CORE JAVA
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Compile and Run First Java Program

In this section we will learn how to write and execute a simple Java program.

  • To write a Java program you can use any text editor like Notepad, Notepad++ (Windows) or gedit (Linux).

  • You can also use popular IDEs such as BlueJ, NetBeans, or Eclipse to write and execute Java programs.

  • To compile your Java source code, you need a Java Compiler, and to run it, you need a Java Interpreter. Both are included in the JDK. Ensure JDK is installed and configured in your system.

Steps in Programming with Java

1. Create the Source Code

Write your program using a text editor and save it with the extension .java.

Example: HelloWorld.java

2. Compile the Program

Use the command:

javac HelloWorld.java

After successful compilation, a HelloWorld.class file (bytecode) will be generated.

compilation process
3. Execute the Program

Run the program using:

java HelloWorld

Note: Do not include .class in the execution command.

execution process

Example: HelloWorld.java

HelloWord.java Copy Code 
public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello World");
    }
}
HelloWorld output

Explanation

  • class: keyword to declare a class.
  • public: access specifier, visible to all.
  • static: method belongs to class, no object needed.
  • void: return type, means no value returned.
  • main: entry point of the program.
  • String[] args: stores command-line arguments.
  • System.out.println(): outputs text to console.

Source Code Facts

  • Written by programmer in high-level language.
  • Saved with .java extension.
  • May contain multiple classes, but only one public class.
  • If a public class exists, filename must match the class name.
  • Structure: 
    package declaration;
import statements;
class ClassName {
   // class members
}

Bytecode Facts

  • Compiler generates bytecode (.class file).
  • Bytecode is intermediate and platform-independent.
  • Only JVM can interpret and run bytecode.
  • Each class generates its own bytecode file.
  • Example: HelloWorld.class
  • If a class is run directly by JVM, it must contain the main() method.
Bytecode generation

Next: JDK, JRE, and JVM


🚀 Quick Knowledge Check

Topic: Writing_executing_java | Language: Java

Q1. What is the purpose of 'System.out.println()' in Java?
Q2. In Java, which method serves as the entry point of execution?
Q3. After compiling HelloWorld.java successfully, which file will be generated?
Q4. Which tool is responsible for interpreting and running Java bytecode?
Q5. Which file extension must be used when saving a Java source code file?
Q6. Which statement about bytecode is correct?
Q7. Which command is used to execute a compiled Java program?
Q8. Which of the following is NOT true about Java source code?
Q9. What is Java bytecode?
Q10. Which command is used to compile a Java program?