi-line arms:

java

  String s = switch (n) {
      case 1 -> "one";
      case 2 -> {
          System.out.println("two!");
          yield "two";
      }
      default -> "?";
  };

3) The four loop forms

java

for (int i = 0; i < 3; i++) { ... }

int j = 0;
while (j < 3) { j++; }

int k = 0;
do { k++; } while (k < 3);

int[] nums = {1, 2, 3};
for (int n : nums) { System.out.println(n); }   // enhanced for

4) `break` / `continue`

java

for (int i = 0; i < 10; i++) {
    if (i == 3) continue;   // skip this iteration
    if (i == 6) break;      // exit the loop
    System.out.print(i + " ");   // 0 1 2 4 5
}

Examples

Example 1 — `IfElseDemo.java`: basic branching

java

public class IfElseDemo {
    public static void main(String[] args) {
        int score = 72;
        String 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("score " + score + " -> " + grade);
    }
}

**Output**

text

score 72 -> C

**Note:** single-line `if` bodies can omit braces, but **the official convention is to use braces**.

Example 2 — `SwitchExpression.java`: `->` / `yield`

java

public class SwitchExpression {
    public static void main(String[] args) {
        int day = 3;
        String kind = switch (day) {
            case 1, 2, 3, 4, 5 -> "weekday";
            case 6, 7 -> "weekend";
            default -> "error";
        };
        System.out.println("day " + day + ": " + kind);

        int n = 2;
        String word = switch (n) {
            case 1 -> "one";
            case 2 -> {
                System.out.println("two!");
                yield "two";
            }
            default -> "?";
        };
        System.out.println(word);
    }
}

**Output**

text

day 3: weekday
two!
two

**Note:** a switch expression **must produce a value** (so it needs a `default`).

Example 3 — `Loops.java`: for / while / enhanced for

java

public class Loops {
    public static void main(String[] args) {
        int sum = 0;
        for (int i = 1; i <= 5; i++) sum += i;
        System.out.println("1+...+5 = " + sum);

        int j = 0;
        while (j < 3) {
            System.out.print(j + " ");
            j++;
        }
        System.out.println();

        int[] nums = {10, 20, 30};
        for (int n : nums) System.out.print(n + " ");
        System.out.println();
    }
}

**Output**

text

1+...+5 = 15
0 1 2
10 20 30

**Note:** the **enhanced for** is safest when you don't need an index.

Example 4 — `BreakContinue.java`: break / continue

java

public class BreakContinue {
    public static void main(String[] args) {
        for (int i = 0; i < 10; i++) {
            if (i == 3) continue;
            if (i == 6) break;
            System.out.print(i + " ");
        }
        System.out.println();

        outer:
        for (int i = 0; i < 3; i++) {
            for (int k = 0; k < 3; k++) {
                if (i + k == 3) break outer;
                System.out.print("(" + i + "," + k + ") ");
            }
        }
        System.out.println();
    }
}

**Output**

text

0 1 2 4 5
(0,0) (0,1) (0,2) (1,0) (1,1)

**Note:** **labeled break** is useful for escaping deeply-nested loops, but overusing it hurts readability.

Full example code (src/)

src/BreakContinue.java

java

public class BreakContinue {
    public static void main(String[] args) {
        for (int i = 0; i < 10; i++) {
            if (i == 3) continue;
            if (i == 6) break;
            System.out.print(i + " ");
        }
        System.out.println();

        outer:
        for (int i = 0; i < 3; i++) {
            for (int k = 0; k < 3; k++) {
                if (i + k == 3) break outer;
                System.out.print("(" + i + "," + k + ") ");
            }
        }
        System.out.println();
    }
}

src/IfElseDemo.java

java

public class IfElseDemo {
    public static void main(String[] args) {
        int score = 72;
        String 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("score " + score + " -> " + grade);
    }
}

src/Loops.java

java

public class Loops {
    public static void main(String[] args) {
        int sum = 0;
        for (int i = 1; i <= 5; i++) sum += i;
        System.out.println("1+...+5 = " + sum);

