Java - Day 3: Understanding Errors, Security, and More

Introduction

Welcome to Day 3 of learning Java! Today, we'll cover key concepts like errors, security, memory management, JDBC, OOP vs Functional Programming, and Java versions. We'll also explore why machines understand only 0s and 1s, and how Java ensures security with bytecode verification.

1️⃣ Compilation vs Runtime Errors

In Java, a program may compile successfully but fail to run if there are runtime errors.

Example: Code Compiles but Does Not Run

class Home {
    // No main method
}

✅ Compiles Successfully

❌ Fails at Runtime because Java looks for a main() method to execute.

Code with an Empty main() Method

class Home {
    public static void main(String[] args) {
        // Empty main method
    }
}

✅ Compiles Successfully

✅ Runs Successfully, but does nothing because main() is empty.

2️⃣ Platform Independence & JVM Architecture

JDK - Platform Independent

• Java source code is written once and compiled into bytecode (.class files).
• This bytecode can run on any OS with a compatible JVM.

JVM - Platform Dependent

• Each OS has its own version of JVM (Windows JVM, Linux JVM, etc.), making JVM platform dependent.
• The JVM translates bytecode into machine code that the OS can understand.

3️⃣ Java Security & Bytecode Verification

Why is Java Secure?

✔ No Pointers – Prevents direct memory access.

✔ Bytecode Verification – Ensures no malicious bytecode is executed.

✔ Security Manager – Restricts unauthorized access to system resources.

Bytecode Verification Process

1. Java compiler converts source code into bytecode.
2. JVM’s Class Loader loads the bytecode.
3. Bytecode Verifier checks for security risks (e.g., invalid access).
4. JVM executes the verified bytecode.

4️⃣ Memory Management & Garbage Collection

Memory Management in Java

• Heap Memory: Stores objects.
• Stack Memory: Stores method calls and local variables.

Garbage Collector (GC) - Automatic Memory Cleanup

• Daemon Thread: Runs in the background.
• Frees unused objects automatically, preventing memory leaks.

5️⃣ Why Java Doesn’t Use Pointers

• Java avoids pointers to prevent direct memory manipulation, reducing security risks.
• Instead, Java uses references, making code safer and easier to manage.

6️⃣ Binary & Decimal Conversion

Binary to Decimal Conversion

Example: Convert 1011 (binary) to decimal.

(1 × 2³) + (0 × 2²) + (1 × 2¹) + (1 × 2⁰) = 8 + 0 + 2 + 1 = 11

1011 (binary) = 11 (decimal)

Decimal to Binary Conversion

Example: Convert 13 (decimal) to binary.

13 ÷ 2 = 6 remainder **1**  
6 ÷ 2 = 3 remainder **0**  
3 ÷ 2 = 1 remainder **1**  
1 ÷ 2 = 0 remainder **1**

13 (decimal) = 1101 (binary)

7️⃣ Template in Java

• A template is a blueprint that allows reusable code structures.
• Java Generics are an example of templates:

class Box<T> {
    T item;
    void add(T item) { this.item = item; }
}

✔ Allows Box and Box without rewriting code.

8️⃣ Error vs Exception vs Bug

9️⃣In Java, **"instance" and "object" are closely related but not exactly the same.

1️⃣ Object

• An object is a specific entity created from a class.
• It has identity (name), state (values), and behavior (methods).
• Objects are stored in heap memory when created.

Example of Object

class Car {
    String color;
    void drive() {
        System.out.println("Car is driving");
    }
}

public class Main {
    public static void main(String[] args) {
        Car myCar = new Car();  // Object creation
        myCar.color = "Red";  
        myCar.drive();
    }
}

✔️ Here, myCar is an object of the Car class.

2️⃣ Instance

• An instance refers to a specific realization of a class in memory.
• When we create an object, we say "this is an instance of the class."
• Every object is an instance, but the term 'instance' emphasizes its existence in memory.

Example of Instance

Car car1 = new Car();  // Instance 1
Car car2 = new Car();  // Instance 2

✔️ Here, car1 and car2 are instances of the Car class.

Key Differences Between Instance and Object

3️⃣ Another Example to Understand

class Student {
    String name;
}

public class Main {
    public static void main(String[] args) {
        Student s1 = new Student(); // Instance/Object 1
        Student s2 = new Student(); // Instance/Object 2
    }
}

✅ s1 and s2 are both objects and instances of the Student class.

Conclusion

✔️ "Object" focuses on the real-world entity and its behavior.

✔️ "Instance" emphasizes that the object exists in memory as a specific realization of a class.

🔟 Java: OOP vs Functional Programming

✔ Java is OOP-first but also supports functional programming (since Java 8).

✔ Functional Features: Lambdas, Streams, and Functional Interfaces.

Example: OOP vs Functional Java

// OOP Style
class Car { 
    void drive() { System.out.println("Driving"); }
}
Car car = new Car();
car.drive();

// Functional Style
Runnable r = () -> System.out.println("Running");
r.run();

1️⃣1️⃣Java Versions (J2SE, J2EE, Spring Boot)

✔ Core Java (J2SE) – Standard Edition for general applications.

✔ J2EE – Used for enterprise applications (Spring, Hibernate).

✔ Most Popular Versions: Java 1.8, Java 11, Java 17.
✔ As of March 2025, the latest version of Java is **Java 24 (JDK 24), released on March 18, 2025. The most recent Long-Term Support (LTS) version is Java 21 (JDK 21), released in September 2023. **

1️⃣2️⃣To be discussed...

1.Why Do Machines Understand Only 0s and 1s?**

• Computers use binary (0 and 1) because transistors can only be ON (1) or OFF (0).
• Other bases (2, 3, etc.) would require different hardware structures, which aren't practical.

2.Why Did Java Evolve from OOP to Both?
OOP is structured and works well for large applications.

Functional Programming (FP) allows concise and efficient code using lambda expressions and streams.

The demand for parallel processing and data-driven programming made FP features necessary.

3.✅ If the class is NOT public, the filename doesn’t have to match, and it runs without issues.
✅ If the class IS public, the filename must match, or you get a compilation error.

4.Java: OOP vs Functional Programming

Reference Sources

1️⃣ Classroom Lessons

2️⃣ ChatGPT Discussion