Java: Introduction

Java is a high-level programming language. A high-level programming language is easy to write and read for humans, and it’s portable, which means it can run on different computers and systems. Java, Python, C++, C, and Perl etc. are all high-level languages. In contrast, a low-level language such as machine language and assembly language, is hard to write or read for humans. They can run directly by a computer, while high-level languages need to be translated to low-level language before running. A low-level language isn’t portable, which means it can only run on one kind of computer.

There are two ways of translating high-level language to low-level language: interpreting and compiling. Interpreting is to translate a program line by line, alternatively reading lines and carrying out commands in an interpreter. Compiling is to translate a program all at once before running any of the commands in a compiler. In compiling, the high-level language is called the source code and the low-level language is called the object code or executable. Java is both interpreted and compiled. Java’s compiler generates byte code, a kind of machine language, for Java Virtual Machine (JVM), which is easy to interpret and portable.

+-------------+    +----------+    +--------------+    +-------------+    +------------------+
| source code |--> | compiler |--> | byte code    |--> | interpreter |--> | result on screen |
| x.java {io} |    +----------+    | x.class {io} |    +-------------+    | {io}             |
+-------------+                    +--------------+                       +------------------+

The process of analyzing the structure of a sentence is called parsing.

Basic operations in a program for all languages:

• Input
• Output
• Math
• Testing
• Repetition

The hierarchy of Java the programming language:

Library: a collection of method and class definitions
└── Package: a collection of classes, e.g. java.lang, java.awt 
    └── Class: a collection of related methods; a set of objects 
        └── Object: a collection of related data with a set of methods operating on it 
            └── Method: a named collection of statements 
                └── Statement

Java is also a development platform which aims at “Write once, run everywhere”, i.e. the same program written in Java can run on various platforms. Java has different editions:

• Java Standard Edition (Java SE)
  • Java Development Kit (JDK)
  • Java Runtime Environment (JRE)
• Java Enterprise Edition (Java EE)
• Java Mobile Edition (Java ME)

Installation

For Windows:

1. Download the package and install it as instructed.

2. Create the JAVA_HOME environment variable: In CMD type:

   setx /m java_home C:\Program Files\Java\jdk1.8.0_101
   

3. Add %JAVA_HOME%\bin to your execution path (classpath). See path and class path.

4. Check the JDK version installed on your OS using javac -version.

For RHEL-like linux distros (Centos, Fedora), the packages are named as below:

• bin: java-1.8.0-openjdk-devel
• bin: java-11-openjdk-devel.x86_64
• src: java-11-openjdk-src.x86_64
• doc: java-11-openjdk-javadoc.x86_64

You can install them with yum or dnf:

sudo dnf install java-devel
sudo dnf install java-src
java -version

See more at openjdk and Fedora docs.

Quick start

Your first “Hello World!” program.

Open a text editor and put the following code in it, and save as “HelloWorld.java”:

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

Open the shell on your system, e.g. bash on Linux and CMD on Windows, run these commands in the directory where the file is saved:

javac HelloWorld.java
java HelloWorld

You should see the output Hello World!.

If you bump into this error:

Exception in thread "main" java.lang.UnsupportedClassVersionError: HelloWorld has been compiled by a more recent version of the Java Runtime (class file version 55.0), this version of the Java Runtime only recognizes class file versions up to 52.0.

See How to Fix java.lang.UnsupportedClassVersionError.

As you see in the directory there are two files, HelloWorld.java and HelloWorld.class. Files with .java suffix are Java source code files and .class are bytecode files compiled from the .java files. There’s another file format the jar files which have the suffix .jar. These are Java archive files, where the standard classes are stored. Most of the standard Java classes are stored in rt.jar in your JAVA_HOME.

• Regular jar files:
  • In Eclipse:
    1. In package explorer right click the project and select “Export”;
    2. Select “JAR File” and click “Next”;
    3. In the box labeled “JAR file” enter a name for the jar file. The name should end with “.jar”. Click “Finish”.
  • Commands:
    1. Compile the source code files;
    2. Execute jar cf <name>.jar *.class. This works only if all the classes are in the default package; .class means every class in the package is included in the jar file. You can specify the class names individually and separate them with spaces to include only certain classes.
• Executable jar files:
  • In Eclipse:
    1. In the 3rd step of creating a regular jar file, instead of clicking “Finish”, click “Next” twice to get to the “Jar Manifest Specification” screen;
    2. In the box labeled “Main class” enter the name of the class that will be run when the jar file is executed. Click “Finish”.
  • Commands:

    1. Compile the source code files;

    2. Create a manifest file (a plain-text file without an extension) that contains the line

       Main-Class: <class name> 
       

       where class name is the name of the class that contains the main() method. This line must end with a new line. Then put the manifest file in the same directory where you will issue the jar command;

    3. Create the jar file by executing:

       jar cmf <manifest file> JarFileName.jar *.class
       

    4. Run the jar file:

       java -jar JarFileName.jar
       

Developing tools

Java programs are normally developed with an IDE, i.e., Integrated Developing Environment. Here are the most commonly used IDEs:

• Intellij IDEA
• Eclipse
• NetBeans

You can also use plain text editors such as Emacs or Vim. After all, codes are just plain texts!