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:
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:
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:
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:
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:
- Initialization:
int i = 0 - runs once
- Condition:
i < 5 - checked before each iteration
- Update:
i++ - runs after each iteration
Counting backwards
for (int i = 5; i > 0; i--) {
System.out.println(i);
}
Result:
Stepping by more than one
for (int i = 0; i <= 10; i += 2) {
System.out.println(i);
}
Result:
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:
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:
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:
continue - skip to the next iteration
for (int i = 0; i < 6; i++) {
if (i % 2 == 0) {
continue;
}
System.out.println(i);
}
Result:
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:
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:
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.