Compiling Java Program

Java Development Kit (JDK)

Modern versions of Java are available in two primary forms:

• Oracle JDK – Downloadable from: www.oracle.com/java/technologies/downloads

• OpenJDK – Open-source version available at: https://jdk.java.net

The Java Development Kit (JDK) includes two essential command-line tools:

1. javac – The Java compiler, which translates source code into bytecode.

2. java – The Java interpreter (also known as the application launcher) that runs compiled bytecode.

TIP

The JDK operates through the command line. It is not a graphical IDE (like Eclipse or IntelliJ IDEA), nor is it a windowed application.

Sample Java Program

/* file name: Example.java */

class Example {
    // A Java program begins with a call to main()
    public static void main(String[] args) {
        System.out.println("Java drives the Web.");
    }
}

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Three Basic Steps to Execute a Java Program

1. Enter the program

2. Compile the program

3. Run the program

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Entering the Program

• Use a text editor (e.g., Notepad, VS Code), not a word processor like MS Word. Word processors add formatting that can confuse the Java compiler.

• The source file name must match the name of the main class. For example, the class Example must be saved in a file called Example.java.

• Java source files must use the .java extension.

• A source file is called a compilation unit and may contain one or more class definitions.

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Compiling the Program

Use the Java compiler (javac) from the command line:

javac Example.java

This creates a file called Example.class, which contains the compiled bytecode which is not executable code.

NOTE

Bytecode is not directly executable. It must be interpreted or compiled at runtime by the Java Virtual Machine (JVM).

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Running the Program

Use the Java interpreter (java) by passing the class name (without the .class extension):

java Example

The interpreter will look for a file named Example.class, then execute the bytecode.

NOTE

Every class is compiled into its own .class file, so naming consistency is important.

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Key Features Common to All Java Programs

Comments

• /* ... */ – Multi-line comment

• // – Single-line comment

Class Declaration

class Example {
    ...
}

• Declares a new class named Example.

• All Java code must reside inside a class.

The main() Method

public static void main(String[] args) {
    ...
}

• public: Accessible from outside the class (required by JVM).

• static: Can run without an object of the class.

• void: The method does not return a value.

• String[] args: Command-line arguments are passed as a string array.

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Output with System.out.println()

System.out.println("Java drives the Web.");

• System is a predefined class.

• out is the output stream connected to the console.

• println() is a method to display output with a newline.

• print() is similar but does not add a newline.

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/* File: Example2.java */

class Example2 {
    public static void main(String[] args) {
        int myVar1;
        int myVar2;

        myVar1 = 1024;
        System.out.println("myVar1 Contains " + myVar1);

        myVar2 = myVar1 / 2;
        System.out.print("myVar2 contains myVar1 / 2: ");
        System.out.println(myVar2);
    }
}

$ javac Example2.java
$ java Example2
myVar1 Contains 1024
myVar2 contains myVar1 / 2: 512

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Working with Floating-Point Numbers

Java supports two floating-point types:

• float (single-precision)

• double (double-precision, more common)

/*
GalToLit.java
Program to convert gallons to liters
*/

class GalToLit {
    public static void main(String[] args) {
        double gallons;
        double liters;

        gallons = 10;
        liters = gallons * 3.7854;

        System.out.println(gallons + " gallons is " + liters + " liters");
    }
}

$ javac GalToLit.java
$ java GalToLit
10.0 gallons is 37.854 liters

NOTE

To print a blank line in Java, simply call System.out.println(); with no arguments.

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