Your first Java program

Java runs much of the world's business software, Android apps and large web services. It is typed and structured: the compiler checks a great deal before your program ever runs, which catches mistakes early and makes large programs manageable.

Hello, world#

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

There is more ceremony than in Python, and every word has a job:

• Everything lives in a class. public class Hello declares one. The file must be named after it: Hello.java.
• public static void main(String[] args) is where the program starts. void means it returns nothing; args holds any command-line arguments.
• System.out.println prints a line. System.out.print prints without ending the line.
• Statements end with ;, and braces { } group code.

Compiling and running#

Java compiles to bytecode, which the Java Virtual Machine (JVM) then runs. That is what lets the same compiled program run on any operating system.

javac Hello.java      # produces Hello.class
java Hello            # runs it: no .class on the end

Since Java 11 you can also run a single file directly, which is handy while learning:

java Hello.java

Printing values#

+ joins text and values:

int age = 21;
System.out.println("Age: " + age);
System.out.println("Sum: " + (2 + 3));   // brackets: otherwise "Sum: 23"

Without the brackets, Java reads left to right: "Sum: " + 2 is the text "Sum: 2", and adding 3 gives "Sum: 23".

For formatted output, printf works like C's:

double price = 99.456;
System.out.printf("Price: %.2f%n", price);    // Price: 99.46

%n is a new line.

Comments#

// to the end of the line
/* across
   several lines */
/** A documentation comment, read by tools that generate API docs. */

Reading an error#

Compile errors name the file and line:

Hello.java:3: error: ';' expected
        System.out.println("Hello, world")
                                           ^

The caret points at the spot. Errors while running look different, with the exception's name and a stack trace:

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: Index 5 out of bounds for length 5
    at Hello.main(Hello.java:6)

Read the first line for what and the first at line in your own file for where.

Key points#

• Every Java program is in a class, in a file of the same name.
• main is the starting point; statements end with ;.
• javac compiles, java runs, or java File.java does both.
• + joins text, so bracket arithmetic inside a print.

Next: variables and Java's types.