Control Flow Statements

// 1.1

A straight line or a fork in the road

A flowchart draws a program as shapes and arrows. Until now your programs were a straight line. A decision adds a fork: the program takes one path or the other, never both.

// 1.2

Anatomy of an if

An if statement runs its block only when the condition is true. When it is false, Java skips the block and carries on after the }. Click any part.

if (balance >= amount) {
    balance = balance - amount;
}

int balance = 120;
int amount = 50;
if (balance >= amount) {
    balance = balance - amount;
}
System.out.println(balance);

$ java Main
70

public class Main {
    public static void main(String[] args) {
        int count = 3;
        if (count) {
            System.out.println("You have messages");
        }
    }
}

$ javac Main.java
Main.java:4: error: incompatible types: int cannot be converted to boolean
        if (count) {
            ^
1 error

// 1.3

Braces are optional. Write them anyway.

Without { }, an if controls only the one statement right after it. Indentation means nothing to Java. Run the code, then add the braces.

// 1.4

if and else: exactly one of two

Add an else block and the program always does one of two things: never both, and never neither. Pick a scenario and step through it like a debugger.

// 1.5

Writing good conditions

A condition is any expression whose value is a boolean. Everything from lesson 3 works here: comparisons, &&, ||, ! and .equals.

if (age >= 18 && hasTicket) {
    System.out.println("Come in");
}
if (day == 6 || day == 7) {
    System.out.println("Weekend");
}

if (city.equals("Baku")) {
    System.out.println("Capital");
}

// 1.6

Checkpoint: making decisions

Eight quick questions on Part 1. Pick an answer to see why it is right or wrong.

// 2.1

else if: the first true condition wins

Java checks the conditions from top to bottom and runs the block of the first one that is true. The conditions below it are not even checked. Move the slider, then try the other versions.

// 2.2

Nested ifs: a login check

An if can sit inside another if. Here a login passes three checks in order. Flip the facts, then compare three ways to write the same logic.

// 2.3

The dangling else: indentation lies

Nested ifs without braces raise a question: which if does the else belong to? Java’s rule: an else belongs to the nearest if above it that has no else yet. Indentation has no say.

// 2.4

Seven classic traps

Two of these don’t compile. The dangerous ones compile fine and quietly do the wrong thing. Pick a trap to see what really happens and how to fix it.

// 2.5

Game: which line prints?

Fourteen short programs. Read the code, predict the exact output and pick it. Every answer was checked with Java 25.

// 2.6

Checkpoint: chains, nesting and traps

Eight questions on Part 2. Several of them hide one of the traps.

// 3.1

An if that gives you a value

condition ? valueIfTrue : valueIfFalse. If the condition is true, the whole expression becomes the first value, otherwise the second. It has three operands, so it is called the ternary operator. Click any part.

String status =
    age >= 18 ? "adult" : "minor";

// 3.2

From if/else to ?:

When both branches only choose a value for the same thing, a ternary says it in one line. Pick an example and morph it. The colors show which part goes where.

// 3.3

Rules and traps of ?:

The ternary is small, but it follows strict rules. Pick a rule to see what goes wrong and how to write it instead.

// 3.4

Game: pick the ternary

Each round gives you an if/else or a task. Pick the one ternary that does exactly the same. The wrong ones swap the branches, flip the condition, get the type wrong or forget the brackets.

// 3.5

Checkpoint: the ternary operator

Eight questions on Part 3.

// 4.1

Anatomy of a switch

A switch looks at one value, jumps straight to the case with the same value, and runs from there. Click any part, and change day to see which lines run.

// 4.2

What can you switch on?

The value in switch (…) is called the selector. Only some types are allowed. Try each one.

// 4.3

case labels: fixed values, each one once

javac decides where each case jumps to before the program runs, so a label must be a value it already knows. Try the three versions.

// 4.4

Fall-through lab

A switch jumps in at the matching case, then runs line after line, straight through the next case labels. Only break gets you out. Remove some breaks and press Run.

