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01 running java programs

Class in Java

In Java, a class is a fundamental unit of object-oriented programming. A class defines the behavior and characteristics of an object. Generally, a class can contain attributes (variables) and methods (functions).

Example of defining a simple class in Java:

public class MyClass {
    // Attributes (variables) can be defined here

    // Methods can be defined here
}

main Method in Java

In Java programming, the main method plays a crucial role as the entry point of any standalone Java application. When you run a Java program, the Java Virtual Machine (JVM) looks for the main method to start the execution of the program.

Structure of the main Method

The main method has a specific signature, and its structure looks like this:

public static void main(String[] args) {
    // Code to be executed when the program starts
}

  • public: The main method is declared as public, which means it can be accessed from outside the class.

  • static: The main method is static, allowing it to be called without creating an instance of the class. This is a requirement for the JVM to invoke the method without creating an object of the class.

  • void: The main method does not return any value.

  • main: The name of the method, which is recognized by the JVM as the entry point.

  • String[] args: The method takes an array of strings as parameters, allowing you to pass command-line arguments to the program.

Steps for Compilation and Execution:

  1. Open IntelliJ IDEA:

    • Open IntelliJ IDEA and make sure your Java project is loaded.

  2. Create or Open a Java File:

    • Create a new Java file (e.g., MyProgram.java) or open an existing one.

  3. Write Your Java Code:

    • Write your Java code, including the main method.

  4. Add Arguments in IntelliJ:

    • If your program expects command-line arguments, you can specify them in IntelliJ.
    • Navigate to Run > Edit Configurations...
    • In the "Program arguments" field, enter your arguments separated by spaces.

  5. Compile and Run:

    • Click on the green "Run" button in the toolbar or press Shift + F10 to compile and execute your program.

Running from the Command Line

  1. Open a Terminal or Command Prompt:

    • Open a terminal on Linux or MacOS, or a Command Prompt / PowerShell on Windows.

  2. Navigate to the Project Directory:

    • Use the cd command to navigate to the directory where your Java file is located.

  3. Check Java Installation:

    • Ensure that Java is installed on your system by running: 
      java -version

  4. Check Java Compiler Installation:

    • Ensure that the Java compiler (javac) is installed by running: 
      javac -version

  5. Set JAVA_HOME (if not set):

    • If JAVA_HOME is not set, set it to the path of your JDK installation directory. For example:
      • Linux / MacOS: 
        export JAVA_HOME=/path/to/your/jdk
export PATH=$PATH:$JAVA_HOME/bin
      • Windows: 
        set JAVA_HOME=C:\Path\To\Your\JDK
set PATH=%PATH%;%JAVA_HOME%\bin

  6. Compile the Java File:

    • Compile your Java file using the javac command. For example: 
      javac MyProgram.java

  7. Run the Java Program:

    • Execute your Java program using the java command. If your program expects arguments, provide them after the class name. For example: 
      java MyProgram arg1 arg2 arg3