Java-21.0.1

About Java

Java is different from most of the packages in LFS and BLFS. It is a programming language that works with files of bytecode to obtain instructions and executes them in a Java Virtual Machine (JVM). An introductory java program looks like:

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

This program is saved as HelloWorld.java. The file name, HelloWorld, must match the class name. It is then converted into byte code with javac HelloWorld.java. The output file is HelloWorld.class. The program is executed with java HelloWorld. This creates a JVM and runs the code. The 'class' extension must not be specified.

Several class files can be combined into one file with the jar command. This is similar to the standard tar command. For instance, the command jar cf myjar.jar *.class will combine all class files in a directory into one jar file. These act as library files.

The JVM can search for and use classes in jar files automatically. It uses the CLASSPATH environment variable to search for jar files. This is a standard list of colon-separated directory names similar to the PATH environment variable.