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Java Basics  ›  Lesson

Basic Syntax: Classes, main and Statements

Basic Syntax 12 minEvery Java statement ends with a semicolon and sits inside a method, every method sits inside a class, and the program starts at main.

Basic Syntax: Classes, main and Statements

Brew Bar's first program only needs to greet customers. Even that one line of output needs a surrounding structure, because Java is strict about where code may live. Learn the structure once and every Java file you open will look familiar.

The rule

Statements live in methods; methods live in classes.

Here is the whole greeting program, saved as Cafe.java:

public class Cafe {
    public static void main(String[] args) {
        // Greet the first customer of the day
        System.out.println("Welcome to Brew Bar");
        System.out.println("What can we make for you?");
    }
}

Reading from the outside in:

  • The class. public class Cafe { ... } is the container. A public class must be saved in a file with exactly the same name, so Cafe goes in Cafe.java.
  • The method. main is where the JVM starts. Its header is always public static void main(String[] args). You will learn every word of it later; for now, copy it exactly.
  • The statements. Each instruction ends with a semicolon ;. System.out.println prints its text and then moves to a new line. System.out.print prints without the line break.

A few more rules to keep in mind:

  • Braces group code. Every { needs a matching }. Indentation is for humans; the braces are what Java reads.
  • Case matters. System and system are different names, and only the first one exists. String is a type; string is an error.
  • Text goes in double quotes. "Welcome" is a piece of text, called a string.
  • Comments are ignored. // to the end of the line, /* across several lines */, and /** documentation comments */ that tools turn into reference pages.

The shortcut

Read any Java file like a set of boxes: class box → method box → statements. If a statement sits directly inside a class box, it is in the wrong place.

Adding a closing-time messageStep 1 of 4

  1. Find the class box

    public class Cafe { ... } in Cafe.java. The names match, so the file will compile.

The most common first error is a missing semicolon. javac points at the line after the real problem, so look one line up.

Drills

Answer fast — trust the rule.

quick drill

A file declares public class BrewBar. What must the file be called?

quick drill

What is printed by System.out.print("A"); System.out.println("B"); System.out.print("C");?

quick drill

Why does system.out.println("Hi"); fail to compile?

quick drill

Where may System.out.println("Open"); be placed?

Remember

  • Statements live in methods; methods live in classes; a public class matches its file name.
  • Read outside in: class box, method box, statements ending in ;.
  • Traps: a missing semicolon, and wrong capital letters.

Practice — level up

0/1 solved

Write your answer, then compare it with a rubric and a model answer.

ExplainWhy won't it compile?

A classmate saved public class Menu in a file called menu.java, wrote System.out.println("Latte") directly inside the class with no method around it, and left off the semicolon. List each problem and explain how to fix it.