Structure of a Java Program

Java source code is contained in methods that are part of a class. Here is a simple class:

public class Greeter {
    // a variable referring to a string
    static String who = "Nerd";

    // a method. 'static' means its owned by the class, not an object
    public static void main(String[] args) {

        // Ridiculously verbose syntax for printing to the console
        System.out.println("Hello, " + who);  // prints: Hello, Nerd
    }
}

This code should be in a file named Greeter.java. Java requires that source file name exactly match the name of the class it contains.

Wrong: greeter.java (lowercase “g”), Greet.java (name doesn’t match)

If you have a class named Person then it should be in a file named Person.java. If not, the compiler will give an error message.

Compiling and Running a Program

You must compile Java code before you can run it. An IDE will compile for you, or you can compile code in a command window using the command javac. First verify that javac is on your shell (or command window) PATH by typing:

cmd>  javac -version

The output print the version number (e.g., java 1.8.0_121). If you get “command not found”, it means the JDK programs directory isn’t on your command interpreter (or “shell”) PATH. The Java commands are in the directory JAVA_INSTALL_DIR/bin. You should add this directory to your path. This depends on OS.

To compile the Greeter class on the command line enter:

cmd>  javac Greeter.java

The compiler (javac) will create a file named Greeter.class containing Java “byte code” for your class. Java byte code files always have .class extension.

Run the byte code in the Java Virtual Machine (JVM). The command is java. Just type:

cmd>  java Greeter

note that you type just the class name (Greeter), not Greeter.class.

What’s a Java Virtual Machine and why bother?

The Java compiler produces hardware-independent byte code. The “byte code” is instructions for a virtual machine, called the Java Virtual Machine (JVM). The JVM works the same on all computers, so you can run the same byte code on any computer that has a JVM.

As a developer, this means you can write and compile your code once, and users can run it on any platform that has a JVM. The only requirement is that the JVM must be at least the minimum version you used to compile your source code. (If your code is compiled for Java 8 and someone only has a Java 7 JVM then it won’t run. But you can correct this by specifying optional flags to the Java 8 compiler to tell it to create Java 7 byte code.)

Experiments

Compile Greeter.java to produce Greeter.class. Give the Greeter.class file to someone using a different kind of computer (e.g. if you have a PC running Windows, give it to someone using a Mac). Verify that they can run it!
Modify Greeter.java to display the Java version (see below). Compile it for an old version of java using the -source and -target flags. Install an old version of the JRE (or find a computer with one) and run it.

Modified Greeter to show OS and Java version:

public class Greeter {
    public static void main(String[] args) {
        String ver = System.getProperty("java.version");
        String os  = System.getProperty("os.name");
        System.out.println("Hello, Human. You are running Java "+ver
                +" on "+os);
    }
}

To compile for Java 1.6 (aka Java 6) use:

cmd> javac -source 1.6 -target 1.6 Greeter

Suppose Java 6 JRE is in the directory /java/jdk6/jre/bin (Java 7 would also work) then you can run Greeter.class using:

cmd> /java/jdk6/jre/bin/java  Greeter

It should print:

Hello, Human. You are running Java 1.6.0_24 on Windows

The update (24) and OS (Windows) may vary.