Compilation, execution, and program structure

Compiling and running in a terminal

Create a directory named hello and a file named Hello.java inside it. Your text editor must save UTF-8 and must not add a hidden .txt extension. For a public class, the filename must match the class name, including case.

Example 1. Greeting and arguments

public class Hello {
    public static void main(String[] args) {
        String name = args.length == 0 ? "student" : args[0];
        System.out.println("Hello, " + name + "!");
        System.out.println("Arguments: " + args.length);
    }
}

javac -encoding UTF-8 Hello.java
java Hello Olena

Expected output:

Hello, Olena!
Arguments: 1

The javac command creates Hello.class. To run it, specify the class name without the .class extension. The directory containing the class file must be accessible through the classpath. You can explicitly specify the current directory:

java -cp . Hello Olena

Figure 1.3. Stages of compiling and running a class

The command java Hello.java Olena runs the source file. It still compiles internally during launch but does not require a separate preceding javac invocation. This mode is convenient for short examples; it does not turn the language into an “untyped script.”

Do not confuse JVM arguments, the program name, and the program's own arguments. JVM options go before the class name or -jar. Words after the class name are passed to args. Enclose an argument containing a space in quotes:

java Hello "Olena Kovalenko"

Here the args array contains one element. Quoting rules differ between shells; specify the shell and exact command in your report. Start with ASCII arguments and check Ukrainian output separately.

Structure of a regular program

public class Hello declares a class. The class body contains the main method. public allows access, static marks a class method, and void means there is no return value. The String[] args parameter is an array of strings. We will study classes in detail starting in Topic 5; for now, this skeleton lets us run programs.

Braces mark block boundaries. A semicolon terminates a statement, and indentation helps readers see the structure. Case matters: System and system are different identifiers. Class names usually start with an uppercase letter; method and variable names start with a lowercase letter.

A // comment continues to the end of the line. A block comment goes between /* and */; these comments cannot be nested. A comment should explain why a decision was made rather than simply restating an instruction.

A literal in double quotes is a string. println prints and adds a newline; print does not add one. Concatenation with + depends on operand types: "Sum: " + 2 + 3 produces text containing the digits 23, while "Sum: " + (2 + 3) produces text containing the number 5. Here parentheses change how the calculation is grouped.

Example 2. Compact source file

Since Java 25, compact source files and instance main methods have been a finalized feature. They let a teaching program start without an explicit class declaration. Save this separate example as Welcome.java:

void main() {
    IO.println("Hello from a compact file!");
    IO.println("JDK " + Runtime.version().feature());
}

java Welcome.java

Output on the target JDK:

Hello from a compact file!
JDK 27

The compiler still creates an implicit class. A compact file automatically imports accessible public types from java.base; in a regular file, import rules remain explicit. java.lang.IO is available for simple line-based input and output. Do not mix IO.readln and Scanner on the same input stream: both may buffer bytes.

Feature specification: https://openjdk.org/jeps/512. The compact format is convenient for a first attempt, but the course's main multi-file examples use explicit classes, packages, and a regular main. Do not call a finalized feature a preview or add --enable-preview unnecessarily.