Syntax and control flow

In plain English

A Java program is made of classes, and a runnable program needs a main method, which is the entry point where the program starts. The JVM, or Java Virtual Machine, is the program that runs your compiled bytecode, so the same Java file can run on any computer that has a JVM. A variable stores one value of a fixed type, such as int for whole numbers, boolean for true or false, or String for text. An if and else statement chooses between branches based on a condition, while a switch matches one value against several fixed cases. A for loop repeats a block a known number of times and a while loop repeats until a condition becomes false, and scope is the region where a variable can be seen before it disappears when its block ends.

Step by step

1. 1Create a class with a main method that prints one line.
2. 2Declare variables with int, boolean and String types.
3. 3Add an if/else branch and a switch for a single value.
4. 4Write a for loop and a while loop.
5. 5Move a variable into a block and notice its scope shrink.

public class ControlFlow {
    public static void main(String[] args) {
        int minutes = 12;
        String level = minutes < 10 ? "short" : "long";

        if (minutes >= 14) {
            System.out.println("deep dive");
        } else if (minutes >= 8) {
            System.out.println("steady lesson");
        } else {
            System.out.println("quick intro");
        }

        switch (level) {
            case "short" -> System.out.println("under 10 minutes");
            case "long" -> System.out.println("10 minutes or more");
            default -> System.out.println("unknown");
        }

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

        while (total < 5) {
            total += 1;
        }

        System.out.println(total);
    }
}

Common mistakes

• Using a variable outside the block where it was declared fails to compile, because its scope has already ended.
• Writing a single = where == belongs in a condition fails to compile, since an assignment does not produce a true or false value.
• Using a switch when you actually need range comparisons is a mistake, because switch matches a single value exactly.
• Declaring every variable at the top of a method widens its scope and makes bugs harder to trace.