// 4.5

Fall-through on purpose, and other selectors

Fall-through is not always a bug. Stack labels to share one block. Then switch on text and characters, and see the one value a switch can’t handle.

// 4.6

Checkpoint: the switch statement

Eight questions on Part 4. Watch out for missing breaks.

// 5.1

Arrow labels: no fall-through, no break

Write case … -> instead of case …: and a case runs only its own code. There is nothing to fall through and no break to forget. Same logic, two forms:

int day = 6;
switch (day) {
    case 6, 7 -> {
        System.out.println("Weekend");
        System.out.println("Open at 10");
    }
    default -> System.out.println("Open at 9");
}

$ java Main
Weekend
Open at 10

// 5.2

Switch expressions: a switch with a value

Put a switch after = and it becomes an expression, like the ternary: every case gives a value. It must give a value for every possible input, and the statement ends with ; after the }.

int day = 6;
int hours = switch (day) {
    case 6, 7 -> {
        System.out.println("Weekend!");
        yield 0;
    }
    default -> 8;
};
System.out.println("Work hours: " + hours);

$ java Main
Weekend!
Work hours: 0

int day = 2;
int hours = switch (day) {
    case 6:
    case 7:
        yield 0;
    default:
        yield 8;
};
System.out.println(hours);

$ java Main
8

// 5.3

if, switch or ?:?

All three make decisions, and each is best at a different job. Read the table, then sort eleven real tasks.

A switch can’t test a range: there is no case >= 90, because labels are fixed values. In new code, prefer arrow labels and switch expressions to the classic form.

Object value = 42;
String text = switch (value) {
    case Integer i when i > 0 -> "positive number";
    case Integer i -> "number";
    case String s -> "text: " + s;
    default -> "something else";
};
System.out.println(text);

$ java Main
positive number

// 5.4

Game: what does this switch print?

Twelve switches: classic and arrow, statements and expressions. Some fall through, some don’t. Every answer was checked with Java 25.

// 5.5

Checkpoint: modern switch

Eight questions on Part 5.

// 6.1

Mini-program: a cinema ticket

A switch expression picks the base price, an else if chain applies the age rules, and a ternary handles the student discount. Change the customer and watch which lines run.

// 6.2

Break it on purpose

These are the mistakes you will make this week. Most of them javac catches for you, with the exact messages below. The last one compiles and is simply wrong. Pick a mistake to see the error and the fix.

// 6.3

Checkpoint: putting it together

Eight questions that mix the whole lesson: the cinema program, compiler errors and one logic bug.

// 7.1

Key takeaways

1. A condition is a boolean expression. A number is not a condition: if (count) doesn’t compile.

2. if runs its block when the condition is true, else when it is false. Exactly one of the two runs.

3. In an else if chain the first true condition wins and the rest are not checked. Put the most specific condition first.

4. Always write braces. Without them an if controls one statement, an else binds to the nearest if, and a stray ; becomes the whole body.

5. Compare Strings with .equals, write if (isActive) instead of == true, and never use = where you mean ==.

6. ?: picks one of two values. It is an expression: use it where a value goes, not to run code.

7. A classic switch falls through from case to case until a break. Arrow labels -> never fall through.

8. A switch expression gives a value, must cover every input (usually with default) and uses yield inside a block.

// 7.2

Check yourself

Twenty questions on the whole lesson. You see the answer right after you pick.

Your checkpoints

// 7.3

Homework

You don’t know how to read input from the keyboard yet, so put the input in variables at the top of main, run the program, change the values and run it again. Tick items as you finish them. Your ticks are saved in this browser only.

// 7.4

for (int i = 1; i <= 5; i++) {
    System.out.println(i % 2 == 0 ? i + " is even" : i + " is odd");
}

$ java Main
1 is odd
2 is even
3 is odd
4 is even
5 is odd