        int j = 0;
        while (j < 3) {
            System.out.print(j + " ");
            j++;
        }
        System.out.println();

        int[] nums = {10, 20, 30};
        for (int n : nums) System.out.print(n + " ");
        System.out.println();
    }
}

src/SwitchExpression.java

java

public class SwitchExpression {
    public static void main(String[] args) {
        int day = 3;
        String kind = switch (day) {
            case 1, 2, 3, 4, 5 -> "weekday";
            case 6, 7 -> "weekend";
            default -> "error";
        };
        System.out.println("day " + day + ": " + kind);

        int n = 2;
        String word = switch (n) {
            case 1 -> "one";
            case 2 -> {
                System.out.println("two!");
                yield "two";
            }
            default -> "?";
        };
        System.out.println(word);
    }
}

Common Mistakes

Writing `if (a = 1)` with `=` instead of `==` → compile error (int where boolean is required)
Off-by-one bugs from `<` vs `<=`
Forgetting `break` in a classic `switch` and falling through
Modifying a collection inside enhanced for → `ConcurrentModificationException`
Missing `default` in a switch expression

Summary

Branch with `if` or a switch expression
Iterate with `for` / `while` / enhanced for
`break` / `continue` and labeled jumps are occasionally useful

Practice

# Practice - 04. Control Flow

## Exercise 1 — FizzBuzz

File: `Homework01.java`
Key concepts: `for`, `if/else if`

Requirements

Print numbers 1..15.
Multiples of 3 print `Fizz`, multiples of 5 print `Buzz`, multiples of 15 print `FizzBuzz`, everything else prints the number.

Expected output

text

1
2
Fizz
4
Buzz
Fizz
7
8
Fizz
Buzz
11
Fizz
13
14
FizzBuzz

Hint

Check the multiple-of-15 case **first**.

## Exercise 2 — Menu selection

File: `Homework02.java`
Key concepts: switch expression

Requirements

Given `menu = 2`, use a switch expression to return:
1 → "New", 2 → "Recommended", 3 or 4 → "Bestseller", else → "None"
Print `Selected: Recommended`.

Expected output

text

Selected: Recommended

## Exercise 3 — Multiplication table 2 ~ 5

File: `Homework03.java`
Key concepts: nested `for`

Requirements

Print the multiplication tables for 2..5.
Format like `2 * 1 = 2`.
Separate tables with a blank line.

Expected output

text

2 * 1 = 2
2 * 2 = 4
...(truncated)

Hint

Outer loop is the multiplicand, inner loop is the multiplier.

## Solutions After trying it yourself, compare with [`answer/`](./answer/).

Solution code (homework/answer/)

answer/Homework01.java

java

/** FizzBuzz 1..15. */
public class Homework01 {
    public static void main(String[] args) {
        for (int i = 1; i <= 15; i++) {
            if (i % 15 == 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);
        }
    }
}

answer/Homework02.java

java

/** Menu via switch expression. */
public class Homework02 {
    public static void main(String[] args) {
        int menu = 2;
        String label = switch (menu) {
            case 1 -> "New";
            case 2 -> "Recommended";
            case 3, 4 -> "Bestseller";
            default -> "None";
        };
        System.out.println("Selected: " + label);
    }
}

answer/Homework03.java

java

/** Multiplication tables 2..5. */
public class Homework03 {
    public static void main(String[] args) {
        for (int dan = 2; dan <= 5; dan++) {
            for (int i = 1; i <= 9; i++) {
                System.out.println(dan + " * " + i + " = " + (dan * i));
            }
            System.out.println();
        }
    }
}

Try It Yourself

bash

cd 01_basics/04_control_flow/src
javac SwitchExpression.java
java SwitchExpression

Next Lecture

[05_Methods](../05_메서드/) — method declaration, overloading, `static`, varargs.

Example code / lecture materials

All lecture materials and example code are openly available on GitHub.

View on GitHub ↗