Java Keywords

Reserved words that have special meaning in Java

🔑 What are Java Keywords?

Java keywords are reserved words with special meanings that cannot be used as variable names. They control program flow, define data types, and specify access levels in your Java code. 


// Keywords in action
public class MyClass {
    private int number;
    public static void main(String[] args) {
        if (true) {
            System.out.println("Hello Java!");
        }
    }
}

Types of Java Keywords

🔒

Access Modifiers

Control visibility of classes and methods

public private protected
📊

Data Types

Define what kind of data to store

int String boolean
🔄

Control Flow

Control program execution flow

if for while
🏗️

Class Keywords

Define classes and inheritance

class extends implements

🔹 Access Modifier Keywords

Control who can access your classes, methods, and variables: 

public class Example {
    public int publicVar = 1;      // Accessible everywhere
    private int privateVar = 2;    // Only within this class
    protected int protectedVar = 3; // Within package and subclasses
    int defaultVar = 4;            // Within package only
    
    public void showAccess() {
        System.out.println("Public: " + publicVar);
        System.out.println("Private: " + privateVar);
    }
}

Output:

Public: 1

Private: 2

🔹 Data Type Keywords

Specify what type of data your variables can hold: 

public class DataTypes {
    public static void main(String[] args) {
        int age = 25;              // Whole numbers
        double price = 19.99;      // Decimal numbers
        boolean isActive = true;   // True or false
        char grade = 'A';          // Single character
        String name = "John";      // Text
        
        System.out.println("Age: " + age);
        System.out.println("Price: $" + price);
        System.out.println("Active: " + isActive);
    }
}

Output:

Age: 25

Price: $19.99

Active: true

🔹 Control Flow Keywords

Control how your program executes: 

public class ControlFlow {
    public static void main(String[] args) {
        // if-else statement
        int score = 85;
        if (score >= 90) {
            System.out.println("Grade: A");
        } else if (score >= 80) {
            System.out.println("Grade: B");
        } else {
            System.out.println("Grade: C");
        }
        
        // for loop
        for (int i = 1; i <= 3; i++) {
            System.out.println("Count: " + i);
        }
    }
}

Output:

Grade: B

Count: 1

Count: 2

Count: 3

🔹 Common Keywords Reference

Essential keywords every Java beginner should know:

Most Used Keywords:

  • public: Makes code accessible everywhere
  • static: Belongs to class, not instance
  • void: Method returns nothing
  • class: Defines a new class
  • new: Creates new objects
  • return: Exits method with value
  • this: Refers to current object
  • super: Refers to parent class 
public class KeywordDemo {
    private String message;
    
    public KeywordDemo(String msg) {
        this.message = msg;  // 'this' refers to current object
    }
    
    public void display() {
        System.out.println(this.message);
        return;  // Optional for void methods
    }
    
    public static void main(String[] args) {
        KeywordDemo demo = new KeywordDemo("Hello Keywords!");
        demo.display();
    }
}

🧠 Test Your Knowledge

Which keyword is used to create a new object in Java?