Program Structure and Output

Java Program Structure and Output

By the end of this lesson, you will be able to write a complete Java program, produce output using print statements, and understand the structure of a Java class.

Anatomy of a Java Program

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

Key elements:

• public class HelloWorld — declares a class named HelloWorld. The filename MUST match the class name exactly (HelloWorld.java).
• public static void main(String[] args) — the entry point; execution starts here.
• System.out.println(...) — prints to the console; adds a newline at the end.

Print Statements

System.out.println("Hello");   // prints "Hello" then newline
System.out.print("Hi ");       // prints "Hi " (no newline)
System.out.print("there!");    // prints on the same line → "Hi there!"
System.out.println();          // prints just a newline

System.out.println(value): prints the value followed by a newline.

System.out.print(value): prints the value WITHOUT a newline.

System.out.printf(format, args): formatted output.

String Concatenation in Output

String name = "Alice";
int age = 17;
System.out.println("Name: " + name + ", Age: " + age);
// Output: Name: Alice, Age: 17

Use + to combine strings and variables:

Watch: adding a number to a string vs. adding numbers:

System.out.println(1 + 2 + " hi");   // → "3 hi" (adds numbers first)
System.out.println("hi " + 1 + 2);   // → "hi 12" (concatenates left to right)

Comments

// This is a single-line comment

/* This is a
   multi-line comment */

/** This is a Javadoc comment — for documentation */

Comments are ignored by the compiler. Good comments explain WHY, not WHAT.

Escape Sequences in Strings