Examples

• 181

  Creating a new Java program

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  181

  Create a new text file named HelloWorld.java and paste this code in it:

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

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  Note: In order for Java to recognize this as a class (and not throw a compiler error), the filename must comprise the name of the class (HelloWorld in this example) with a .java extension. Convention states that Java classes should begin with an uppercase character.

  To compile your code, open the directory where HelloWorld.java is located in a terminal window:
  cd /path/to/containing/folder/ and enter javac followed by the file name and extension as follows:

  $ javac HelloWorld.java
  

  Note: The javac command invokes the Java compiler.

  The compiler will then generate a bytecode file called HelloWorld.class which can be executed in the Java Virtual Machine (JVM). The Java programming language compiler, javac, reads source files written in the Java programming language, and compiles them into bytecode class files. Optionally, the compiler can also process annotations found in source and class files using the Pluggable Annotation Processing API. The compiler is a command line tool but can also be invoked using the Java Compiler API.

  Next enter java, followed by the name of the class with the main method (HelloWorld in our example). Notice how the .class is omitted:

  $ java HelloWorld
  

  Note: The java command runs a Java application.

  This will output:

  Hello, World!

  You have successfully programmed and built your very first Java program!

  Note: In order for Java commands (java, javac, etc) to be recognised, you will need to make sure:

  • A JDK is installed (e.g. Oracle, OpenJDK and other sources)
  • Your environment variables are properly set up

  You will need to use a compiler (javac) and an executor (java) provided by your JVM. java -version and javac -version on the command line, will report which versions (e.g. 1.8.0_73) of these are installed.

  ---

  A closer look at the Hello World application

  The Hello World application consists of a HelloWorld class definition and a main method.

  The class keyword begins the class definition for a class named HelloWorld. Every Java application contains at least one class definition (Further information about classes).

  The main method:

  This is an entry point method (defined by its name main) from which the JVM can run your program. Every Java program should have one. It is:

  • public: meaning where the method can be called from isn't limited to inside your program.
  • static: meaning it exists and can be run by itself (at the class level without creating an object).
  • void: meaning it returns no value. Note: This is unlike C where a return code such as int is expected (Java's way is System.exit()).

  It accepts :

  • An array (called args) of Strings used to pass arguments into your program (e.g. from command line arguments).

  Almost all of this is required for a Java entry point method. The exceptions being:

  • The name args is a variable name, so can be called anything.
  • Whether its type is an array or Varargs (String... args).

  Note A single application may have multiple classes containing an entry point (main) method, and the entry point of the application is determined by the class name passed as argument to the java command.

  edited Oct 4 at 16:26

  

  

  

  

  

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