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Programming and Compiling

Most Java applications only require the JRE (Java Runtime Environment). But to write and compile you need the JDK (Java Development Kit). While the JRE provides Java’s standard libraries and exceptions as well as a JVM, the JDK provides all the above as well as javac, the compiler. Java source code is written in text files labeled with .java extension. It is then compiled into bytecode in .class files by javac. Then the bytecode is executed by the JVM, which translates the Java commands into low-level instructions to the operating system.

Since Java 6, all Java programs not run inside a container (such as a Servlet Web Container) start and end with the main method. The class containing the main method can have any name, but the method itself should always be named main

class Example {
    public static void main(String[] args) {
        System.out.println("Num args:" + args.length);
    }
}

• public is a Java access modifier keyword that means the main method can be accessed from any method during the program’s execution.
• static is a Java keyword that means the method can be invoked without creating an instance of the class that contains it, making it a global method.
• void is a Java return type keyword that means the method doesn’t return any values of any data type.
• args is a Java variable of type String array which means the method can take command line arguments as an array of Strings

We can compile this code into a .class file of the same name:

javac Example.java

And to run the resulting Example.class file:

java Example

The java and javac commands require the full directory path or class path to any source code or binary file respectively. If you have a package com.demo in the first line of Example, then you would nest the java file into a com/demo/ directory and then run:

javac com/demo/Example.java

java com.demo.Example

From here we can add packages and imports, expanding the application into a set of interacting objects. By default, the javac compiler implicitly imports several base packages from the standard library.

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