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Control Flow

Control flow statements let your program make decisions and repeat actions. Java's control flow is similar to other C-family languages, but with some Java-specific features like enhanced switch and for-each.

if / else if / else​

int temperature = 25;

if (temperature > 30) {
    System.out.println("It's hot");
} else if (temperature > 20) {
    System.out.println("It's warm");
} else if (temperature > 10) {
    System.out.println("It's cool");
} else {
    System.out.println("It's cold");
}

Result:

It's warm

Rules:

  • The condition must be a boolean expression. Java does not coerce integers or objects -- if (1) is a compile error.
  • Only the first matching branch executes.
  • Always use braces {}, even for single-line bodies.

Combining conditions​

int age = 25;
boolean hasLicense = true;

if (age >= 18 && hasLicense) {
    System.out.println("You can drive");
}

boolean isWeekend = true;
boolean isHoliday = false;

if (isWeekend || isHoliday) {
    System.out.println("Day off!");
}

boolean isLoggedIn = false;
if (!isLoggedIn) {
    System.out.println("Please log in");
}

Result:

You can drive
Day off!
Please log in

Ternary operator​

A one-line if/else:

int age = 20;
String status = (age >= 18) ? "adult" : "minor";
System.out.println(status);

Result:

adult

Use it for simple assignments. Do not nest ternaries -- they become unreadable.

switch -- classic​

Compare a value against multiple options:

String day = "Monday";

switch (day) {
    case "Monday":
    case "Tuesday":
    case "Wednesday":
    case "Thursday":
    case "Friday":
        System.out.println("Weekday");
        break;
    case "Saturday":
    case "Sunday":
        System.out.println("Weekend");
        break;
    default:
        System.out.println("Unknown day");
}

Result:

Weekday

Always include break -- without it, execution falls through to the next case.

Supported types: byte, short, int, char, String, and enums.

Enhanced switch (Java 14+)​

The modern switch uses arrow syntax and eliminates the need for break:

String day = "Saturday";

String type = switch (day) {
    case "Monday", "Tuesday", "Wednesday", "Thursday", "Friday" -> "Weekday";
    case "Saturday", "Sunday" -> "Weekend";
    default -> "Unknown";
};

System.out.println(type);

Result:

Weekend

Key differences:

  • Arrow -> instead of colon :
  • No fall-through -- no break needed
  • Can return a value (switch expression)
  • Multiple values per case with commas

Switch with blocks​

int dayNumber = 3;

String name = switch (dayNumber) {
    case 1 -> "Monday";
    case 2 -> "Tuesday";
    case 3 -> {
        System.out.println("Processing day 3...");
        yield "Wednesday"; // 'yield' returns from a block
    }
    case 4 -> "Thursday";
    case 5 -> "Friday";
    default -> "Weekend";
};

System.out.println(name);

Result:

Processing day 3...
Wednesday

Use yield to return a value from a multi-statement block in a switch expression.

for loop​

for (int i = 0; i < 5; i++) {
    System.out.println("Iteration " + i);
}

Result:

Iteration 0
Iteration 1
Iteration 2
Iteration 3
Iteration 4

The three parts:

  1. Initialization: int i = 0 -- runs once
  2. Condition: i < 5 -- checked before each iteration
  3. Update: i++ -- runs after each iteration

Counting backwards​

for (int i = 5; i > 0; i--) {
    System.out.println(i);
}

Result:

5
4
3
2
1

Stepping by more than one​

for (int i = 0; i <= 10; i += 2) {
    System.out.println(i);
}

Result:

0
2
4
6
8
10

Enhanced for-each​

Iterates over arrays and collections without an index:

String[] fruits = {"apple", "banana", "cherry"};

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

Result:

apple
banana
cherry

Read as "for each String fruit in fruits". Use for-each when you do not need the index.

while loop​

Repeats while a condition is true:

int count = 0;

while (count < 3) {
    System.out.println("Count is " + count);
    count++;
}

Result:

Count is 0
Count is 1
Count is 2

Use while when the number of iterations is unknown:

int number = 1;

while (number < 100) {
    number *= 2;
}

System.out.println(number);

Result:

128

do-while loop​

The body runs at least once before the condition is checked:

int attempt = 0;

do {
    attempt++;
    System.out.println("Attempt " + attempt);
} while (attempt < 3);

Result:

Attempt 1
Attempt 2
Attempt 3

break and continue​

break -- exit the loop early​

for (int i = 0; i < 10; i++) {
    if (i == 5) {
        break;
    }
    System.out.println(i);
}

Result:

0
1
2
3
4

continue -- skip to the next iteration​

for (int i = 0; i < 6; i++) {
    if (i % 2 == 0) {
        continue;
    }
    System.out.println(i);
}

Result:

1
3
5

Labeled loops​

Labels let break and continue target an outer loop:

outer:
for (int i = 0; i < 3; i++) {
    for (int j = 0; j < 3; j++) {
        if (i == 1 && j == 1) {
            break outer;
        }
        System.out.println("i=" + i + ", j=" + j);
    }
}

Result:

i=0, j=0
i=0, j=1
i=0, j=2
i=1, j=0

Nested loops​

for (int row = 1; row <= 3; row++) {
    StringBuilder line = new StringBuilder();
    for (int col = 1; col <= 3; col++) {
        line.append("(").append(row).append(",").append(col).append(") ");
    }
    System.out.println(line.toString().trim());
}

Result:

(1,1) (1,2) (1,3)
(2,1) (2,2) (2,3)
(3,1) (3,2) (3,3)

Common patterns​

Summing values​

int[] numbers = {10, 20, 30, 40, 50};
int sum = 0;

for (int num : numbers) {
    sum += num;
}

System.out.println("Sum: " + sum);

Result:

Sum: 150

Finding a value​

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

for (String name : names) {
    if (name.startsWith("C")) {
        found = name;
        break;
    }
}

System.out.println("Found: " + found);

Result:

Found: Charlie

FizzBuzz​

for (int i = 1; i <= 20; 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);
    }
}

Result:

1
2
Fizz
4
Buzz
Fizz
7
8
Fizz
Buzz
11
Fizz
13
14
FizzBuzz
16
17
Fizz
19
Buzz

Multiplication table​

int n = 5;
for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= n; j++) {
        System.out.printf("%4d", i * j);
    }
    System.out.println();
}

Result:

   1   2   3   4   5
   2   4   6   8  10
   3   6   9  12  15
   4   8  12  16  20
   5  10  15  20  25

Summary​

  • if/else if/else for branching decisions -- conditions must be boolean.
  • Ternary ? : for simple one-line conditionals.
  • Classic switch requires break; enhanced switch (Java 14+) uses arrows and eliminates fall-through.
  • for loop when you know the iteration count.
  • Enhanced for-each for arrays and collections -- cleaner than indexed for.
  • while when the iteration count is unknown; do-while when the body must run at least once.
  • break exits a loop; continue skips to the next iteration; labels target outer loops.

Next up: Methods -- reusable blocks of code.