العبارات والفاصلة المنقوطة والكتل

Written Explanation

What a statement is

A statement is one complete instruction to the computer. In this module, every statement you write is a call to System.out.println or System.out.print:

System.out.println("Order received");

The Java Language Specification defines many kinds of statements. What matters now is the shared rule: a simple statement ends with a semicolon. The semicolon tells the compiler "this instruction is finished, the next one starts after this." It plays the part of a full stop in a sentence.

The compiler reads your file as a stream of words and symbols and uses semicolons and braces to find where each instruction ends. That is why the layout of your lines does not matter to it, but the semicolons do.

What a block is

A block is a group of statements inside a pair of braces. The specification describes a block as statements between braces that are carried out in order, first to last.

You have already written blocks without naming them:

public class Tidy {                                   // the class body is a block of members
    public static void main(String[] args) {          // main's body is a block of statements
        System.out.println("Order received");
        System.out.println("Order saved");
    }
}

The braces after main(String[] args) contain the method body, and the statements inside run top to bottom. Later in the course you will meet if, for, and other constructs that each use a block of their own. The idea is always the same: braces group things together.

Which pieces end with a semicolon?

Here is the rule of thumb for this course:

A semicolon right after the main line is an error. If you write public static void main(String[] args); and then the opening brace on the next line, javac complains that the method has no body, because the semicolon said the method ended there.

An extra semicolon after a statement is not an error. The specification calls a lone ; an empty statement, which does nothing. So System.out.println("A");; compiles. Do not write it on purpose, but do not panic if you find one.

Matching braces

Every opening { needs a closing }. A good way to check is to count: walk through the file and add one for each { and subtract one for each }. At the end of the file the count must be zero, and it should never go negative on the way.

If the count ends above zero, you forgot a closing brace, and the compiler will tell you it "reached end of file while parsing". If the count goes below zero, you wrote an extra closing brace, and the compiler will complain that a class, interface, enum, or record was expected. Lesson 5 explains these messages.

Indentation: for people, not for the compiler

Java ignores extra spaces, tabs, and blank lines between tokens. The specification lists spaces, tabs, and line breaks as white space that only separates tokens. Compare these two programs. They compile to the same behaviour:

public class Tidy {
    public static void main(String[] args) {
        System.out.println("Order received");
        System.out.println("Order saved");
    }
}

public class Tidy2 { public static void main(String[] args) { System.out.println("Order received"); System.out.println("Order saved"); } }

Both print:

Order received
Order saved

So why do we indent? Because programs are read far more often than they are written, mostly by other people and by your future self. A common style, used throughout this course, is:

• Put each statement on its own line.
• Indent each nested block four spaces further than the line that opened it.
• Put the closing } on its own line, lined up with the line that opened the block.
• Use blank lines to separate groups of steps.

One exception to "spaces are free": spaces inside quotation marks belong to the text. "a b" prints three spaces between a and b. We return to this in Lesson 3.

Most IDEs, including IntelliJ IDEA, can reformat a file for you. The shortcut and menu name vary by version and operating system, so look for a "Reformat Code" action in the menus. If the IDE's re-indentation looks wrong, that is often a clue that a brace is missing.

Where the compiler points for a missing semicolon

The compiler cannot know a semicolon is missing until it reads the start of the next thing. So it usually reports the problem at the end of the line that needs the semicolon, with a caret ^ placed right after the last character. In some cases it can look as though the compiler points at the following line. Always check the line it names and the line above it.