First Java Application: Setup and Execution

• The document outlines the steps to prepare and execute a first Java application

Steps to Prepare First Java Application

• Install Java Software
• Select a Java Editor
• Write a Java Program
• Save the Java File
• Compile the Java File
• Execute the Java Application

Installing Java Software

• Download the JDK (Java Development Kit) executable file from the internet, e.g., jdk-8-windows-i586.exe
• Double-click the downloaded .exe file to start the installation
• During installation, you might need to click "Yes," "Next," and "OK" buttons
• Change the JDK installation location to "C:\Java\jdk1.8.0" (or similar) by clicking "Change" and "OK"
• Change the JRE (Java Runtime Environment) installation location similarly, to "C:\Java\jre8" (or similar)
• After installation, set the "path" environment variable to include the JDK's bin directory (e.g., C:\Java\JDK1.8.0\bin)
• This makes Java commands available to the operating system
• You can temporarily set the path variable using the command prompt with: set path=C:\Java\JDK1.8.0\bin;
• This change is only valid for the current command prompt session.
• To set the path permanently, use the following steps:
  • Right-click "This PC" or "Computer" on the desktop
  • Select "Properties"
  • Select "Advanced System Settings"
  • Click on the "Environment Variables" button
  • In the "User variables" Section, click the "New".
  • In the "Variable Name" field set the value to "path"
  • Set the variable value to the JDK's bin directory (e.g., C:\Java\jdk1.8.0\bin;)
  • Click "OK" on all open windows

Handling Multiple Java Versions

• If multiple Java versions (e.g., JAVA6, JAVA7, JAVA8) are set in the "path" environment variable, the system will use the first one in the order specified
• For example, if path=C:\Java\jdk1.6.0\bin;C:\Java\jdk1.7.0\bin;C:\Java\jdk1.8.0\bin;, java -version will show JDK 1.6.0
• Batch files can simplify switching between Java versions
• Create a text file with the "path" command for a specific Java version, and save it with a ".bat" extension (e.g., java6.bat)
• To switch versions, open the command prompt, navigate to the directory containing the ".bat" file, and run the batch file (e.g., java6.bat)

Selecting a Java Editor

• A Java editor is a software used for writing and saving Java programs
• Examples include Notepad, Notepad++, and Editplus
• However, for real-time application development, using Integrated Development Environments (IDEs) like Eclipse, MyEclipse, or Netbeans is recommended

Writing a Java Program

• Use predefined libraries provided by the JAVA API (Application Programming Interface)
• A basic Java program structure includes:

class Test {
    public static void main(String[] args) {
        System.out.println("First Java Application");
    }
}

Saving a Java File

• If the Java file contains a public element (class, abstract class, interface, enum), it must be saved with the same name as the public element (e.g. "Test.java" for public class Test)
• Violating this condition will cause a compilation error
• If the Java file contains no public element, it may be saved with any name (e.g., abc.java).
• It is suggested to save the java file with the main() method class name.

Restrictions on Public Classes

• It's not possible to have more than one public class in a single Java file
• If attempted, a compilation error will occur because the filename must match the public class name, which is impossible with multiple public classes

Compiling Java File

• The main purposes of compiling a JAVA file are to convert the JAVA program from a High-Level Representation to a Low-Level Representation, and check for compilation errors
• Use the command javac File_Name.java (e.g., javac FirstApp.java) in the command prompt
• The operating system searches for the "javac" command in predefined locations specified by the "path" environment variable
• If the javac program is not found, an error message will appear; otherwise, the Java Compiler software is activated
• The compiler searches for the specified .java file in the current location
• If the file is not found, an error message is displayed
• If the file is found, the compiler starts the compilation process from beginning to end
• If syntax errors are found during compilation, the compiler show error messages
• If no syntax errors are found, a .class file (or files) will be generated in the same directory
• The number of .class files depends on the number of classes, abstract classes, interfaces, enums, and inner classes in the Java file
• To compile from the current location but put the .class files in a different directory: javac -d target_location File_Name.java

Package Declaration

• Using the -d option with javac is also used when you use package declaration statement in java file
• The generated .class files for each member of the package will be stored in a directory structure w.r.t the package name
• Consider the following example

package com.durgasoft.core;
enum E {
}
interface I {
}
abstract class A {
}
class B {
    class C {
    }
}
class FirstApp {
    public static void main(String[] args) {
    }
}

• If you use command: javac -d c:\abc FirstApp.java
• The compiler will compile the file and generate E.class, I.class, A.class, B.class, B$C.class, FirstApp.class files to the location "C:\abc\com\durgasoft\core"
• You can compile all files in a directory by using: javac *.java, however, this will create the package directoy structure at your curent directory

Executing JAVA Application

• Use the command java Main_Class_Name (e.g., java FirstApp) to execute the program
• The operating system will run the "java" program (the JVM)
• The JVM will then execute the following actions:
• JVM takes the Main_Class_Name from the command prompt
• JVM searches for that class in: current location, java libraries, locations in "classpath" variable
• If the .class file isn't found in those locations, either java.lang.NoClassDefFoundError (Java 6) or "Error: Could not find or load main class FirstApp" (Java 7+) will occur
• To make the .class file available from another location, set "classpath" (e.g., set classpath=E:\XYZ;).
• After finding the main class, JVM will load/read bytecode to the memoery using "Class Loaders".
• Next JVM will search for the main() method.
• If the main() method is not found, a java.lang.NoSuchMethodError:main (Java 6) or an "Error: Main method not found in class FirstApp, please define main method as: public static void main(String args[])" (Java 7+) will be displayed
• If the main() method exists, then JVM will create a thread called "Main thread" and execute the main() method
• Once the main() method execution completes, the main thread is destroyed, and the JVM shuts down.