Hello, World

Every Java program is a class with one special method the JVM starts from. By the end of this lesson you can print a line, print part of a line, and build an output line out of fixed text plus a value held in a variable.

The smallest program

public class Main {
    public static void main(String[] args) {
        System.out.println("Starting up");
    }
}

Read it from the outside in:

• public class Main -- all Java code lives inside classes; there are no free-floating functions as in Python or JavaScript. This platform compiles your file as class Main, so keep that name.
• public static void main(String[] args) -- the entry point, and the JVM looks for exactly this signature. public so the JVM can reach it, static so it can run before any object exists, void because it returns nothing, String[] args for command-line arguments (unused here, but required).
• System.out.println(...) -- write the argument to standard output, then end the line. System is in java.lang, which every file imports automatically.

The ceremony is deliberate: from line one, every program has an unambiguous starting point and every statement has a home. Small scripts pay a tax; large codebases collect the refund.

print versus println

println writes the text and a line break. print writes only the text, so the next output continues on the same line:

System.out.print("3 ");
System.out.print("2 ");
System.out.println("1");
System.out.println("liftoff");

3 2 1
liftoff

The first three calls build one line, and only the println on "1" ends it. Change that call to print and the output becomes 3 2 1liftoff -- two lines silently fused into one, which an exact-match grader will reject. Every line you intend to be a line must end with println (or a %n, which a later lesson covers).

Joining text and values with +

+ between a string and anything else builds a longer string. This is how a fixed message and a value from the program share one line:

int points = 42;
System.out.println("Score: " + points + " points");

Score: 42 points

The quotes mark literal text; points without quotes is the variable. Put points inside the quotes and you print the word, not the number. A variable holding text works the same way: "Hi " + user joins the two.

Where input comes from

Most exercises here read standard input. The starter code already does it with a Scanner and gives you a variable to work with, so for now treat a line like String name = sc.nextLine(); as "the program received a line of text and stored it in name". Scanner gets its own treatment in the next lessons.

What the compiler says

system.out.println("hi");    // error: cannot find symbol -- Java is case-sensitive; it is System
System.out.println("hi")     // error: ';' expected -- every statement ends with a semicolon
System.out.println('hi');    // error: unclosed character literal -- single quotes hold ONE char

One more sneaky one: public static void Main(...) with a capital M compiles, because it is a perfectly legal method, but the JVM then reports that no main method was found.

Common mistakes

• Using print where println was needed. Two intended lines come out as one fused line.
• Quoting a variable. "name" prints the five letters n-a-m-e; name prints what it holds.
• Missing spaces around joined pieces. "Hello," + name prints Hello,Ada; the space has to be inside the quotes.
• Forgetting the ;. The error message points at the line after the real problem.

Your exercise

Read one line, a name, and print a two-line greeting built from it. The starter already reads the name; the two output lines are yours. The mistake the tests catch is gluing the lines together or dropping a space or punctuation mark.